Product Guide

High Voltage Gapless ZnO Surge Arresters

ABB Power Technologies

High Voltage Products
Surge Arresters
Ludvika, Sweden

2004 Edition

ABB
TABLE OF CONTENTS Page

1. INTRODUCTION 3

2. DEFINITIONS 4

3. OVERVOLTAGES 10
3.1 External Overvoltages
3.2 Internal overvoltages

4. HISTORICAL BACKGROUND 13
4.1 Operation of gapped and gapless surge arresters

5. FEATURES OF ZnO ARRESTER DESIGN 18

5.1 ZnO varistor
5.2 Housing of a surge arrester
5.3 Polymer arrester designs

6. DESIGN REQUIREMENTS FOR ZnO SURGE ARRESTERS 29

6.1 Designing for continuous stresses
6.2 Designing for non-continuous stresses

7. STANDARDS AND TESTING 42

7.1 Type Tests
7.2 Routine Tests
7.3 Acceptance Tests

8. ARRESTER CLASSIFICATION AS PER STANDARDS 66

9. ARRESTER SELECTION 68
9.1 Matching the electrical characteristics
9.2 Matching the mechanical characteristics

10. INSTALLATION GUIDELINES 80

11. MAINTENANCE AND MONITORING 82

11.1 Condition monitoring
11.2 Replacement of gapped surge arresters

12. SPECIAL APPLICATIONS 86

12.1 Reduced clearance distances
12.2 Station protection
12.3 Lightning protection of transmission lines
12.4 Switching surge control in EHV systems
12.5 Shunt capacitor banks
12.6 Series capacitors
12.7 HVDC arresters
12.8 Current sharing considerations

13. REFERENCES 94

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1. INTRODUCTION

All electrical systems and equipment are subjected to electrical stresses caused by higher than
normal voltages many times during their lifetime. Such overvoltages are caused by atmospheric
disturbances (lightning), switching phenomena as well as system disturbances, and these
cannot be avoided.

It is vital that the electrical equipment operates fault-free during such abnormal conditions.
However, for economic reasons, it is not possible to insulate the electrical equipment with a
sufficiently high withstand level to survive all these overvoltages, particularly those resulting
from lightning or switching surges. Consequently, these pose a very real danger for causing
failure of the electrical equipment. An economical and safe on-line network therefore requires
extensive protection against unacceptable overvoltage loads.

Overvoltage protection is not new, and has been used in one form or another for well over
100 years. Today, overvoltage protection can basically be achieved in two ways (sometimes in
combination):
• Avoid or limit the overvoltages at the point of origin. For example, through the use of
overhead shield earth wires and lower tower footing resistance as countermeasures
against atmospheric overvoltages and pre-insertion resistors and/or controlled switching
against switching overvoltages.
• Limit overvoltages near the electrical equipment with surge arresters

In isolation, shield earth wires and pre-insertion resistors offer a degree of protection. However,
by their nature, surge arresters provide the primary protection against different types of
overvoltages (atmospheric and switching). They are generally connected between each phase
and ground, in parallel with the equipment to be protected and function to divert the surge
current safely to earth; thereby limiting the overvoltage seen by the protected object.

Insulation co-ordination is the art and science of choosing the right insulation strength of
electrical equipment taking into account normal and abnormal service conditions as well as the
characteristics and location of suitable surge arresters.

Despite being a well-established technology, there remains a degree of mysticism about the
design, selection and application of surge arresters in electrical networks. This is not made
easier through the continual improvement and development of the active elements by leading
manufacturers as well as the designs and housing material, ultimately leading to new
applications for surge arresters.

This guide is intended to clear away some of this mystification, and guide the reader to a better
understanding of how to select and use modern day surge arresters. It is principally limited to
the common application of the protection of transformer insulation between phase and ground in
outdoor air-insulated substations. Other applications are briefly discussed, but are, for the most
part, considered beyond the scope of this Guide. Instead, the reader is referred on to additional
technical literature which covers the topic in more detail. In addition, International Standard
IEC60099-5 “Surge arresters - Selection and application recommendations” is recommended
reading.

Finally, the reader is referred to the ABB surge arresters “Arresters Online” web page
(www.abb.com/arrestersonline) for continually updated information on surge arresters.

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2. DEFINITIONS

To permit the reader to understand the basis for the selection and application of surge arresters,
it is important to make a brief review of some of the common terminology used throughout this
Guide.

The surge arrester standards referred to herein are the prevailing editions of:

• IEC 60099-4 Second edition, 2004-05

Metal-oxide surge arresters without gaps for a.c. systems

• IEEE C62.11 1999

Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV)

Backflashover
Occurs when lightning strikes the transmission line tower structure or overhead shield wire. The
lightning discharge current, flowing through the tower and tower footing impedance, produces
potential differences across the line insulation. If the line insulation strength is exceeded,
flashover occurs, i.e. a backflashover. Backflashover is most prevalent when tower footing
impedance is high.

Continuous current (Ic)

The current that flows through the arrester at continuous operating voltage (Uc or MCOV).
This current is predominantly capacitive (in the range of mA) and is generally expressed as a
peak value.

Continuous operating voltage

It is the maximum permissible r.m.s. power frequency voltage that may be applied continuously
between the arrester terminals. This voltage is defined in different ways (verified by different test
procedures)in IEC and IEEE.

IEC (U c)
IEC gives the manufacturer the freedom to decide Uc . The value is
verified in the operating duty test. Any uneven voltage distribution in the
arrester shall be accounted for.

IEEE (MCOV)
IEEE lists the maximum continuous operating voltage (MCOV) for all
arrester ratings used in a table. The value is used in all tests specified by IEEE.
Note! MCOV is less stringent as regards uneven voltage distribution in an arrester.

Duty-cycle voltage rating (IEEE)

The designated maximum permissible voltage between its terminals at which an arrester is
designed to perform its duty cycle.

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Earth-fault factor (ke )
The ratio of the voltages in the healthy phases during and prior to earth-fault conditions.

Energy capability
The energy that a surge arrester can absorb in one or more impulses, without damage and
without loss of thermal stability. The capability is different for different types and duration of
impulses.

Standards do not explicitly define the energy capability of an arrester. The only measure
specified is the Line Discharge Class in IEC. Often, this is not enough information to compare
different manufacturers. Therefore ABB presents energy capability also in kJ/kV (Ur). This is
done in 3 different ways:

Two impulses as per IEC switching surge operating duty test

This is the energy that the arrester is subjected to in the switching surge
operating duty test while remaining thermally stable thereafter against the specified TOV
and Uc .

Routine test energy

This is the total energy that each individual block is subjected to in production tests.

Single-impulse energy
This is the maximum permissible energy, which an arrester may be
subjected to in one single impulse of 4 ms duration or longer and remain
thermally stable against specified TOV and Uc .

Follow current
The current from the connected power source which flows through an arrester with series gaps
following the passage of discharge current.

Hydrophobicity Classification
The superior electrical performance of composite insulators and coated insulators stems from
the hydrophobicity (water-repellency) of their surfaces. The hydrophobicity will change with time
due to exposure to the outdoor environment and partial discharges (corona).

Seven classes of the hydrophobicity (HC 1-7) have been defined (STRI Guide 1, 92/1).
HC 1 corresponds to a completely hydrophobic (water-repellent) surface and HC 7 to a
completely hydrophilic (easily wetted) surface. These classes provide a coarse value of the
wetting status and are particularly suitable for a fast and easy check of insulators in the field.

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Impulse (of current or voltage)
A unidirectional wave which rises rapidly to a maximum and falls, a little less rapidly, to zero. Its
waveshape is expressed by two numbers (T1/T2). T1 refers to the virtual front-time and T2 to the
virtual time to half-value of the tail; both expressed in microseconds. Some important current
impulses are defined below.

Impulse Waveshape (T 1/T2)

Steep current impulse T1 = 1 µs T2 < 20 µs
Lightning current impulse T1 = 8 µs T2 = 20 µs
Switching current impulse 30µs < T1 < 100 µs T2 ~ 2T1
(usually designated 30/60 µs)
High current impulse T1 = 4 µs T2 = 10 µs

A special impulse is the rectangular current impulse which is in the shape of a

rectangle. A common duration is 2000µs.

Insulation withstand characteristic

A general term for the equipment insulation withstand voltages and comprises:

Withstand level Voltage waveshape

Lightning impulse withstand level (LIWL) 1.2/50 µs
Switching impulse withstand level (SIWL) 250/2500 µs
Power-frequency withstand (PFW) 50 Hz or 60 Hz sinusoidal

Lightning classifying current (IEEE)

The designated lightning current used to perform the classification tests.

Maximum system voltage (U m )

The maximum voltage between phases during normal service expressed in kV r.m.s.

.
Nominal discharge current (In according to IEC)
The peak value of the lightning current impulse which is used to classify the arrester.

Normal service conditions

The service conditions which the surge arresters should normally be suitable to operate under
without any special consideration in design, manufacture or application.

Ambient temperature -40 °C to +40 °C

Solar radiation < 1.1 kW/m 2
Altitude above sea level < 1000 m (< 1800 m according to IEEE)
Power system frequency 48 – 62 Hz

This should be seen as the minimum requirement for compliance with the Standards, and
individual designs may operate in wider extremes, even without the need for special
consideration.

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Overvoltage
A voltage level exceeding the maximum allowable continuous operating voltage for an electrical
system.

Protective characteristic
The combination of the arrester’s residual voltages for different current impulses. For good
protection, the arrester characteristic should lie well below the equipment insulation withstand
characteristic at all points.

Lightning impulse withstand level (LIWL or BIL) is the equipment’s insulation

withstand level against lightning impulses

Switching impulse withstand level (SIWL or BSL) is the equipment’s insulation

withstand level against switching impulses

Lightning impulse protection level (LIPL or Upl) of the arrester is the

residual voltage for the nominal discharge current

Switching impulse protection level (SIPL or Ups ) of the arrester is the residual
voltage for a specified switching impulse current

Note! IEEE standards refer to LIWL as BIL and SIWL as BSL

Parameters Parameters
of the of the
system surge
arrester
Voltage
BIL / BSL

LIWL / SIWL Protective margin

Protection level
LIPL / SIPL
Upl / Ups

TOV
TOV capability

COV

Fig. 1 Protective function of a surge arrester

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Protective margin
The protective ratio minus 1 and expressed as a percentage. As an absolute minimum, the
margin should cover the voltage increase due to the connections between the arrester and the
protected equipment as well as the increase in the residual voltage due to the discharge
amplitude and front-time being different from the nominal discharge current of the arrester.

Protective ratio
The ratio of the equipment insulation withstand level to the corresponding protection level of its
arrester.

Rated voltage (U r )
For other apparatus, the voltage that may be applied continuously is usually called its rated
voltage. However, this is not the case for surge arresters. An arrester fulfilling the IEC standard
must withstand its rated voltage (Ur) for 10 s after being preheated to 60 °C and subjected to
two long duration current impulses , corresponding to its line discharge class as defined in the
standard. Thus, Ur shall equal at least the 10 second TOV capability of an arrester. Additionally,
rated voltage is used as a reference parameter.

Reference current (Iref)

The peak value of the power frequency resistive current at which the reference voltage is
measured.

Reference voltage (U ref)

The peak value divided by √2 of the voltage measured across the arrester at reference current .

Residual voltage/ Discharge voltage

This is the peak value of the voltage that appears between the terminals of an arrester during
the passage of discharge current through it. Residual voltage depends on both the magnitude
and the waveform of the discharge current.

Shielding
Protection of phase conductors from direct lightning strokes; generally by means of additional
conductor(s) running on the top of the towers and grounded through the tower structures to
earth. Stations can also be shielded by earth wires or lightning masts.

Shielding failure
Occurs when lightning strikes a phase conductor of a line protected by overhead shield wires.

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Short circuit (pressure relief) capability
The ability of the arrester, in the event of its overloading due to any reason, to conduct the
resulting system short-circuit current through it without a violent explosion which may damage
nearby equipment or injure personnel. After this operation, the arrester must be replaced.

Temporary overvoltages (TOV)

Temporary overvoltages, as differentiated from surge overvoltages, are oscillatory power
frequency overvoltages of relatively long duration (from a few cycles to hours or longer). The
most common form of TOV occurs on the healthy phases of a system during an earth-fault
involving one or more phases. Other sources of TOV are load-rejection, energisation of
unloaded lines, etc.

Temporary overvoltage withstand strength factor (T r or T c)

This is the TOV capability of the arrester expressed in multiples of Ur or Uc respectively.

Tower footing impedance

The impedance seen by a lightning surge flowing from the tower base to true ground (earth).
The risk for backflashover increases with increasing footing impedance.

Travelling wave
Occurs when lightning strikes a transmission line span and a high current surge is injected onto
the struck conductor. The impulse voltage and current waves divide and propagate in both
directions from the stroke terminal at a velocity of approximately 300 meters per microsecond
with magnitudes determined by the stroke current and line surge impedance.

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3. OVERVOLTAGES

An overvoltage is defined as a voltage level exceeding the maximum allowable continuous

operating voltage for an electrical system. Overvoltages may be of different types, which can
be divided into three generic groups:
• atmospheric overvoltages (lightning)
• switching overvoltages
• temporary overvoltages

Depending on the origin of the overvoltage, a differentiation is made between external

overvoltages (caused by lightning), and internal overvoltages originating from switching
operations in the network (switching overvoltages) or faults and other abnormal system
disturbances. Overvoltages can cause severe problems for the operation of the system, which is
why it is essential to limit these to a low and harmless level. One way of limiting overvoltages is
to use surge arresters to protect important apparatus.

3.1 External Overvoltages

Atmospheric overvoltages are normally divided into two different groups: those arriving from
direct lightning strokes to the lines or equipment and those induced from nearby strokes to
ground or between clouds.

3.1.1 Direct lightning strokes

A direct lightning stoke to a transmission line will result in two identical travelling
waves propagating in either direction along the line. Arrester currents of extreme amplitude and
steepness can occur in arresters located on an unshielded transmission line. For lower system
voltages, the current in these cases will be approximately a third of the stroke current since
flashover to all three phases is likely to occur.

When lightning strikes a transmission line, the line itself is usually not damaged but the
overvoltage generated may result in flashovers of the line insulators and can also cause
insulation breakdowns in apparatus in sub-stations connected to the line. A lightning impulse
has a very short front time, microseconds (µs), and the voltage on the transmission line can
rapidly increase to several thousands of kilovolts when lightning strikes the line. If the earthing
impedances of the towers are not sufficiently low, a lightning stroke to the tower or to the
overhead shield wires (if any) might cause a so called “backflashover” across the insulator
strings to the phase conductors. Travelling waves are generated at the location where the
lightning hit the line and these waves propagate along the line. The insulation is stressed
further if the travelling waves reach an open end of the line where they are reflected; causing a
doubling of the voltage.

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3.1.2 Induced overvoltages
The induced overvoltage on the line resulting from an indirect lightning stroke is
• proportional to the stroke current
• inversely proportional to the distance between the line and the location of the stroke
• proportional to the height of the line above ground

Induced surges are lower in magnitude and the front of the wave is usually substantially longer
than for direct strokes. The front steepness is therefore seldom decisive from a protection point
of view.

However, despite the fact that discharge currents are mostly lower than for direct strokes,
induced strokes can nevertheless be decisive for arrester duty requirements. This is especially
true for low voltage systems in areas where the lightning intensity is high. Due to a substantially
larger collection area compared to direct strokes, the number of arrester operations per year
can be substantial. For distribution and low voltage systems it is often the induced overvoltages
which cause the most damage to unprotected equipment.

3.1.3 Protection measures

Atmospheric overvoltages are particularly dangerous for low voltage, distribution and even sub-
transmission systems. Transmission lines for 300 kV and above are usually equipped with
overhead shield wires as a protection against direct lightning strokes. These overhead shield
wires are installed along the entire transmission line and are earthed at each tower and
connected to the common earthing system in the substations at the ends of the line. Lines for
lower systems voltages usually lack overhead shield wires along the entire line length. Instead,
they are only used in close vicinity (1 – 2 km) out from the substations in order to prevent direct
strokes to the phase conductors close to the stations. The amplitudes of incoming lightning
surges to the stations will thus be limited.

Surge arresters are used as protection in the stations against incoming overvoltages. In close
vicinity to the arresters, these overvoltages are reduced to low and harmless levels. However, at
some distance away from the arresters, high overvoltages may still occur, which is why it is
essential to position the arresters as close as possible to important equipment. More recently,
special arresters have been taken into use out on the transmission lines. These so called
Transmission Line Arresters (TLA) are installed at selected towers along the line in order to
prevent lightning and/or switching related faults on the line itself.

3.2 Internal overvoltages

Internal overvoltages, i.e. switching overvoltages and temporary overvoltages, are caused by
transient phenomena including, for example, switching of transmission lines or transformers,
faults between phases and earth, etc. The duration for these overvoltages can range anywhere
from milliseconds to days, depending on the cause of the overvoltage and the system
parameters.

Due to the common insulation practice with relatively low insulation levels for higher system
voltages, switching overvoltages will normally only be of interest for system voltages above
245 kV.

Switching overvoltages occur in connection with all kinds of switching operations in a network.
The waveshape can be of practically any form, with the fundamental frequency normally in the
order of some hundred to some thousand Hertz.

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Large overvoltages can occur in connection with switching operations, particularly with the
following types of loads:
• Interruption of short circuits
• Disconnection of unloaded transformers and shunt reactors
• Switching of long unloaded lines

The switching overvoltages are usually defined in terms of per-unit of the system voltage. The
overvoltage factor is defined as the ratio of the peak value of the overvoltage to the peak value
of the maximum phase-earth voltage. In EHV networks, for example, it is desirable for cost
reasons to reduce the insulation level as much as possible, and thus overvoltages higher than
2.5 p.u. are usually not accepted.

A commonly used method for limiting line switching overvoltages is to use pre-insertion resistors
on the line breakers. Other means, such as synchronized control of breaker closing times and
the use of surge arresters (alone or in combination), can also be used to limit these kinds of
overvoltages.

Temporary overvoltages (TOV’s) can be defined as overvoltages which are sustained for a
number of cycles. The frequency can either be the network fundamental or a higher frequency
determined by system resonances superimposed on the power frequency.

Temporary overvoltages typically arise from events such as:

• Earth faults
• Sudden change of load
• Resonance phenomena

These overvoltages can normally be kept to acceptable levels with the help of a high short-
circuit power in the supply network, line compensation with shunt reactors, suitable generator
control, automatic fault clearing, etc. Hence, this type of overvoltage is normally not of concern
for the system equipment itself (although at system voltages of 550 kV and above it may
become significant).

Conversely, temporary overvoltages can be decisive in the selection of rated voltage for the
surge arresters. Arresters are not normally required to protect against TOV’s (although special
cases exist), but they must survive them and the arrester rated voltage needs to be chosen
accordingly.
Um x √2
p.u 1 p.u =
√3
6 Lightning over -voltages
Fig. 2
5 Classification of
overvoltages showing
4 Switching over-voltages duration and amplitudes of
stress on insulation in
3 Temporary over-voltages HV networks
2 System voltage
1
t
10-6 10 -4 10-2 10 0 102 104

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4. HISTORICAL BACKGROUND

Surge arresters constitute the primary protection for all equipment in a network against
overvoltages which may occur as the result of lightning or switching operations in the network.

The earliest overvoltage protection devices were introduced during the last decade of the
19th Century and consisted of a simple air gap for which the sparkover voltage changed with
weather conditions, i.e. temperature, air pressure and humidity. One major disadvantage with this
device was that its operation led to a power arc and consequent interruption of power supply on
systems having earthed neutral points.

The next significant step in the development was the so called conventional arrester, or gapped
arrester, developed during the 1930’s. The arrester comprised of voltage dependent silicon-
carbide (SiC) resistor blocks in series with spark gaps, mounted together in a porcelain housing.

The gapped arrester was improved through several generations

during the subsequent decades. The voltage across the series
connected spark gaps was controlled with grading components
comprising non-linear resistors and capacitors and the protection
characteristics were improved by introduction of current limiting
(active) gaps around 1960. Better protection was achieved through
the active gaps permitting the use of SiC resistors with a lower
residual voltage.

The conventional spark-gap assembly consisted of stacked brass

electrodes with steatite spacers and grading resistors (if present)
between them. Between each electrode was a device for pre-
ionisation of the ignition point. This ensured that the ignition was
distinct and as free as possible from variations resulting from
different surge steepnesses.

Active gaps were formed between electrodes riveted to discs of arc-

Fig. 3
resistant material, with several assembled to form a stack. The Conventional
stack also comprised a blow-out coil with a parallel-connected spark-gap
voltage-dependent resistor. Active gap arresters had better assembly
extinguishing capacity, a lower discharge level and a greater
discharging capacity for switching surges than conventional gapped
arresters.

The most advanced gapped SiC arresters in the middle of the 1970’s gave good protection
against overvoltages, but the technique had reached its limits. It was difficult, for example, to
design arresters with several parallel columns to cope with the very high energy requirements
needed for HVDC transmissions. The statistical scatter of the sparkover voltage was also a
limiting factor with respect to the accuracy of the protection levels.

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The metal-oxide (also called MO, zinc -oxide or ZnO) surge arrester was introduced in the mid to
late 1970’s and proved to be a solution to the problems which could not be solved with the old
technology. The protection level of a surge arrester was no longer a statistical parameter, but
could be accurately given. The protective function was no longer dependant on the installation or
vicinity to other apparatus - as compared to SiC arresters, who’s sparkover voltage could be
affected by surrounding electrical fields. The ZnO arrester could be designed to meet virtually any
energy requirements by connecting ZnO varistors in parallel (even though the technique to ensure
a sufficiently good current sharing, and thus energy sharing, between the columns is
sophisticated). The possibility to design protective equipment which could handle extremely high
energy stresses also opened up new application areas; protection of series capacitors, for
example.

Some of the first arresters with ZnO blocks utilised spark gaps in series with the ZnO blocks or in
parallel with sections of the block column (shunt gaps). These designs reflected, to some extent,
a concern for the long-term stability of the ZnO material. Using spark gaps in series or parallel
consequently decreased the voltage stress on the blocks. These designs are not found on the
market any longer for HV applications. With experience, the elimination of gaps permitted the
building of very compact, reliable, low profile arresters compared to what was possible with the old
technology.

The ZnO technology was developed further during the 1980’s and 1990’s through to present day,
towards improved protection levels, higher permissible voltage stresses on the material, greater
specific energy absorption capabilities and better current withstand strengths.

New polymeric materials, superseding the traditional porcelain housings, started to be used in the
mid 1980’s for distribution arresters. By the end of the 1980’s, polymer-housed arresters were
available up to 145 kV system voltages, and today polymer-housed arresters have been accepted
even for 800 kV system voltages.

Many of the early polymeric designs utilized EPDM rubber as an insulator material, but during the
1990’s more and more manufacturers changed to silicone, which is less affected by
environmental conditions, including UV radiation and pollution.

4.1 Operation of gapped and gapless surge arresters

A non-linear resistor type gapped arrester, commonly known as a silicon carbide (SiC) arrester,
comprises SiC valve resistor blocks in series with either passive or active (current limiting) spark
gaps. The purpose of the gaps is to protect the valve elements, give an exact sparkover voltage,
carry the arc during the discharge without being damaged and to deionize the arc sufficiently at
the short time at zero passage to avoid a reignition of the gap. The active gap has the additional
function to create an arc voltage drop resulting in a counter voltage, and thus a current limitation,
during the follow current and extinction interval. In series with the active gaps, a coil is connected
electrically in parallel with a non-linear resistor valve block. See Figure 4.

The operating principle for SiC arresters with passive (non current limiting) gaps and active
(current limiting) gaps differs. For the passive gaps, the overvoltage wave creates an increasing
voltage across the gaps until sparkover occurs and, during a short period of time, an impulse
current rushes through the arrester. Thereafter, the normal power frequency voltage will force a
follow current through the arrester of several hundreds of amperes. Due to the non-linearity of the
resistor blocks, the current is reduced much faster than the voltage, and when the voltage
approaches zero, the current is choked and the arc extinguishes.

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Practically the entire voltage is across the blocks, with only some small percentage being across
the gaps as an arc voltage drop. When the current has been reduced to below about 1 ampere,
occurring some electrical degrees before zero, the arc voltage drop is suddenly increased since
the low current cannot support any plasma. The arc is transformed to a corona discharge and is
extinguished some hundred microseconds before the zero crossing.

The function of an arrester with active gaps is somewhat different. A lightning overvoltage, which
has a high steepness, causes a sparkover of the gaps and the impulse current passes through
the non-linear resistor blocks in parallel with the coils, since the impedance of the coil for the steep
wave is much higher than that for the non-linear resistor. The follow current is, however, much
lower, both in steepness and magnitude, and the current is forced into the coil and a magnetic
field is built up.

A. Stack of spark-gaps
B. Coil
C. Shunt resistor
D. Valve resistor
E. Grading resistor

Fig. 4 One section of an active-gap arrester

The magnetic field results in an electromagnetic force acting on the arc, which is forced from the
initial ignition point out into a narrow chamber where the arc is lengthened 50 - 100 times. The arc
is cooled against the walls and starts to take up voltage. The resulting voltage reduces the follow
current and, as soon as the momentary value of the power frequency voltage falls below the arc
voltage, the follow current ceases. This is in contrast to a passive gap, which must wait until the
voltage is almost zero before it can interrupt the current.

Voltage distribution for steeper waves is determined by the capacitance of the arrester. The
function of the grading resistors in gapped arresters is to distribute the voltage evenly across the
gaps in the event of relatively slow voltage variations. The sparkover voltage at power frequency
and for switching surges is then determined by these grading resistors. There are two kinds of
grading resistors, those with linear resistance and those with non-linear resistance. Generally,
the sparkover voltages for this frequency range needs to be fairly high to prevent false
operations for normal service voltage variations.

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Service under polluted conditions has always been a problem for gapped arresters. The
formation of so called “dry bands” on the porcelain surface under such conditions leads to a
disturbed voltage gradient, which affects the internal gaps by means of coupling capacitance
between gaps and porcelains. As a result, some arresters may then even sparkover at service
voltage during periods of heavy pollution. Repeated sparkover may result in overheating when
the gaps fail to reseal, leading to complete failure of the arrester. Improved reliability under
conditions of high contamination requires a strong grading, which can be achieved with highly
non-linear grading resistors.

Should a SiC resistor be placed on high service voltage without series gaps, it would draw a
continuous current of some hundreds of amperes and thus quickly destroy itself. A “gapless SiC
arrester” is therefore not a possibility.

Zinc-oxide (ZnO) varistors, in contrast, represent a high impedance at normal service voltage and
draw only a small leakage current (predominantly capacitive), with the resistive component of the
current in the order of only 50 to 250µApeak (depending on the varistor diameter). Such a low
“leakage” is neither dangerous to the varistor nor uneconomic for the system. Therefore ZnO
varistors can be placed directly on voltage, and it is possible to remove the series gaps entirely
from the arrester.

ZnO varistors have an extremely non-linear, but well defined, volt-amp operating characteristic.
The working principle of a gapless ZnO arrester is therefore very simple: When an impulse
occurs, the arrester’s impedance reduces via its operating characteristic and subsequently
changes over from conducting a small, predominantly capacitive current to a large resistive
current. Due to the passage of the impulse current, a voltage is consequently built up across the
arrester (residual voltage), the magnitude of which is determined by the volt-amp operating
characteristic of the arrester for the applied impulse current and waveshape. Once the impulse
has been dissipated, the arrester thereafter immediately returns back along its operating
characteristic to its non-conducting state.

Even though a lightning overvoltage causes an impulse current through a gapless ZnO arrester
as for the gapped arrester, the normal power frequency voltage after the discharge is not high
enough to force a follow current through the arrester. Hence, a ZnO arrester is only subjected to
the energy from the lightning, in contrast to the SiC arrester, where a large energy contribution is
obtained from the follow current.

Protection levels for gapless ZnO arresters depend only on the residual voltages determined by
the operating characteristic for the respective waveshapes and currents and thus are better
defined and more stable compared with gapped types. In general, the protection levels are lower
(i.e. better) than for gapped SiC arresters of equal rated voltage. This improvement is particularly
marked when steep-fronted impulses and switching surges are considered.

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 Function of a silicon carbide Function of a metal oxide
(SiC) arrester (ZnO) arrester
with passive gaps without series gaps

Fig. 5 Comparison in operation of a silicon carbide (SiC) arrester with passive gaps
and a gapless metal-oxide (ZnO) arrester without series gaps

The following Table 1 gives a summary of the major differences between gapless ZnO and
gapless SiC arresters.

Metal-oxide type (gapless) Gapped type

No sparkover, current flows as per U-I Sparkover, afterwards power frequency follow
characteristic current
Small scatter band for residual voltages, Usual scatter band for spark-gaps (up to 15%
typically ± 3% scatter; even higher for poorly graded arresters)
Excellent steep-front wave characteristics Strong rise (>25%) in sparkover voltage due to
(only approx. 10%) steep-front overvoltages
Temporary power frequency load above Continuous voltage at power frequency, always
Uc possible lower than rated voltage
Energy absorption capability can be Restricted energy absorption capacity, parallel
increased (arresters in parallel) connection has no effect
Simple active part with few components Complex structure for active part
Practically no ageing effect Ageing of spark-gaps due to arc erosion

Table 1 Summary of the major differences between gapless ZnO and gapped SiC arresters

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5. FEATURES OF ZnO ARRESTER DESIGN

A zinc-oxide (ZnO) surge arrester for high voltage applications comprises the following main
components:
• ZnO varistors (blocks)
• Internal parts
• Housing of porcelain or polymeric material with end fittings of metal (e.g. flanges)
• A grading ring arrangement where necessary

The internal parts can differ considerably between a porcelain housed arrester and a polymer-
housed arrester. The only certain commonality between these two designs is that both include a
stack of series connected zinc oxide varistors, together with components to keep the stack
together.

Surge arrester with porcelain housing (left) Surge arresters with silicone-housing
in an open-cage (centre) and
tubular design (right)

Fig. 6 Cut-away view of three principal designs for ZnO surge arresters

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5.1 ZnO varistor
The most important component in the arrester is the zinc-oxide (ZnO) varistor itself, which gives
the arrester its protective characteristics. All other components are simply used to protect or keep
the ZnO varistors in place.

The ZnO varistor is a densely sintered block, pressed to a cylindrical body. The block consists of
approximately 90% zinc oxide and 10% of other rare earth oxides (additives). During the
manufacturing process a powder is prepared, which is then pressed to a cylindrical body under
high pressure. The pressed bodies are sintered in a kiln for several hours at a temperature in the
order of 1200 °C. During the sintering, the oxide powder transforms to a dense ceramic body with
varistor properties, whereby the additives form an intergranular layer surrounding the zinc oxide
grains.

ZnO Grains
10–15 µm

Fig. 7 ZnO varistor blocks and their microstructure

These layers, or barriers, give the varistor its non-linear characteristics. Metal is applied on the
end surfaces of the finished varistor to improve the current carrying capability and to secure a
good contact between series -connected varistors. An insulating layer is also applied to the
cylindrical surface to give protection against external flashover and chemical influence.

Before the blocks are assembled in an arrester, they must be subjected to a variety of tests to
verify their protection performance, energy and current capability as well as long term electrical
stability.

5.1.1 How does a ZnO varistor work?

With reference to the following Figure 8, the voltage-current characteristic for the varistors can be
divided into three different regions with respect to the slope.

In the low current region, called the ”prebreakdown region” (Region 1), the resistivity of the
material is temperature dependant. The normal continuous operating voltage is found in this
region. Here the surge arrester acts as a capacitor, with only small resistive currents through it.
This is mainly due to the metal-oxide barrier (intergranular layers) between the zinc-oxide grains
acting as insulating barriers. However, the varistor’s temperature influences the insulation
capability slightly, i.e. an increased temperature leads to a higher resistive current.

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In the “breakdown region” (Region 2), when the voltage stress has increased due, for example,
to temporary overvoltages or switching overvoltages, the intergranular layers switch from
insulating barriers to conducting layers and the current carrying capability of the varistor
increases many-fold. For example, if the voltage stress increases from 200V/mm to 300V/mm,
the current increases 10 000 times. This acts as a voltage limitation, and gives the arrester its
protective characteristics.

At even larger current densities, the arrester is working in the “high current region” (Region 3)
and the curve turns upwards, which determines the impulse behaviour of the surge arrester.
The barriers between the ZnO grains are electrically broken down and the current increase is
solely limited by the resistivity of the ZnO grains.

When the voltage across the arrester is reduced to a normal level, the working point returns again
to Region 1, without delay.

Voltage (p.u.)
Min protection levels in kV (peak)
according IEC60099-4

Region 1 Region 2 Region 3

Protection against lightning overvoltages

2.3
Protection against switching overvoltages
2.0

Rated voltage (Ur )

√2
1.0 x ? Ires , resistive current
Continuous operating voltage (Uc)
√2
0.8 x ? Effect of increased
Ires block temperature
Icap
on Ires

Icap, capacitive
current (no influence
from temperature)

10-5 10-3 102 103 104 Log scale

Current (Ampere)

Fig. 8 Current-voltage characteristic of a ZnO-varistor.

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5.2 Housing of a surge arrester
The main purpose of the insulator housing is to :
• Keep the internal parts together
• Protect against external flashovers
• Secure that the function of the arrester is independent of external influences

An arrester must also be equipped with fastening devices to ease the erection. This is achieved
by assembling flanges (or similar) at one or both ends of the insulator. If the arresters consists of
several series connected units, the flanges are also used to mechanically and electrically secure
arrester units to each other.

Insulators can be manufactured with different mechanical fracture values. The required fracture
value for a specific insulator is determined by the design and intended use of the arrester.

5.2.1 Mechanical design

A surge arrester consists internally of series -connected ZnO varistors blocks, plus additional
hardware as necessary for individual designs: metal spacers, assembly plates, sealing rings,
pressure relief device, etc. To ensure a controlled environment for the blocks, the internal parts
must be shielded against the ambient environment, and this is achieved by housing the blocks in
a well designed and securely sealed insulator.

The insulator housings for surge arresters have traditionally been made of porcelain. However,
today there is a strong trend, and even a preference, towards the use of silicone insulators for
arresters at all system voltages.

There are a number of reasons why silicone is seen as an attractive alternative to porcelain,
including:
• Better behaviour in polluted areas
• Better short-circuit capability with increased safety for other equipment and personnel
• Low weight
• Better earthquake withstand capability

It is incorrect, however, to believe that all polymer-housed arresters automatically have these
features just because the porcelain has been replaced by a polymeric insulator. The design must
be scrutinised carefully for each specific type, which can be grouped generally into the following
categories:
• Open or cage design
• Closed design
• Tubular design

These are discussed in more detail at the end of this chapter.

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5.2.2 Pollution performance
The creepage distance is the total length of the outer contour of the insulator. Simplified, the
longer this length is, the more severe environmental conditions the arrester will be able to operate
under without increasing the risk for an external flashover. Surge arres ters can be supplied with
different creepage distances, and one type of insulator frequently used has a long-short
shed-form, thus enabling a short assembly height for a given creepage distance.

Silicone insulators generally perform better in polluted

environments compared to a porcelain insulator. This is mainly due
to the hydrophobic behaviour of the silicone material, i.e. the ability
to bead water and prevent wetting of the insulator surface.
Hydrophobicity results in reduced creepage currents during heavy
pollution episodes, minimising electrical discharges on the surface;
thereby reducing the effects of material ageing. However, it should
be noted that whilst most polymer materials are hydrophobic when
new, not all polymeric insulators necessarily retain their
hydrophobic properties over their service lifetime.

Two commonly used polymeric materials for the arrester housing

Fig. 9
are silicone and EPDM rubber (Ethylene-Propylene Diene
Insulators made from silicone Monomer), and both exhibit hydrophobic behaviour when new.
retain their hydrophobic Polymer materials may lose their hydrophobicity during an
properties over their extended period of severe pollution, such as salt in combination
in-service lifetime with moisture. Silicone, however, will ultimately recover its
hydrophobicity, through diffusion of low molecular silicone oils to
the surface restoring the original material behaviour. EPDM
rubber, in contrast, lacks this ability. Hence the material is very
likely to lose its hydrophobicity completely with time, and is
consequently often regarded as a hydrophilic insulator material,
similar to porcelain.

Polymeric materials can potentially be more affected by ageing due to partial discharges and
leakage currents on the surface, UV radiation, chemicals, etc, compared to porcelain, which is a
non-organic material. For this reason, the raw material is often blended with a variety of additives
and fillers to achieve the desired material features: UV stability, anti-tracking, flame-retardancy,
etc. Silicone, as a material, has a natural resistance against these effects, and thus such
additives simply aid in further improving the material’s inherent properties.

5.2.3 Short-circuit capability

A correctly selected arrester can divert surges to ground almost endlessly, provided the energy
to be dissipated is within the capability of the arrester.

In the event that an arrester is required to dissipate more energy than it is capable of, it will
sacrifice itself by failing short circuit. Most commonly, arresters are connected between phase-
ground and the resultant earthfault will immediately collapse the voltage on that phase, thereby
protecting other equipment on the same phase. The upstream protection will initiate a breaker
trip to clear the fault, and the failed arrester can then be replaced.

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If steps are not taken, there is a likelihood that arresters which contain an enclosed gas volume
might explode due to the internal pressure increase caused by the heat generated from the
short circuit arc. This leads to the need for these arresters to be fitted with some type of
pressure relief system which will open quickly to release the enclosed gas volume to the
outside. Such arresters are normally supplied with devices at the top and bottom of each unit,
which operate as soon as the internal pressure reaches a certain value. The ionised gas will
subsequently be evacuated to the outside of the arrester, and when the two gas streams meet
the internal arc will commute to the outside, thus preventing a continual internal pressure
increase.

Fig. 10
Operating principle of the pressure
relief device of an ABB type EXLIM
porcelain housed arrester.

(1) Arrester in its healthy state

(2) Arrester has failed short-circuit,

pressure relief plates open and gas
begins to be expelled through the
venting ducts

(3) The two gas streams meet and the

internal arc is commuted safely to
the outside

(1) (2) (3)

The sealing cover in ABB’s high voltage EXLIM porcelain-housed arresters also acts as an
overpressure relief device. Other manufacturers may have other solutions; a blast plate for
example.

During normal service, the sealing

Pressure relief
cover tightens against the porcelain. and Sealing plate Flange cover
At an internal short-circuit of the
arrester, an open arc occurs across Venting duct
the block column. Due to the heat
from the arc, the internal pressure
increases and would soon reach a
O-ring
value that could cause an explosion of
the insulator if no pressure relief
Indicating cover
device was present. The sealing
cover is designed such that it will Flange
open, both at the top and bottom, as
soon as the internal pressure reaches Cementing

a certain value (significantly below the

bursting pressure of the porcelain) Fig. 11
and the enclosed gas volume can be The position of the overpressure relief device on an ABB
type EXLIM T porcelain housed arrester. The figure shows the key
evacuated to the outside of the
parts of an arrester with the pressure relief and sealing plate,
arrester. The internal pressure is thus block column, spring device and the cemented metallic flange.
relieved, and a violent shattering of
the porcelain is avoided.

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Safer short-circuit performance is not, however, automatically achieved simply by replacing the
porcelain housing with one made of polymer. In the past, there has been the incorrect belief that
all polymer-housed arresters, irrespective of design, were capable of carrying enormous
short-circuit currents. Standardised short circuit test procedures within IEC (for both porcelain and
polymer-housed arresters) now take into consideration what might happen at failure of the ZnO
blocks for individual designs.

Fig. 12
Operating principle of “ pressure relief” for an ABB type PEXLIM
moulded open-cage design.

(1) Arrester has failed short-circuit and gas begins to be expelled

through the soft silicone housing

(2) The gas streams trigger an external flashover and the internal
arc is commuted safely to the outside

The short circuit capability for surge arresters (porcelain and polymer) is verified by tests to
minimize the risk for damage to surrounding equipment and personnel. However, the risks
related to an open arc in service can also be influenced by the physical positioning of the
equipment as well as by the circuit connections .

5.2.4 Internal corona

A low corona (partial discharge, PD) level during normal service conditions is essential for all
apparatus designs intended for high voltage applications. Arresters with an annular gas -gap
between the active parts and the external insulator may have large voltage differences between
the outside and inside of the arrester during external pollution and wetting of the housing surface.
To fully avoid corona under such conditions is not technically or economically feasible. Instead
the internal parts, including the ZnO blocks, must be able to withstand these conditions.

In order to prevent internal corona during normal service conditions for these type of arresters, the
distance between the block column and insulator must be sufficiently large to ensure that the
radial voltage difference between the blocks and insulator will not create any partial discharges.

For polymer-housed arresters lacking such annular space in their design, the radial voltage
difference is entirely across the rubber insulator. In order to avoid puncturing of the insulator, the
rubber must be sufficiently thick. It is also very important that the insulator is free from voids to
prevent internal corona in the material which might lead to problems in the long term.

The maximum voltage stress occurring across the polymer material is proportional to the length of
the insulator. A longer insulator therefore requires that the thickness of the material is
proportionally increased with respect to the increase in length. Another solution is to reduce the
height of the individual units in a multi-unit arrester, since the maximum voltage across each unit
is limited by the non-linear current-voltage characteristic of the ZnO blocks.

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5.2.5 Voltage grading
The performance of a ZnO surge arrester is defined by its protective levels, its temporary
overvoltage and energy discharge capabilities and the long term-stability of the zinc oxide
material.

The temporary overvoltage and energy handling capabilities are closely related to the
temperature of the ZnO blocks during normal operation. This temperature depends on the
power losses, which increase rapidly with voltage stress, due to the block material ’s non-linear
voltage-current characteristics. Therefore, the most essential parameters to minimize the
temperature during normal operation are inherently low power losses in the zinc oxide material,
together with a linear voltage distribution along the block column.
Under normal operating conditions and voltage, the ZnO blocks act like a capacitor. The voltage
distribution along the block column then depends on the capacitance of the ZnO blocks and the
influence of stray capacitances. The stray capacitances are strongly dependent on the height of
the block column. Short arresters - up to about one meter in height - usually have a sufficiently
linear voltage distribution along the block column, as the self-capacitance of the ZnO blocks is
relatively high. For taller arresters, the influence of stray capacitances makes the voltage
distribution less linear. If no measures are taken to prevent an uneven voltage distribution on a
tall arrester, the local voltage stress at the top may reach (or even exceed) the knee-point of
the voltage-current characteristic of the zinc oxide material. This leads to a localized increase in
the power losses, with high temperatures in the block column as a consequence.
Corona
Above the knee-point of the current-voltage characteristics, the
ring
blocks start to conduct large currents, which would ultimately lead
to the failure of the arrester. The amount of this current is
determined by the applied voltage and the total stray-capacitance
of the arrester to earth and can be considerable; particularly for
high-voltage arresters. Further, the localized heating of the ZnO
blocks (hot-spots) leads to a reduced energy absorption capability
of the arrester.

Tall arresters therefore must be equipped with some type of voltage

grading. This can be achieved by additional grading capacitors Grading
and/or grading rings. Provision of suspended grading rings is the rings
most common way of improving the voltage distribution.

It should be noted that it is only grading rings hanging down from its
electrical connection point that helps to improve the voltage grading
of the arrester. Large metallic electrodes, including metallic flanges
or rings to reduce corona without any suspension from its electrical
contact point to the arrester, actually increases the stray-
capacitances to earth, thereby amplifying the uneven voltage
distribution.

An important point, which often remains unconsidered, is that an

actual surge arrester installation constitutes a three-dimensional
problem with three phase-voltages involved together with certain
stipulated minimum distances between phases and to grounded
(earthed) objects. All this must be considered when making Fig. 13
electrical field calculations. To not consider the influence of Example of grading ring
adjacent phases, for example, will lead to an underestimation of and corona ring arrangement
on an ABB type EXLIM surge
the maximum uneven voltage distribution of up to 10%. arrester for 550KV system
voltage

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Both IEC and IEEE standards require the maximum voltage stress to be taken into
consideration in accelerated ageing tests on ZnO blocks. However, it is not possible to
determine the correct voltage stress to be used in these tests without proper calculations of the
maximum voltage stress occurring in practical three-phase installations. If no such calculations
have been performed, the tests should therefore be carried out with a voltage stress
corresponding to the knee-point of the voltage-current characteristics, i.e. at the reference
voltage.

Type tests in accordance with Standards to verify the long-term stability of the ZnO blocks are
hence not valid if the actual voltage stress on the arrester during service is allowed to exceed the
applied voltage stress proven in the type tests.

When grading arrangements for surge arresters are based on complete electrical field
calculations for each arrester design at the maximum continuous operating voltage and with the
maximum possible three-phase influence taken into account, this guarantees that the voltage
stress remains below the critical level at all points along the block column. This maximum
voltage stress level is then used in accelerated ageing tests on the ZnO blocks. In this way, the
long-term stability of the ZnO blocks is verified at the highest possible voltage stress found in
any installation under normal service conditions.

A guide for the determination of the voltage distribution along surge arresters using simplified
representations of arrester geometries and boundary conditions (applied voltage, proximity and
voltage applied to other objects in the vicinity) is given in IEC60099-4.

5.3 Polymer arrester designs

The potential weight reduction for polymer arresters can be considerable compared to porcelain
housed arresters. As an example, one of the standard ABB type EXLIM arresters with porcelain
insulator for a 362 kV system voltage has a mass of approximately 430 kg. A PEXLIM silicone-
housed arrester for conventional up-right erection, with the same rated voltage, has a mass of
only approximately 125 kg.

This leads to the obvious benefit of lighter structures with subsequent reduced costs, and even
the possible complete elimination of the need for a structure at all if alternative mounting
arrangements are acceptable; e.g. suspended mounting.

Fig. 14
Two examples of possible mounting arrangements for ABB type
PEXLIM silicone housed surge arresters
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Since the soft outer polymeric insulator does not have the necessary mechanical strength to keep
the ZnO column together, other insulator materials must be used in the design. The most common
material used for this purpose is glass-fibre reinforced plastic.

There are then several types of mechanical designs in common use: loops or rods, cross-winding
and tubes. These designs can be grouped generally into three basic categories:
• Open or cage design
• Closed or wrap design
• Tubular design

5.3.1 Open or cage design

This design may consist of loops of glass-fibre, a cage of glass-fibre weave or glass-fibre rods
around the block column. What defines this type of design is that the active components are not
fully enclosed by hard materials. Instead, a body of soft polymer material directly surrounds the
internal components.

An outer insulator with sheds is required over the inner body, with two common methods for
achieving this being:
• A pre-moulded polymer insulator is made in a separate process, and then slipped over the
internal component assembly (which itself may be enclosed in soft polymer). The
boundary between the internal assembly and the outer polymer insulator is usually filled
with grease or gel, generally of silicone.

• The outer housing is moulded directly onto the internal components to form a void-free,
sealed housing along the entire length of the insulator.

Such designs lack enclosed gas volume. Should the arrester be stressed in excess of its design
capability, an internal arc will be established. Due to the open design, the arc will tear or burn its
way through the polymer material, permitting the arc, along with any resultant gases, to escape
quickly and directly. Hence, special pressure relief vents or diaphragms are not required for this
type of design.

It is of great importance that no air pockets are present in these designs, otherwise partial
discharges might occur, which would lead to the destruction of the insulator over time.

Penetration of water and moisture must also be prevented, which places strict requirements on
the sealing of the insulator at the metallic flanges (in the case of a pre-moulded housing) and
adherence or bonding of the rubber to all internal parts (in the case where the polymer is directly
moulded onto the inner body).

ABB employs a unique, patented design for the PEXLIM arrester to enclose the ZnO blocks of
each module under pre-compression in a cage formed of glass-fibre reinforced loops fixed
between two yokes which form the electrodes. An aramide fibre is wound over the loops
resulting in an open cage design for the module. This results in high mechanical strength and
excellent short circuit performance, through the aramide windings preventing explosive
expulsion of the internal components.

Each module is then passed through a computer-controlled cleaning and priming process.
Thereafter, the module is loaded in a highly automated vulcanising press, where silicone is
injected at high pressure and temperature to completely bond to the active parts, leaving no
internal voids or air spaces.

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5.3.2 Closed or wrap design
Surge arresters in this category incorporate a “void-free” (partial or total) polymer housing around
the internal assembly, while surrounding the active components themselves with hard material. In
contrast to the open design, they have been mechanically designed to not include a direct path for
externalising the arc during internal short circuit.

Typical designs include a glass-fibre weave wound directly on the block column or a separate
tube in which the ZnO blocks are mounted. A soft polymer insulator is then fitted (either
pre-moulded or directly moulded) over this internal component assembly; often together with
grease or gel to fill the interfaces.

In order to obtain a good mechanical strength, the weave/tube must be made sufficiently strong,
which, in turn, might lead to a too strong design with respect to short-circuit strength. The internal
overpressure could rise in the tube design to a high value before cracking the tube, which may
lead to an explosive failure with parts being thrown over a wide area. To prevent a violent
shattering of the housing, a variety of work-around solutions have been utilised, e.g. slots in the
tube. When glass-fibre weave is used, an alternative has been to arrange the windings in a
special manner to obtain weaknesses that may crack. These weaknesses are intended to ensure
a pressure relief and commutation of the internal arc to the outside; thus preventing an explosion.

Note that such alterations do not then make these an “open design”, as the arc path is not
considered to be direct and the internal components are still, in practical terms, surrounded by
hard material.

Sealing and partial discharge issues also require consideration in a similar manner as for the open
or cage design.

5.3.3 Tubular design

The tubular design incorporates a distinct annular gas -gap between the active parts and the
external insulator. It is designed in more or less the same way as a standard porcelain arrester,
but with the porcelain housing having been substituted by an insulator of a glass-fibre reinforced
plastic tube, moulded with an outer insulator of silicone or EPDM rubber.

The internal parts are, in general, almost identical to those used in an arrester with porcelain
housing. The arrester must therefore obviously be equipped with some type of sealing and
pressure relief devices, similar to what is used on porcelain-housed arresters.

This design has the prime advantage that high mechanical strength is possible (potentially even
higher than for porcelain). Among the disadvantages compared to other polymeric designs is less
efficient cooling of the ZnO blocks and, if appropriate precautions are not taken in the design, an
increased risk of exposure of the polymeric material to corona that may occur between the inner
wall of the insulator and the block column during external pollution.

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6. DESIGN REQUIREMENTS FOR ZnO SURGE ARRESTERS

There are a variety of parameters influencing the dimensioning of an arrester, but the demands as
required by a user can be divided into two basic categories:
• Protection against overvoltages
• High reliability and a long service life

Additionally, there is the requirement that the risk of personal injury and damage to adjacent
equipment shall be low in the event of an arrester overloading. Users are also beginning to put
requirements on environmental aspects, for example that arresters should be separable,
recyclable and only contain non-hazardous materials.

The above two main requirements are somewhat in contradiction to each other. Aiming to
minimise the residual voltage normally leads to the reduction in the capability of the arrester to
withstand power-frequency overvoltages. An improved protection level may therefore be achieved
by slightly increasing the risk of overloading the arresters. The acceptance for increase of this risk
is, of course, dependent on how well the amplitude and duration of the temporary overvoltages
(TOV’s) can be predicted. The selection of an arrester is therefore always a comprom ise between
protection levels and reliability.

A more detailed classification could be based on what stresses a surge arrester is normally
subjected to and what continuous stresses it shall withstand. For example:
• Continuous operating voltage
• Ambient temperature
• Rain, pollution, sun radiation
• Wind and possible ice loadings as well as forces in line terminal connections

and additionally, non-frequent, abnormal stresses, for example:

• Temporary overvoltages, TOV’s
• Overvoltages due to transients, which affect
• thermal stability and ageing
• energy and current withstand capability
• external insulation withstand
• Large mechanical forces (e.g. from earthquakes)
• Severe external pollution

and finally, what the arrester can be subjected to only once:

• Internal short-circuit

For transient overvoltages, the primary task for an arrester is to protect. But it must also normally
be dimensioned to handle the current through it, as well as the heat generated by the overvoltage.
The risk of an external flashover must also be very low.

Detailed test requirements are given in International and National Standards, where the surge
arresters are classified with respect to various parameters such as energy capability, current
withstand, short-circuit capability and residual voltage.

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6.1 Designing for continuous stresses

6.1.1 Continuous operating voltage

Maximum continuous operating voltage, denoted as Uc in the IEC standard, is the maximum
r.m.s. voltage level the arrester is designed to operate under during its entire lifetime. The arrester
shall act as an insulator against this voltage. The entire voltage is across the ZnO varistors and
these must be able to maintain their insulating properties during their entire lifetime.

The continuous operating voltage for AC surge arresters is mainly at power frequency, i.e. 50 Hz
or 60 Hz with some percent of superimposed harmonics. For other applications, e.g. HVDC, the
waveform of the voltage might be very complex or even a pure DC voltage. It must therefore be
verified for all applications that the ZnO varistors are able to withstand the actual voltage under
their technical and commercial lifetime; normally stated to be in the order of 30 years.

The basis for the dimensioning is the result from ageing procedures where possible ageing effects
are accelerated by performing tests at elevated temperature.

6.1.2 Ambient temperature

All arresters, according to the IEC standard, must be designed to withstand an ambient air
temperature of -40 °C to +40 °C without impairing the surge arrester’s function. Due to the varistor
current-voltage characteristic, higher temperatures may be decisive for the arrester’s design as
resistive leakage current increases with higher temperatures at Uc . In order for a manufacturer to
verify that the arresters are capable of withstanding the highest possible temperatures, certain
type tests must be performed. It is, however, worth noting that the ambient air temperature
surrounding the arrester is not necessarily the temperature of the ZnO blocks themselves.
Arresters installed outdoors, for example, will always have a proportion of their housing in the
shade. Thus, even if the ambient temperature is considered higher than +40 °C, it is the average
ambient temperature of the blocks themselves which should be the determining factor for the
evaluation of verifications made for thermal stability during the type tests. Further, this
temperature is normally considered to be the average over a 24 hour period.

6.1.3 Rain, pollution, sun radiation

A contaminated insulator surface in combination with moisture causes a creepage (external
leakage) current on the insulator surface that can reach high values. This leakage current may
negatively influence the arrester with respect to internal corona, heating of ZnO blocks and
external flashovers. Thus the arrester must be designed in such a way that the internal parts will
endure, during a limited time, a high internal corona level (for arresters with an annular gap
between insulator and block column), and that the blocks will withstand a higher grading current,
and subsequent higher power losses, than normal without failing. The risk of an external flashover
must also be minimized.

Heating of the arresters due to direct sun radiation and self-heating is normally a minor problem.
The influence from the sun radiation is sometimes thought to be significant, as one might
assume that sun radiation can result in considerably high surface temperatures. However, it is
the average surface temperature of the complete arrester that counts, and sun radiation falls on
less than half of the insulator surface at any point in time. In fact, the closer to the equator an
arrester is situated, the smaller the fraction of the insulator surface that is subjected to direct
radiation due to the sheds. The effects of direct sun radiation are included in the Operating Duty
test of arrester sections. Heating from sources other than sun radiation must be checked
separately for each case.

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For a given pollution level, the performance of gapless arresters can be generally improved by
employing any or all of the following measures:
• Increasing the creepage distances of the housings to reduce external leakage current
• Using ZnO blocks of larger volume to improve the energy absorption capability
• Improving the TOV capability, i.e. by increasing the rated voltage (Ur) for the same
arrester type
• Improving the heat transfer mechanism
• Using blocks with lower losses at Uc

A well designed arresters should already employ blocks with very low losses and the heat
transfer mechanism should be optimized for these blocks. Thus, in practical terms, pollution
performance is usually improved by using one or more of the first three methods. In this regard,
it is important to note that an increased Uc without corresponding improvement in TOV capability
is not effective.

6.1.4 Wind, ice, external forces

A surge arrester is not a post insulator and should not be used as such, since normally its
mechanical strength is limited. In all but extreme cases, wind and ice loads are usually not a
problem for surge arresters. It is only if hurricanes (gales) could be expected at the arrester
location, that a detailed check is necessary of whether or not a standard arrester will mechanically
withstand the wind forces. By suitable selection of the housing strength or its physical mounting, a
surge arrester may be designed to withstand very high winds. Similarly, severe ice storms are
normally required to build up sufficient ice to load the arrester significantly.

The most suitable way to connect an arrester to the overhead line is to arrange the tee-off
vertically and slack to the line terminal of the arrester to minimize the bending moment on the
arrester. Since surge arresters have a certain maximum bending moment for each design type,
expressed in Nm, the maximum force at the line terminal is lower for a tall arrester than for a
shorter one of the same type.

6.1.5 Considerations for polymer arrester designs

The design for continuous stresses on polymer arresters must also take into consideration their
effect on the behaviour and characteristics of the polymer material. For example, polymeric
materials can potentially be more affected by ageing due to partial discharges and leakage
currents on the surface, UV radiation, chemicals, etc, compared to porcelain. Further, polymers,
as a rule, become softer at higher temperatures with a higher degree of creeping (cold flowing),
while at cold temperatures the material becomes brittle.

Many of these characteristics are strongly dependent on temperature and load time. It therefore
is of great importance that the arrester design is tested with different temperature and load
combinations to verify that all possible sealings operate adequately over the entire temperature
interval.

Composite materials, such as glass-fibre joined in a matrix with epoxy or other polymeric
materials, can exhibit behaviour changes at high loading. The rate of this material degradation is
determined by temperature, applied force, velocity of the applied force, humidity and the time
during which the load is applied. It is therefore not sufficient to simply dimension the arrester with
respect to its breaking force, but rather consideration must also be taken to how the arrester
withstands cyclical stresses.

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6.2 Designing for non-continuous stresses

6.2.1 Temporary overvoltages (TOV)

TOV’s in networks are primarily caused by earth faults, load rejection, energising of unloaded
lines and resonance. By definition, a TOV is above Uc and normally will last from some few cycles
up to some seconds. However, in certain isolated systems, the duration of an earth-fault may last
several days. Further, the TOV's may be preceded by a switching surge.

A ZnO arrester is considered to have withstood a TOV if:

• the ZnO-blocks are not destroyed due to energy under the TOV i.e. cracking, puncturing
or flashover of the blocks does not occur, and
• the surge arrester is thermally stable against Uc after cessation of the TOV

Since the resistive leakage current through the arrester is temperature-dependent, achieving
thermal stability is also dependent on the final block temperature. If, for example due to a prior
switching surge, the arrester already has a high starting temperature before being subjected to a
TOV, it will naturally have a lower overvoltage capability.

This is exemplified in the TOV characteristic given below (Fig. 15), which shows the ability of a
specific ZnO arrester to withstand overvoltages with and without a preceding energy absorption.
The lower curve is valid for an arrester which has been subjected to maximum allowable energy,
for example from a switching surge prior to the TOV. The upper curve is valid for an arrester
without prior energy duty.

For ZnO arresters, the TOV amplitudes are normally at, or immediately above, the knee-point of
the current-voltage characteristic. If the arrester is designed to fulfil the IEC standard, it shall be
able to withstand a TOV equal to the rated voltage of the arrester for at least 10 seconds after
being subjected to an energy injection corresponding to two line discharges as per relevant line
discharge class of the arrester. This voltage level is also designated as the ”rated voltage” of the
arrester in compliance with IEC.

Fig. 15
Example of TOV-capacity for
a specific ZnO surge arrester.

The upper curve is valid if the

arrester has not been
subjected to any energy prior
to the TOV and the lower
curve is valid if the arresters
has absorbed maximum
allowable energy prior to the
TOV.

The TOV capability is

normally based on the lower
curve, being the “worst
case”.

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The TOV is generally regarded as a stiff voltage source, i.e. the surge arrester cannot influence
the voltage amplitude. For the dimensioning to fulfil a certain TOV level, the varistor characteristic
must be chosen such that the current through the arrester, and consequently the energy
dissipation, will not result in a temperature above the thermal instability point.

The TOV capability given for a certain surge arrester should always be assumed with a stiff
voltage source. However, if this is not the case, the TOV capability of the arrester is, generally,
significantly higher.

An important parameter concerning the dimensioning for TOV's is to accurately control the knee-
point voltage, since the non-linearity of the characteristic is at its most extreme in the TOV range.
This is best achieved by defining a reference voltage close to the knee-point on the voltage-
current characteristics, and then chec king through routine tests that every arrester has a
reference voltage above a guaranteed minimum voltage.

A manufacturer is relatively free to assign any data for the arresters. A given arrester with ZnO
blocks capable of absorbing a certain amount of high energy could therefore be assigned a high
line discharge class with low TOV capability or, conversely, a low line discharge class with high
TOV capability. The ideal should naturally be to assign the highest line discharge class with the
highest possible TOV capability.

6.2.2 Transient overvoltages - Protective function

The arrester shall, for an expected maximum current, limit an overvoltage to a level well below the
insulation withstand level of the protected equipment.

The protective characteristic for a ZnO varistor is slightly dependent on the steepness of the
expected current. The below Figure 16 shows the characteristics for a specific arrester for three
different current shapes given in the Standards.

Fig. 16
Example of protective
characteristics for a specific
ZnO surge arrester.

The protection level is given

in % of the residual voltage
at a current impulse with
wave-shape 8/20 µs and
amplitude 10 kA.

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As can be noted from the diagram, the protection level for currents having a front time of 1µs are
approximately 10% higher compared to currents with a wave form 8/20µs or longer. However,
even more important than this marginal increase for steep current waves, is the effect of
positioning the arrester in relation to the protected equipment and the length of the connections.
There is also an effect as a result of the arrester’s own height (length). These effects add
inductance into the circuit, typically 1µH/m for outdoor arresters, which results in a further increase
in the overall residual voltage against steep current impulses.

In order to obtain an efficient protection against fast transients, for example caused by
backflashover close to a substation, large margins are therefore required between the protection
level of the surge arrester and the protected equipment’s insulation level.

A ZnO block with larger diameter normally has a better protection level with maintained
overvoltage capability. A better protection level, in this case, also automatically results in a better
energy capability.

Computer programs are used to make accurate calculations of the resulting overvoltages in a
substation originating from lightning and detailed models of the transmission line and substation
are made. In these type of calculations, a ZnO arrester may be modelled as shown below
(Fig. 17).

HIGH FREQUENCY MODEL OF A SURGE ARRESTER

L1 L1=(LENGTH OF CONNECTING CABLES +

ARRESTER HEIGHT)*1 µH/m

L2=0.029 µH/kV RATED VOLTAGE

L2 R1
R1=0.06 Ω/kV RATED VOLTAGE

ZnO=U-I CHARACTERISTIC FOR

ZnO
8/20 µs CURRENT IMPULSES

Fig. 17 Equivalent scheme for ZnO arresters used in computer calculations

Apart from the standard current-voltage characteristic for an arrester (”ZnO” above) a circuit is
included for modelling the increase of the residual voltage for shorter times than 8/20 µs (”R1” and
”L2” above). The effects from connection leads and arrester height is modelled with the inductance
”L1”.

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6.2.3 Transient overvoltages - Energy capability and current withstand strengths
In service, a surge arrester may be subjected to different energy impulses originating from such
sources as lightning, faults in the network, switching of lines or capacitor banks, etc. The
arresters must be designed in such a way that the ZnO blocks will withstand the energy or current
without failing. Additionally, the arrester must be able to withstand the thermal energy,
i.e. it must be able to cool against Uc after an energy absorption.

High voltage arresters are normally designated according to IEC with a specific line discharge
class. The below Figure 18a shows relative energies in kJ/kV rated voltage for the different line
discharge classes. The intention with this classification is naturally that a higher class should
represent a higher energy capability for a given arrester. Hence, the energy absorbed during a
single line discharge is approximately:
• Class 1 1 kJ/kV (Ur)
• Class 2 2 kJ/kV (Ur)
• Class 3 3 kJ/kV (Ur)
• Class 4 4 kJ/kV (Ur)
• Class 5 5 kJ/kV (Ur)

However, this is only valid if the ratio between the switching impulse residual voltage, Ups , to the
rated voltage of the arrester, Ur, is approximately a factor of 2.0. If the ratio differs greatly from
this, the line discharge class becomes a useless measure, i.e. the higher the residual voltage for
a given rated voltage, the less energy the arrester is required to absorb during the line discharge,
and vice-versa.

Specific energy kJ/kV (Ur) Specific energy kJ/kV (Ur)

7 7
Class 1 Class 72.5 - 150kV
Class 2 Class 151 - 325kV
6 Class 3 6 Class 326 - 400kV
Class 4 Class 401 - 600kV
Class 5 Class 601 - 900kV
5 5

4 4

3 3

2 2

1 1

0 0
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
Relative protective level Ua/Ur Relative protective level Ua/Ur

Fig. 18a Relative energy stresses for different line Fig. 18b Relative energy stresses for different line
discharge classes according to IEC 60099-4. discharge classes according to IEEE C62.11.
Ups is designated as Ua . Ups is designated as Ua.

Potentially even more confusing for the ordinary user is the classification as per the IEEE
standard, as depicted in the above Figure 18b. The diagram is drawn for lowest used rated
voltage on highest existing system voltage in each class. The highest relative energy occurs for
the Class 326 to 400 kV. In general, the energy is lower in IEEE than for the IEC classes. On the
other hand, the Line Discharge test as per IEEE shall be performed with 18 impulses in only 3
groups of 6 impulses, compared with IEC which prescribes 6 groups with 3 impulses in each
group. The interval between the impulses in each group shall be 50 - 60 seconds and full cooling
is allowed between groups.

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The energy absorption capability of an arrester is only defined in IEC as per the previously
mentioned Line Discharge classification. Different manufacturers assign the energy capability in
different ways. For example, the energy capability may be given as:

• kJ/kV Ur, kilojoule per kilovolt rated voltage which is possibly complemented with the
shortest time during which the energy can be absorbed

• kJ/kV Uc , kilojoule per kilovolt continuous operating voltage which is possibly

complemented with the shortes t time during which the energy can be absorbed

• the sum of the energy resulting from two line discharges separated 50 – 60 seconds in
compliance with IEC’s line discharge classification

Therefore a surge arrester may be described with at least three different energy values, which is
why it is essential to state how the energy for a specific arrester has been given. As an example,
the following energy capabilities can be given for the same ABB arrester type EXLIM P (Class 4):

• 7.0 kJ/kV Ur, rectangular current impulse with a duration of at least 4 ms

• 8.8 kJ/kV Uc , rectangular current impulse with a duration of at least 4 ms

• 10.8 kJ/kV Ur, two line discharge impulses in compliance with IEC 60099-4

The ZnO blocks are normally able to withstand considerably higher energies with longer durations
(seconds), compared to shorter durations (milliseconds). Expressions like ”kJ/kV Ur” or ”kJ/kV” are
therefore meaningless unless the shortest time for which the arrester can be subjected to the
given energy is also s tated.

As mentioned previously, a high voltage arrester is normally designed in compliance with a

chosen line discharge class as per IEC with respect to energy. For non-standard stresses, such
as capacitor discharges or high energies due to lightning, the design may need to be made with a
lower energy stress per varistor.

Aside from withstanding the energy from current impulses, the ZnO blocks must also have a
sufficiently high dielectric withstand so as to ensure that the voltage across the block will not result
in a puncture or a flashover across the block. To ensure a sufficient insulation withstand margin
for normal stresses, the ZnO blocks (together with all internal parts in a high voltage arrester) are
dimensioned to withstand current impulses with an amplitude of at least 100 kA, having a wave
form of 4/10 µs.

Requirements for high energy absorption capability can be solved by increasing the block volume
- either by using blocks with larger diameter or by paralleling block columns and/or arresters. To
ensure that the latter designs will operate correctly during service, a very careful procedure is
required to ensure a good current sharing between the block columns and/or arresters connected
in parallel. Furthermore, possible changes of the block characteristic due to the normal applied
service voltage as well as energy and voltage stresses must be extremely small.

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6.2.4 Transient overvoltages - External insulation withstand
The primary function of an arrester is to limit, and thus render harmless, overvoltages to which
the protected equipment is exposed. It is obvious therefore, that its own insulation (both external
and internal) is the best-protected of all.

In contrast to other HV apparatus, the insulation level for surge arresters therefore does not
need to fulfil a standardised insulation class since the arrester will effectively protect its own
insulation against overvoltages. Distance effects need not to be considered. Instead, the
Standards stipulate a specific safety margin between the residual voltage of the arrester to the
voltage withstand level of its external insulation.

The voltage across an arrester can never be higher than that determined by the arrester's
protective characteristics. Only the need for an additional (statistical) safety factor (margin)
including correction for installation altitude can technically justify a higher external insulation
strength. Generally, the risk of an external flashover less than or equal to 10-3 is considered as
acceptable; which leads to a factor of approximately 1.10 to 1.15 (excluding altitude correction)
between the arrester protective levels and the LIWL and SIWL of the housing.

Both the IEC and IEEE standards clearly stipulate that such a margin is sufficient. IEEE
stipulates that the external LIWL of the housing shall be 20 % above the discharge voltage at
20 kA, 8/20 µs impulse plus an altitude factor of 9% per every 3000 feet (roughly equal to 10 %
per every 1000 m). IEC stipulates a LIWL margin of 15 % above the discharge voltage at
nominal current plus an altitude factor of 13 % for up to 1000 m.

A longer arrester may, in fact, lead to less effective protection for steeper surges for which the
inductance of the arrester itself becomes more significant. Thus, the stipulation of high external
insulation withstand values (e.g. equal to that for the protected equipment) may thus be
disadvantageous for the protected equipment.

The complete arrester, including possible grading rings, must be designed to give a reasonable
safety margin against external flashovers. With the specified margins in the IEC Standard, an
acceptable low risk for external flashovers is obtained up to an altitude of 1000 m. For higher
altitudes, special consideration needs to be given on a case-by-case basis.

6.2.5 Large mechanical forces

It is relatively simple to calculate the maximum bending moment at the base of a self supported
arrester from loads caused by wind and terminal pull. For the earthquake forces, however, the
situation can be a lot more complicated.

When the earthquake is defined as a maximum horizontal acceleration, the bending moment
can be easily calculated when considering the arrester as a rigid body. When more accurate
calculations are necessary, the elasticity and damping of the arrester must first be determined in
a snap-back test.

Knowledge about elasticity, resonance frequencies and related damping is also required when
the earthquake is specified by a frequency spectrum, In such cases, a specially developed
computer program will need to be used. However, a reliable calculation needs to also have
adequate n i formation about the structure on which the arrester is erected. For example,
mounting the arrester on a support structure which has a sufficiently high natural frequency (e.g.
a large power transformer) may reduce or remove the seismic loading on the arrester.

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Since the polymer-housed arresters are more or less elastic, temporary
loads, including short-circuit forces and earthquake forces, can be looked
upon differently compared to rigid bodies like porcelain insulators. The
reason for this is that the forces do not have time to act fully due to the
elasticity of the material and mass inertia, i.e. the forces are spread out in
time leading to the arrester not encountering any high instantaneous
values.

These advantages, combined with a design with small mass participation,

have been fully utilised by ABB for the 550 kV arrester shown opposite in
Figure 19. This arrester withstands a ground horizontal acceleration in
excess of the highest seismic demands as per IEEE standards.

Seismic qualification testing has also been successfully made on other

standard ABB type PEXLIM surge arresters (without additional bracing),
even at the arduous 1.0g ZPA level.

Alternatively, suspending polymer surge arresters directly from the

overhead line is a viable mounting alternative to eliminate seismic loads
altogether.
Fig. 19 ABB type PEXLIM P
for 550 kV system voltage.
The arrester is designed to meet
extreme earthquake requirements
in the Los Angeles area (USA).

6.2.6 Severe external pollution

High radial voltage stresses may occur between the block column and the outside of the
insulator during severe external pollution. Generally, external pollution may influence a surge
arrester in the following ways:
• Possibility of internal corona
• External flashover
• Heating of the blocks
• Tracking and/or erosion of the insulator

The problems for arresters with porcelain housings installed in extremely polluted areas have
historically been solved by greasing the insulator, thus improving the pollution performance. The
aim of the greasing is to reduce the leakage currents on the insulator surface. Hydrophobic
materials, like silicone, give a similar effect. This is one of the strongest motivations for why
silicone has been seen as an attractive insulator material.

A common belief is that all polymer-housed arresters have better pollution performance
compared to arresters with porcelain housings. However, a more correct statement would be
that hydrophobic materials (like silicone) have better performance in polluted areas due to
reduced external leakage currents. In contrast, EPDM rubber, which can lose its hydrophobic
properties quickly, should be designed in the same manner as porcelain from a pollution
performance point of view.

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It is very difficult to avoid internal corona during severe external pollution on arresters containing
an annular gap between the ZnO blocks and the insulator, irrespective of whether the insulator
is made of porcelain or a polymeric material. The design of such arresters must therefore be
able to withstand corona during such pollution episodes.

Some rules-of-thumb for designs such as these are:

• ”No” corona during dry conditions
• Minimise the use of organic materials in the arrester. When organic materials are used,
they must have been thoroughly tested and subjected to realistic corona tests
• Prevent the possibility of electrical discharges directly onto the ZnO blocks

For polymer-housed arresters which do not have any annular gap, large radial stresses may
occur between the blocks and the outside of the insulator during severe external pollution
episodes. It is therefore very important that the rubber insulator is sufficiently thick to avoid a
puncture of the material. Furthermore, steps need to be taken to avoid large air pockets or
cavities, otherwise corona may occur that would eventually lead to an arrester failure.

To avoid external flashover, the creepage distance of the arrester, i.e. the shed form and the
length of the insulator, is typically designed in compliance with the same criteria valid for other
insulation at the actual site.

Possible thermal stresses are determined by the leakage currents that might be present on the
outer surface of the insulator. For porcelain arresters, it has been shown that the integral of the
leakage current, i.e. the charge, can be regarded as independent of the creepage distance, and
instead is approximately linearly dependent on the diameter of the housing. An insulator with a
larger diameter thus may give rise to higher thermal stress during conditions with external
pollution, provided the service conditions are otherwise the same.

For applications requiring arresters with parallel housings and several units connected in series,
the general rule is that the units should not be connected in parallel except at the top and bottom.
This is because, during pollution episodes, the ZnO blocks in one unit could conduct the external
leakage current from all of the parallel connected arresters which cons equently may give an
increased thermal stress on that unit.

Since the ZnO blocks have a negative temperature coefficient in the leakage-current region, i.e.
the leakage current increases with increased temperature, the heating of one unit will lead to a
reduction of the voltage characteristic with subsequent increase of the current. An increased
current through the unit leads to higher power losses with increased temperature, and so the cycle
continues. Not even a careful current-sharing test (matching) of the arrester units will be of help
below the knee-point of current-voltage characteristic.

For a given pollution level, the performance of gapless arresters can generally be improved by
employing any or all of the following measures:
• Increasing the creepage distances of the housings to reduce external leakage current
• Using ZnO blocks of larger volume to improve the energy absorption capability
• Improving the TOV capability, i.e. by increasing the rated voltage (Ur) for the same
arrester type
• Improving the heat transfer mechanism
• Using blocks with lower losses at Uc

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Lower leakage currents on the insulator surface is achieved with a hydrophobic surface, i.e. the
use of silicone insulators. The below Figure 20 shows leakage currents as measured on a
porcelain insulator and a polymer-housed arrester having a silicone insulator. The values are
taken from testing at NGC’s tes t station at Dungeness on the English Channel.

As can be noted, the amplitudes of the Daily maximum

Daily maximumcurrents in aover
currents 16 days
a 16period at
day period
leakage currents on the silicone Dungeness test station
at Dungeness test station
insulator are roughly half to a third of 30
the corresponding leakage currents on Arrester with silicone insulator
Porcelain insulator
the surface of the porcelain insulator 25
during this specific measuring interval.
20

Current (mA)
Fig. 20 Leakage currents for surge arrester
PEXLIM Q108-VV145M and porcelain 15

insulator at Dungeness test station.

10
The leakage current for the arrester includes
an internal leakage current of around 1 mA. 5
The creepage distance for the polymeric
arrester is 5148 mm and 4580 mm for the
0
porcelain insulator.
Days

6.2.7 Thermal stability

Thermal stability is one of the most important application criteria for ZnO arresters, and hence a
thermally stable arrester is a pre-condition for the safe protection of equipment.

The majority of the previously mentioned stresses are potential sources of heat input to the
arrester, which must withstand them without loss of thermal stability. i.e.
• Continuous operating voltage
• Temporary overvoltages
• Transient overvoltages
• Ambient temperature
• Pollution effects
• Non-linear voltage distribution
• Uneven current sharing between parallel columns

The concept of thermal stability can be depicted with the help of a heat loss – input balance
diagram, as depicted in the following Figure 21 . This shows principally how the ability of an
arrester encapsulation to dissipate heat and the temperature dependent power losses of the
blocks result in a working temperature at a certain ambient temperature and chosen voltage
stress (”A” in the Figure). An upper maximum temperature also exists (”B” in the Figure), above
which the design is no longer thermally stable for a given voltage stress.

It can also be seen from Figure 21 that the instability threshold is very much dependant on the
applied power frequency voltage. As the power losses curve is non-linear, a lower applied
service voltage than verified in test, for example, would shift the upper intersection point further
to the right, thereby increasing significantly the temperature limit at which thermal runaway
becomes a risk.

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 Relative power losses Fig. 21
5 Thermal capacity for an
arrester housing and power
B losses for ZnO blocks at
4 different relative voltage stresses
(ambient temperature +40 °C)
Porcelain curve
3

Losses at 0.9*Uref
2 Losses at 0.8*Uref
Losses at 0.7*Uref
A Losses at 0.6*Uref
1
A = Service temperature at 0.8*Uref
B = Thermal limit at 0.8*Uref
0
40 60 80 100 120 140 160 180 200

Varistor temperature (°C)

To explain the concept further: The power losses of a typical ZnO varistor (curved line) due to a
constant applied power frequency voltage is extremely temperature dependant. At the same
time, the ability of the arrester assembly to dissipate heat is generally linear (straight line) and
proportional to its thermal design and temperature rise above the ambient temperature.
Consequently, there are two intersections of the two curves: one at low temperature – a so
called stable operating point, and the other at high temperature – a so called instability
threshold. To obtain thermal stability, the temperature rise due to power losses in the ZnO
varistors must be balanced against heat dissipation to the environment.

If power losses exceeds heat dissipation, then excess energy is stored in the varistors and their
temperature slowly increases. Conversely, if heat dissipation exceeds power losses, the
temperature of the varistors decreases. The varistor temperature may well increase significantly
due to the application of transient or temporary overvoltages, but will always ultimately settle
back at the stable operating point, as long as the varistor temperature does not exceed the
instability threshold. As the two characteristics diverge beyond the instability threshold point, a
thermal runaway will invariably occur from varistor temperatures above this point, whereby the
temperature will continue to increase until the arrester ultimately fails.

Some “rules-of-thumb” for ensuring a design with good heat dissipation, and thereby low risk for
thermal runaway:
• Low-loss blocks
• Reduced voltage stress/mm
• Increased block size
• Homogenous block material
• Non-ageing blocks
• Good mechanical design with regards to thermal heat transfer

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7. STANDARDS AND TESTING

In order to fulfil the requirements of users, Standards specify uniform tests and test methods
aimed at verifying an arrester’s:
• ability to protect against overvoltages
• reliability and long lifetime

The protective function is verified with different measurements of the voltage level for different
current amplitudes and current wave-forms (residual voltage tests) and the reliability is checked
through a number of electrical and mechanical tests. An important part of the electrical tests is the
operating duty tests in which an arrester, or a pre-scaled model of the arrester, is subjected to a
combination of stresses representing anticipated service stresses that an arrester might be
subjected to during its lifetime. The lifetime is further verified by subjecting the ZnO blocks to an
accelerated ageing test procedure.

According to Standards for testing of arresters, the tests can be divided into three main
categories:
• Type tests (Design tests according to IEEE)
• Routine tests
• Acceptance tests (Conformance tests according to IEEE)

These test categories can be defined as follows:

Type tests are performed after completion of the development of a new arrester design to
establish representative performance and to demonstrate compliance with the relevant
standard. Once made, these tests need not to be repeated unless the design is changed
in a way which may negatively influence the performance. Only the relevant tests need to
be repeated in such a case.

Routine te sts are made on each arrester or arrester unit, as well as components, as a
quality control integrated in the production. Their aim is to ensure that the products meet
the design specification.

Acceptance tests are made on a number of randomly chosen arresters from a delivery
lot when it has been specially agreed between the manufacturer and the purchaser at the
time of ordering. Acceptance tests should not be confused with routine tests.

Specifically how surge arresters shall be tested is defined in detail in the Standards, with the two
most widely accepted being IEC 60099-4 (International Standard) and IEEE C62.11
(American National Standard).

The Standard IEC 60099-4 supersedes the old Standard for gapped silicon-carbide surge
arresters, IEC 60099-1, which is not applicable to ZnO arresters. Many countries also have their
own National Standards which more or less comply with IEC or IEEE. Changes and annexes to
the IEC Standard (from Amendment 2) deal with specific issues of importance, including: polymer
housed arresters, short-circuit tests, accelerated ageing, voltage distribution, environmental and
weather ageing tests.

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The tests in the old IEC Standard, IEC 60099-1 are not generally applicable to ZnO arresters, and
IEC 60099-4 reflects a completely different approach on how to select test sections and verify the
protection characteristics.

Some of the major differences between these two IEC Standards are listed below.

IEC 60099-1 IEC 60099-4

Type Tests

Sparkover voltage test No gaps in a ZnO arrester and thus no

sparkover voltage tests

Conditioning test

Residual voltage tests on prorated tests Residual voltage test at type testing gives
sections verifies the absolute value for a a relation to routine tests values
complete arrester

No accelerated ageing tests procedures Accelerated ageing test for 1000 hours.
prescribed Consideration is taken to possible ageing
in the Operating duty tests

Changes of the residual voltage level Changes of the residual voltage level ≤ 5%
≤ 10%

Tests on open sections Tests on open sections and thermal

models

No pre-heating Pre-heating to +60 °C

Strictly specified how the test sections
shall be selected and how the rated
voltage of the section shall be determined

Routine Tests

50 Hz sparkover voltage test Reference voltage measurement

Residual voltage measurement
Corona test (PD measurement)
Tightness check
Current sharing test on arresters with
parallel block columns

Table 2 Comparison between test requirements according to IEC 60099-1 and IEC 60099-4

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7.1 Type Tests
In general. set requirements on arresters and their dimensioning are considered to be
satisfactorily verified by subjecting the arresters to the following generic tests:

• Residual voltage measurement at different current amplitudes and wave-shapes

• Current impulse withstand
• Operating duty
• Accelerated ageing
• Artificial pollution
• External insulation withstand
• Short circuit (pressure relief)
• Partial discharge
• Radio interference voltage (RIV)
• Sealing
• Mechanical

The above tests are considered to be type tests (design tests) but some of these may instead
be performed during the manufacturing process and/or assembly as part of a manufacturer’s
quality assurance. This is acceptable, and even preferable, provided that the type test criteria
are fulfilled during the routine testing. ABB has chosen to do this for testing of internal partial
discharge, seal leak rate and current distribution (multi-column arresters), as applicable for
specific designs.

Regarding polymer-housed arresters, the test procedures in IEC60099-4 differ somewhat from
previous tests on porcelain designs. The above tests by topic are also generally applicable to
polymer designs, with the main exception being that there si no artificial pollution test yet
specified for polymer arresters. Instead, a Weather Ageing test for the polymer material has
been devised. Further, the sealing test requirements are more well defined in the form of a
Moisture Ingress test, as are the criteria for short circuit safety.

7.1.1 Test sections (prorated test sections)

In order to verify guaranteed arrester data, tests are made on both complete arresters as well as
on units of arresters and on components. It is both customary and accepted that some of the tests
are made on scaled-down models of the arresters, thus making it possible to also scale-down the
requirements on the test equipment. These scaled-down arresters units are called ”section of an
arrester” or ”prorated section”. According to the definition in the IEC Standard, the arrester section
intended for a particular test must correctly represent the performance of the complete arrester
during a specific test.

An arrester section may therefore look different depending on the intended tests. In some tests it
is sufficient, for example, to perform the test on series connected ZnO blocks while other tests
require that the ZnO blocks are encapsulated in a thermally correct model of the complete
arrester. The requirements set on the tests sections, according to IEC 60099-4, are listed in the
following Table.

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 Type of prorated test section Valid for tests

Non-encapsulated ZnO blocks * ) Residual voltage tests

Tests with long duration current impulses

Encapsulated ZnO blocks **) Accelerated ageing test procedure

ZnO blocks in a thermal Operating duty tests where thermal stability

model of an arrester must be verified
*)
Residual voltage and long duration current impulse tests are performed on three new
samples, which may be either resistor elements, arrester sections or complete arresters.
**)
For porcelain housed arresters the blocks must be in the same atmosphere as found in
the actual arrester. For polymer-housed arresters the blocks must be surrounded by the
same m aterial as used in the actual arrester.

Table 3 Requirements on prorated test sections at different tests according to IEC 60099-4

For all tests with energy injections, it is important that the test section fulfils the following
requirements:

• The block volume shall not be greater than the minimum block volume specified for the
complete arrester, scaled down with respect to the rated voltage of the prorated test section

• The energy injected into the test section must correspond to what a test section comprising
ZnO blocks with a minimum voltage-current characteristic would have been subjected to

It is equally important during tests with temporary overvoltages that the test voltage is scaled
down with respect to the reference voltage of the test section and the minimum reference voltage
assigned to the complete arrester.

Test sections comprising non-encapsulated ZnO blocks are well defined, but verification tests are
necessary to design a thermally correct test section. A thermal section shall, in principal, be a
cross section of the complete arrester. However, the heat transfer in the middle of a long arrester
unit takes place mainly in the radial direction, and hence a conservative model of the arrester
must be thermally insulated at both ends to avoid heat transfer axially. The principal design of a
thermal section for polymeric arresters is shown in the following Figure 22.

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 Electrical
Electrical
connection
connection

Sheets of compressed
Sheets of Glass bolt
Glasfiber fibre bolt
wool
compressed wool Insulator
Insulator
Sheets of compressed
wool
ZnO
ZnO blocks
block

Electrical
Electrical
connection
connection

Insulation

Fig. 22 Principal design of a thermally pro-rated section for polymer-housed arresters.

It is, however, not sufficient to specify only the design of a section; it must also be verified through
tests. The verification of the thermal section is made by heating a complete arrester unit and a
thermal section to around +120 °C by the application of voltage. Thereafter, the ZnO block
temperatures in the unit and the section are measured during the cooling time. A correctly
designed thermal section shall not cool faster than the arrester unit. The below Figure 23 shows
cooling curves from a test on a thermal section and a complete arrester. The Figure additionally
shows the cooling for a section designed according to requirements given in IEEE C62.11, which
also requires that verification tests be carried out.
Relative temperature above ambient temperature
1
Completesurge
Complete surgearrester
arrester
Thermalsection
Thermal sectionaccording
accordingto
to IEC
IEC
0.9 Thermalsection
Thermal sectionaccording
accordingto
to IANSI/IEEE
IEEE

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1
0 20 40 60 80 100 120
Time (minutes)

Fig. 23 Verification of thermal section and comparison of specifications between IEEE and IEC

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7.1.2 Residual voltage tests (Discharge-Voltage Tests according to IEEE)
The purpose of these tests is to verify the protection level of the surge arrester. All residual
voltage tests are made by subjecting the arrester or a section of the arrester (usually some ZnO
blocks) to current impulses with different amplitudes and wave forms, and measuring the residual
voltage across the test object. The measured voltage represents the protection level of the
arrester for the actual current and waveshape.

The Standards make a distinction between different current impulses, based on different events in
a network:

• Currents caused by lightning (lightning impulse current)

The testing is made with a current impulse having a front time of 8 µs and a half-
value time of 20 µs. The impulse is normally designated as an 8/20 µs impulse.

• Currents caused by switching overvoltages (switching impulse current)

The testing is made with a current impulse having a front time of 30 – 100 µs and a
half-value time on the tail of roughly twice the virtual front time. The impulse is
normally designated as an 30/60 µs impulse.

• Currents having a steep front (steep current impulse)

The testing is made with an impulse with a front time of 1 µs while the half-value
time may be any value. However, normally a test circuit generating a half-value time
of approximately 2 µs to 20 µs is used, i.e. a 1/(2-20) µs impulse.

It is of course possible that switching events or a fault can result in steeper current pulses than
30 µs, or that the current at lightning overvoltages may show both shorter or longer front times
than 8 µs.

By testing with different current amplitudes for each of the current-shapes, a complete protection
characteristic is obtained for each wave-form. For current impulses with the same amplitude, the
residual voltage level increases slightly for shorter front-times. This frequency dependence is
illustrated below (Fig. 24), showing results from a test with 10 kA for wave-forms 8/20 µs and
1/2µs. The steeper front, 1 µs, may be the result of a lightning stroke very close to a substation
protected by surge arresters. Further, inductance effects can become significant with steep
current impulses, and IEC specifies that the steep current impulse residual voltage tests may
need to be corrected to account for the possible inductive voltage drop between the arrester
terminals.
Residual
Residualvoltage 1/2 current
voltage for 1/2µs currentimpulse
impulse

Residualvoltage
Residual voltage for
for 8/20
8/20µs currentimpulse
current impulse
Voltage (kV) Fig. 24
12
Comparison between residual
8
voltage levels for current
pulses 8/20µs and 1/2µs.
4
The upper curves show the
0 Current impulse
Current impulse
voltages and the lower the
8/20µs
8/20 currents.
-4

Current
Current impulse
impulse
Note that the curves have
-8
1/21/2µs been misaligned for clarity.
-12
The lower time scale is valid
Current (kA) 0 10 20 30 40 50 for the 1/2µs impulse, while
the upper scale is valid for
0 1 2 3 4 5
Time (microseconds)
the 8/20µs impulse
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In order to generate the specified current pulses an impulse generator is needed with the
capability to create currents up to 40 kA. To be able to create such currents through a complete
arrester at high voltages would require very large impulse generators, since the test equipment
must principally be able to simulate full scale lightning.

Tests on complete arresters are however not necessary, nor desirable for reasons of accuracy.
IEC therefore recommends that the residual voltage tests are made on scaled-down models of the
arrester and specifies also how the measured values shall be re-calculated to be valid for a
complete arrester.

According to IEC, the objective of the residual voltage type tests is to verify the claimed protection
levels by checking the relationship of protection levels at different current wave-forms and
amplitudes to a level which is checked in routine tests on all arresters. Normally the residual
voltage at 10 kA with waveform 8/20 µs is used as a reference. This means that the 10 kA level
with this waveform must be verified in a routine test and given for all manufactured arresters. The
requirement for a routine test can be fulfilled by measuring the residual voltage for each individual
block within the arrester and summing up the result. This procedure will be correct, since all
blocks in a single column arrester will be subjected to the same current.

7.1.3 Tests with long impulses (Long Duration Current Impulse Test)
A surge arrester limits incoming overvoltages by diverting the surge current. The energy the
arrester absorbs is given by the equation:

t
W = ∫ (u * i ) dt
0
where u = voltage across the arrester
i = current through the arrester

The arresters must withstand this energy without thermal instability or damage to the blocks in any
way. It is equally important that the characteristics of the arrester are not changed due to repeated
energy stresses. This could not only jeopardize the protection function of the arrester, but also the
current sharing between parallel block columns in an arrester, or between several parallel
arresters, that have been matched with respect to current sharing to cope with large energy
requirements.

Requirements for very high energy capabilities are solved by utilizing many parallel block
columns. For such designs, it is required that changes in the protective characteristic of the blocks
is low. From a protection point of view, it is acceptable that the residual voltage decreases with
repeated current impulses, but if blocks are connected in parallel, the acceptable changes are
much lower than what is allowed by the Standard.

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Aside from discharge of capacitor banks, the highest arrester energies for high voltage AC
arresters are obtained from switching of long transmission lines. Simplified, the arrester will be
subjected to a current impulse of rectangular shape, with the duration of the impulse being
determined by the length of the line. The current amplitude through the arrester is given by the
prospective overvoltage (without a surge arrester), the surge impedance of the line and the
characteristic of the arrester. By tradition, the energy capability of an arrester has been defined
with respect to the withstand capability for rectangular current impulses.

A desired energy capability for the arrester can be given indirectly by defining line parameters, or
directly in kJ/kV rated voltage. However, it must be emphasized that any value given as kJ/kV
rated voltage without specifying test procedures is undefined and thus of little value.

The test with long current impulses is made on arrester sections. For arresters having a nominal
current class 10 kA and 20 kA, the tests are defined as line discharge tests where the test circuit’s
wave impedance, charging voltage and duration of the current impulse are defined in the
Standards. The resulting energy is dependent on the protection level of the arrester, which is why
the energy must be defined and be given in the test report.

Arresters having a nominal current class of 2.5 kA and 5 kA are not tested with line discharges,
and instead tests with rectangular current impulses are specified with given amplitudes and
durations.

7.1.4 Operating duty test

The purpose of this test is to verify that the arrester withstands all the kinds of electrical stresses
which are likely to occur during its lifetime. This is schematically shown in the following Figure 25.

The standardized operating duty test therefore includes different stresses and sequences of
current pulses and voltage amplitudes representing possible events in a power system.

Originally, the operating duty test was used to verify an arresters’ ability to handle lightning
currents while being simultaneously subjected to maximum allowable operating voltage. For
gapped arresters this meant that, apart from the lightning current stress, they were subjected to a
power frequency follow current before the gaps were able to extinguish the arc at voltage zero.
If the arc was not extinguished at the first voltage zero, the arrester normally failed.

ZnO arresters do not contain any gaps, but an operating duty test is nevertheless still useful to
check that the arrester is thermally stable after having absorbed large amounts of energy under
severe ambient conditions with respect to temperature and voltage. These energy inputs could
come from energy discharges as well as from Temporary Overvoltages (TOV) on the system.
How the operating duty test shall be carried out for different Line Discharge Classes is illustrated
in the following Figures 26 and 27.

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 ZnO surge arrester

~ Uf = power frequency voltage

across arrester

Type of fault Stress Duration Voltage profile

Normal service Uf = Um / √3 Continuous

Lightning impulse µs-impulse

Earth fault Uf < Um / √3 ≈ 0.1 - 10 s *)

Breaker operation Uf ⇒ 0 ≈ 0.3 - 1 s

Re-closing switching ms-impulse

Possible faults:

Earth fault on other phase Uf = ke*U m /√3 ≈ 0.1 - 10 s

Breaker operation Uf ≤ Um /√3 ≈ 0.3 - 1 s

Re-closing switching ms-impulse

Normal service Uf = Um / √3 Continuous

*)
In some Countries even longer earth-fault times are allowed.

Fig. 25 Examples of stress sequences on surge arresters during different fault conditions.
The system is directly earthed having an earth-fault factor ”ke” ≤ 1.4.

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 1 Residual voltage measurement at In , 8/20 µs
Time interval not specified
Conditioning test. Four groups of five impulses at
2 In , 8/20 µs, superimposed on the continuous
operating voltage + 20%
Time interval not specified, +20 °C ± 15 °C
High current impulse, 4/10 µs
Pre-heating to +60 °C ± 3 °C

Time as short as possible, not exceeding 100 ms

3 Elevated rated voltage, Ur *, 10 seconds

Elevated continuous operating voltage, Uc *,

30 minutes
Cooling to ambient temperature, +20 °C ± 15 °C

Residual voltage measurement at In , 8/20 µs

4
Visual check of the test objects

Elevated rated voltage Ur* and continuous operating voltage Uc * only if the accelerated ageing
test procedure gives increased power losses. Otherwise, Ur and Uc are applied.

In = Nominal discharge current

Explanation of the numbers:

1 Preparatory measurements 3 Operating duty test with high current impulses

2 Conditioning 4 Measurements and checking

Fig. 26 Operating duty test on 10 kA surge arresters with line discharge class 1 and arresters of class 1.5 kA,
2.5 kA or 5kA.

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 1 Residual voltage measurement at In , 8/20 µs
pulse
Time interval not specified
Conditioning test. Four groups of five impulses at
In , 8/20 µs, superimposed on the continuous
operating voltage + 20%
Time interval not specified, +20 °C ± 15 °C
Conditioning with high current impulse on a
2 pro-rated test section, 4/10 µs
Cooling to ambient temperature
Conditioning with high current impulse on a
pro-rated test section, 4/10 µs
Kept for future testing
Pre-heating to +60 °C ± 3 °C
Line discharge impulse
Time interval 50 - 60 seconds
Line discharge impulse
3 Time as short as possible, not exceeding 100ms
Elevated rated voltage, Ur *, 10 seconds
Elevated continuous operating voltage, Uc *,
30 minutes
Cooling to ambient temperature +20 °C ± 15 °C
Measurement of residual voltage at In, 8/20 µs
4
Visual check of the test objects

Elevated rated voltage Ur* and continuous operating voltage Uc * only if the accelerated ageing
test procedure gives increased power losses. Otherwise, Ur and Uc are applied.

In = Nominal discharge current

Explanation of the numbers:

1 Preparatory measurements 3 Operating duty test with

long duration current impulses

2 Conditioning 4 Measurements and checking

Fig. 27 Operating duty test on 10 kA surge arresters with line discharge class 2 or class 3
and 20 kA arresters with line discharge class 4 or 5

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The test sequence starts with a conditioning phase, where the test objects are subjected to a
large number of current puls es to take into consideration possible changes of the block
characteristic during actual service conditions due to repeated stresses. After this sequence, the
part of the test commences which shall verify the arrester’s thermal stability and designated rated
voltage.

This part of the test shall therefore be made fulfilling the following requirements:

• The prorated test section must be thermally equivalent to a complete surge arrester

• The test objects must be heated before the test to a temperature being representative of
the worst possible service conditions

• The test must be made on previously non-tested blocks. Consideration must be taken of
possible ageing of the blocks by applying correction factors (giving Ur* and Uc*) according
to the guidelines in IEC 60099-4.

Finally, it is required that the arrester withstands the operating duty test without change to its
electrical properties beyond acceptable limits. The residual voltage level at nominal discharge
current is therefore checked before and after the test sequence.

The operating duty test is normally performed on arrester sections. To fulfil the requirement for
thermal equivalency, the section is principally a cross-section of the complete arrester.
IEC 60099-4 requires preheating of the thermal pro-rated section to +60 °C before the energy
injections. This temperature is thought to represent an ambient temperature of +40 °C together
with solar radiation, self-heating of the blocks due to power losses and some influence from
pollution. For ZnO blocks with low power losses at normal service voltage, +60 °C is a
conservative value, and the operating duty test consequently gives a safety margin with respect
to thermal stability limits.

7.1.5 Accelerated ageing test procedure

One of the key basis for the dimensioning of an arrester is the result from the accelerated ageing
test procedure, where an acceleration of possible ageing effects is obtained by performing the test
at an elevated temperature. Surge arresters limit overvoltages by conducting current, but during
most of the arresters’ lifetime it shall act as an insulator. The entire continuous operating voltage
is across the ZnO blocks and these must keep their insulating properties during their lifetime.

IEC specifies an accelerated ageing test during 1000 hours at an elevated temperature of 115 °C
as a type test. For arresters filled with air, the ZnO blocks need not be encapsulated during the
test. If the surrounding atmosphere is something else (e.g. nitrogen or other gas) the test must be
performed with the blocks in that particular atmosphere. For polymer-housed arresters, where the
blocks are in direct contact with other materials, the ageing test must be made including all
materials which are in direct contact with the blocks to show that the blocks are not negatively
affected (i.e. aged) due to influence from the other materials.

The accelerating ageing test is based on the Arrhenius law, which provides good confidence on
life expectancy of ZnO blocks. When tested according to the IEC requirements, the equivalent
minimum demonstrated life time is predicted to be 110 years at the conservative average ambient
temperature of 40 °C.

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An important parameter during the ageing test is the selection of the voltage stress on the
blocks. The test voltage must reflect the highest possible local voltage stress in the arrester
when it is energized at the highest possible continuous operating voltage, Uc , assigned to the
arrester. A thorough electrical field calculation therefore must be made for each arrester type
and rated voltage which, in turn, is the basis for determining the relevant voltage stress during
the accelerated ageing test procedure. Influence from all phases in a three-phase configuration
must also be taken into account when performing the calculations.

Accelerated ageing test on ZnO varistors type PEXLIM Q

at 115 °C with a voltage stress of 0.97 * reference voltage, U ref
Polymeric insulator molded directly on the ZnO block
Test time: 1122 hours

Relative powerlosses P/Po (Po=losses after 1.5 hour)

1.2

0.8

0.6

0.4

0.2

0
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
Time (hours)

Fig. 28 Example of power losses during an accelerated ageing test procedure for ABB’s ZnO blocks.

ZnO blocks are normally manufactured in batches of some thousands of blocks, and variations
(even minor ones) may have a negative influence on the block characteristics. From a quality
point of view, it is thus necessary to perform ageing tests as sample tests on blocks from each
manufactured batch.

Separate from the type test, ABB further verifies the stability of every production batch of ZnO
blocks by routinely performing an accelerated ageing test on some blocks picked out randomly
from the whole batch. Power losses after 1000 hours, extrapolated from a test with shorter
duration, at an elevated temperature of 115 °C at 1.05 x Uc shall not exceed the losses at the start
of the test and not more than 10% above the lowest losses occurring during the test period.
Batches in which unapproved blocks appear are rejected.

It is however not sufficient to check only the characteristics of the blocks, but rather the entire
arrester must be seen as a unity. The ability of the arrester housing to dissipate heat must also be
adjusted to the power losses of the blocks during different service conditions with respect to
voltage, temperature and even frequency. This is necessary to ensure that the average block
temperature will not considerably exceed the ambient temperature, and thereby remain thermally
stable.

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7.1.6 Artificial pollution test
Artificial pollution tests are intended to provide information on the behaviour of external
insulation under conditions representative of pollution in service, although they do not
necessarily simulate any particular service conditions.

A number of different methods have been trialed for artificial pollution tests on surge arresters,
with those specifically intended for porcelain-housed arresters having the intention of
determining
• risk for external flashover
• effect of partial discharges inside the surge arrester due to radial fields between the
external surface and the internal active elements
• adverse temperature rise of the internal active elements due to a non-linear and
transient voltage grading caused by the pollution layer on the surface of the housing

Different methods are intended to test for one or more phenomena. Further, artificial pollution
tests aimed at determining localised temperature rise are only considered applicable to multi-
unit arresters, since single-unit arresters do not have a direct electrical connection between
inside to outside along their length. However, the risk of puncture exists for very long units.

The conclusion is, of course, that it is necessary to have an arrester design (both internal and
external) which minimizes such stresses and/or their effect under all anticipated conditions.

A problem with many of the pollution test methods is that their relevance to real conditions
during arrester life is questionable. Such methods test the arresters behaviour in more-or-less
irrelevant respects, and thus help neither users nor manufacturers to judge between arrester
designs with respect to pollution performance. A meaningful test program for surge arresters
must therefore start with an investigation of the pollution conditions which arresters can see in
real life and what effect these conditions will actually have on arrester designs. Consequently,
field-tests of arresters in areas with severe natural pollution have been performed to sort out the
relevant mechanisms for arrester performance under polluted conditions.

Since 1982, ABB has ZnO surge arresters installed at different sites with known severe pollution
conditions (salt fog, sand, industrial, etc). The testing has been carried out in collaboration with
recognized leading power utilities around the world. The field tests included conductivity
measurement of natural-polluted layer, recording of external and internal currents with counting
of current pulses and temperature recordings. The results and experience gained from these
field-tests has contributed greatly in the designing of ABB type EXLIM porcelain arresters to
ensure their optimum pollution performance, even under the most severe conditions. For
example, the results show that the temperature rise during real pollution episodes has not been
seen to be sufficient to increase the block temperature to such an extent as to create a risk for
thermal runaway in the EXLIM design.

The value of this experience is recognized also in the IEC Standard, with the possibility for
agreement between user and manufacturer to omit performing an artificial pollution test, based
(for example) on service experience in specified environments.

IEC60099-4 further acknowledges that artificial pollution tests, as prescribed for porcelain-
housed arresters, are not strictly applicable to polymer-housed arresters. Instead, for the time
being, only a weather ageing test for the polymer material is specified.

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7.1.7 Weather ageing test
In contrast to porcelain which, as a material, can reasonably be expected to remain unchanged
over its lifetime, there exists concerns (real or perceived) about the longevity of polymer
insulators in extreme weather conditions.

The weather ageing test in IEC60099-4 is thus applicable only to polymer housed arresters.
The test is intended as a continuous test with a duration of 1000 hours under salt fog conditions
at constant power frequency voltage equal to Uc . It shall be performed on the longest electrical
unit with the minimum specific creepage distance and the highest rated voltage recommended
for the arrester type.

This test is primarily intended to age the polymer material so as to cause tracking, erosion or
puncture; although other failure mechanisms may occur. Interruptions due to flashover are
therefore permitted. If they do occur, the arrester shall be washed with clean tap water and the
test restarted with a lower salt content for the fog.

As an alternative, in case of severe environmental conditions - intense solar radiation, frequent

temperature inversion with condensation, heavy or very heavy pollution (as defined in
IEC60815) - a 5000 hour continuous multi-stress test may be performed after agreement
between the manufacturer and the user.

This test consists of constantly energizing the arrester with constant power frequency voltage
equal to Uc and then applying various stresses in a cyclic manner:
• solar radiation simulation
• artificial rain
• dry heat
• damp heat (near saturation)
• high dampness at room temperature (saturation has to be obtained)
• salt fog at low concentration

Furthermore, temperature variations may cause some degree of mechanical stress, possibly
leading to sealing failure, and also give rise to condensation phenomena. As this test is
intended to accelerate ageing from weather conditions seen in service, flashovers should not
occur.

If the 5000 hour test is performed on the longest electrical unit with the minimum specific
creepage distance, then the 1000 hour test may not be considered necessary to perform.

Both tests are considered passed if no tracking occurs, erosion does not occur through the
entire thickness of the external housing to the next material layer, the sheds and housing are
not punctured, and the electrical performance of the arrester is substantially unchanged from
before to after the test.

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7.1.8 External insulation withstand tests
The external insulation of arresters need not fulfil a certain standardized insulation class since the
arrester effectively protects its own insulation against overvoltages, both external as well as
internal.

This is also reflected in the Standards, where the insulation requirements for arresters are based
on the arrester’s protection levels with a reasonable safety margin added. The ZnO blocks can
naturally not be assembled in the arrester during such a test on the housing, since no laboratory
equipment exists which is capable of generating the very high currents that would be needed.
The tests are therefore performed on empty unit housings. For multi-unit arresters, grading
capacitors can be used in place of the ZnO blocks.

The following minimum values for the external insulation must be kept according to IEC 60099-4:

• Arresters with rated voltage < 200 kV

For short impulse, 1.2/50 µs

The arrester housing shall withstand 1.3 times the residual voltage value at nominal
discharge current (10 kA or 20 kA) with wave form 8/20 µs, i.e. the lightning
impulse protection level

For power frequency voltage, 50 Hz or 60 Hz

The housings for 10 kA and 20 kA arresters shall withstand a voltage (peak value) of
1.06 times the residual voltage level at classifying current for switching overvoltage
(0.125 kA up to 2 kA) with wave form 30/60 µs. Arresters with nominal current of
1.5 kA, 2.5 kA and 5 kA shall withstand a voltage (peak value) of 0.88 times the
lightning impulse protection level.

• Arresters with rated voltage ≥ 200 kV

For short impulse, 1.2/50 µs

The arrester housing shall withstand 1.3 times the residual voltage value at nominal
discharge current (10 kA or 20 kA) with wave form 8/20 µs, i.e. the lightning impulse
protection level.

For long impulse, 250/2500 µs

The arrester housing shall withstand 1.25 times the residual voltage value at
classifying current for switching surges (0.125 kA up to 2 kA) with wave form 30/60 µs.

Note: IEEE does not use the same correction factors as IEC, and therefore IEEE requires
other withstand levels; due partly to the difference in maximum required design altitude (1800 m
for IEEE compared with 1000 m for IEC). Refer Table 4.

IEC60071-1, for insulation co-ordination principles and rules, states that when it has been
demonstrated that one condition (dry or wet) or one polarity or a combination of these produces
the lowest withstand voltage, then it is sufficient to verify the withstand voltage for this particular
condition. Hence, insulation withstand tests shall be wet tests for outdoor arresters where wet
conditions are expected to lower the withstand voltages. Experience shows that this is the case
for power frequency tests and switching impulse tests, but not for lightning impulse tests. If the
arresters are intended for indoor use, dry tests are considered sufficient in all cases.

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 IEC (installation < 1000 m) IEEE (installation < 1800 m)

Ur < 200 kV Ur > 200 kV

Power frequency 1.06 /√2 x Ups Not applicable 0.82 x Ups
for arresters with
IEC: In = 10 or 20 kA
50 Hz, 1 minute or
0.88 /√2 x Up l
IEEE: for arresters with
60 Hz, 10 second
In = 1.5, 2.5 and 5 kA

SIWL Not applicable 1.25 x SIPL Not defined

LIWL 1.3 x Upl for all arresters 1.42 x Upl at 20 kA
for all arresters

Table 4 Comparison of IEC and IEEE requirements for insulation withstand voltages

All distances between the arrester’s own parts, e.g. grading ring to flanges, must be checked with
respect to voltage stress and withstand, either by calculation or test. If actual tests are required
on complete multi-unit arresters, the blocks mus t be replaced with something giving the same
internal voltage grading as the blocks would give. Normally capacitors are used to replace the
ZnO blocks during such tests to model actual service conditions as closely as possible.

7.1.9 Short circuit (pressure relief) tests

As the primary requirement for an arrester is to protect under all circumstances, this leads to the
higher possibility for a failure compared to other high voltage equipment. This is also generally
accepted, and should not be considered as a “failure” in the design.

As a result, special requirements are set on arresters to ensure that a possible arrester failure will
not cause consequential damage to other equipment or injury to personnel. The Standards
therefore require tests where a deliberate internal short-circuit has been made to check the short-
circuit / pressure relief capability.

Previously, tests were made as specified in the ”old” IEC60099-1 Standard for gapped SiC
arresters. In these test requirements, it was taken for granted that an arrester fulfilling a certain
current class with respect to pressure relief capability automatically also fulfilled all lower current
classes. It was subsequently realized that this was not always the case (particularly for porcelain-
housings), and a design may include ”grey zones” if it is only tested against the highest possible
current amplitude. In order to avoid this uncertainty, IEC60099-4 requires that arresters must not
only be tested with the highest short-circuit current (100%), but also at approximately 25 % and
50 % of the highest current. In addition, similar to the “old” standard, a low current test shall be
performed at 600 + 200 A.

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IEC60099-4 further recognizes that it is not possible to uncritically use test methods intended for
porcelain designs on polymeric designs lacking internal channels for pressure relief. For example,
to make short-circuit tests by short-circuiting a polymer-housed arrester with a fuse-wire at
random alongside the block column may result in unsafe arresters being incorrectly considered
reliable from a pressure relief point of view.

According to IEC, the samples should be prepared with means for conducting the required
short-circuit current, with the method dependant on the short-circuit current level and arrester
design.

For the rated and reduced short-circuit currents (two currents), the methods of test sample
preparation depends upon the arrester construction:

• For an arrester with pressure relief device (and thus also an enclosed gas volume
surrounding the ZnO block stack or a gas channel in close vicinity to the stack): the ZnO
blocks are externally bypassed by a fuse wire placed along the surface of the blocks
inside the housing.

• For an arrester without a pressure relief device: the ZnO blocks may either be pre-
failed by overvoltage or bypassed with an internal fuse wire installed in a drilled hole
through the blocks. For the overvoltage method, a high voltage is applied to the arrester
leading to an electrical failure of the arrester within 2 - 8 minutes; after which the arrester is
subjected to the short-circuit current within 15 minutes.

The method of preparing arresters for conducting the low short-circuit current is by overvoltage,
regardless of the design.

High current Low current

Rated short- Reduced short-circuit Short-circuit current,

Arrester class = nominal
circuit Current Currents (+ 10 %) with a duration of 1 s
discharge current
Is A A
A
A
20 000 or 10 000 80 000 50 000 25 000 600 + 200
20 000 or 10 000 63 000 25 000 12 000 600 + 200
20 000 or 10 000 50 000 25 000 12 000 600 + 200
20 000 or 10 000 40 000 25 000 12 000 600 + 200
20 000 or 10 000 31 500 12 000 6 000 600 + 200
20 000 , 10 000 or 5 000 20 000 12 000 6 000 600 + 200
10 000 or 5 000 16 000 6 000 3 000 600 + 200
10 000, 5 000, 10 000 6 000 3 000 600 + 200
2 500 or 1 500
10 000, 5 000, 5 000 3 000 1 500 600 + 200
2 500 or 1 500

Table 5 Short circuit (pressure relief) test currents (Source: IEC 60099-4)

The arrangement for connection of the test circuit has also been included in IEC. Either the
so-called C-connection or Z-connection should be used in such a manner (as defined in IEC)
which would represent the worst case scenario for a particular design. Refer following Figure 29.

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 With pressure relief device Without pressure relief device
(C-connection) (Z-connection)

Fig. 29 Circuit layout for short circuit testing (source IEC60099-4)

The test duration for the high current and reduced current tests shall be 0.2 seconds, reflecting
the time it takes for a circuit breaker to disconnect a fault. To avoid an explosion of the housing it
must be ensured, in most cases, that the internal arc is transferred (commuted) to the outside of
the housing within the first half-period of the short-circuit current. Since this time is critical, a
certain amplitude for the first peak of the current is defined in the test procedure for the rated
short-circuit current, which must be at least 2.5 times the r.m.s value of the symmetrical
component of the prospective short-circuit current. For the reduced short-circuit currents,
however, there is no asymmetrical requirement on the first peak.

For the low current test, the current shall flow through the arrester for 1 second, or until venting
occurs for arresters fitted with a pressure relief device.

One type of design (including significant volume of gas/liquid) makes use of the internal
overpressure, which is built up due to the internal arc coming from the short-circuit of the
arrester elements. The overpressure is created by heating the enclosed volume of gas or liquid,
which expands, leading to bursting or flipping of a pressure relief device (in this case the tests
are sometimes called “pressure relief tests”). The arrester housing is dimensioned to not break
before the overpressure is relieved.

Another design, usually of a compact polymer type (closed, open or cage design) with no
significant enclosed volume of gas or liquid, does not have any pressure relief device. The
short-circuit performance of this design depends on the arc directly burning through or tearing
the housing without explosively expelling the internal components.

For polymer-housed arresters, the tests shall also demonstrate the ability of the arrester to
self-extinguish any fire caused by the arc.

For the high current tests, the test samples should be the longest mechanical section with the
highest rated voltage of each different design of arrester. It is accepted that approved high
current tests made on the longest housing also covers all shorter insulators of the same design.
For the low current test, however, the test sample may be a mechanical section of any length
with the highest rated voltage used for each different design and chosen length of test sample.

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7.1.10 Partial discharge and RIV tests
A low corona level – both internal and external – is essential for all surge arresters to achieve
during normal operating conditions. As a consequence, manufacturers may put more
significance on checking for corona as a routine test on all produced units rather than as a
single type test.

The IEC partial discharge test is intended to detect internal corona which could lead to problems
in the long term. The arrester unit must be first pre-stressed at significantly higher than its
normal operating voltage in order to create the potential for initiation of discharges. Thereafter
the voltage is reduced to a value somewhat higher than the arrester unit’s continuous operating
voltage, at which the internal partial discharge level is recorded (measured as apparent charge
in pico-coulomb, pC).

Radio interference voltage (RIV) testing, as the name suggests, is aimed primarily at detecting
external corona which can cause interference with communication equipment. In contrast to
internal partial discharge tests, which are performed on individual arrester units, an RIV test
needs to be performed on a complete arrester, fully assembled with all fittings (since the aim is
to detect discharges from sharp edges, bolts, pins etc). After voltage pre-stress, the value of
RIV is measured at different applied voltage levels. RIV instruments measure the voltage drop
(recorded in microvolts) caused by a partial discharge just within a narrow frequency band,
transforming it by a weighting circuit according to the sensitivity of the human ear.

IEC60099-4 permits that RIV testing may be omitted if the same arrester has passed a partial
discharge test; provided both internal and external discharges are recorded.

7.1.11 Environmental tests

The environmental tests are intended to demonstrate by accelerated test procedures that the
sealing mechanism and the exposed metal components, e.g. flanges and terminals, are not
impaired by environmental conditions.

These test requirements consist briefly of the following, with the criteria described in more detail
in the relevant IEC60068-2 documents:

• Temperature cycling test (IEC60068-2-14)

The specimen is exposed to changes of temperature in air by exposure in a chamber to
prescribed temperatures varied at a controlled rate. This test is applicable only to
porcelain-housed arresters, as polymer housed arrester are instead subjected to
temperature cycling as part of the moisture ingress test.

• Sulphur dioxide test (IEC60068-2-42)

This test is intended to provide accelerated means to assess the corrosive effects of
atmospheres polluted with sulphur dioxide. The test is not considered suitable as a
general corrosion test, but may be suitable for giving information on a comparative basis.

• Salt mist test (IEC60068-2-11)

This test is applied to compare the resistance to deterioration from salt mist of
specimens of similar construction. It can be useful for evaluating the quality and the
uniformity of protective coatings.

The arresters shall be considered satisfactory provided there is no visible damage, partial
discharge levels do not exceed specified levels (for polymer arresters without enclosed gas
volume) or no degradation in the sealing has occurred (for arresters with enclosed gas volume
and a separate sealing system).
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7.1.12 Sealing and Moisture ingress test
Sealing breakdown has historically been a major cause of arrester failure, particularly for
distribution arresters.

Sealing tests on arresters with enclosed gas volume and a separate sealing system should be
made using a sensitive method which can detect very low leakage rates (for example,
max 1µW = 1 x 10-6 Pa. m 3/s = 1 x 10-5 mbar.litre/s according to IEC60099-4). Example of test
methods which are commonly used include:
• Helium-mass spectrometer
• Vacuum over water
• Pressure or vacuum decay
• Halogen detection

Provided the method and criteria used during routine testing of seal tightness on individual units
also fulfils the type test criteria, many manufacturers prefer to waive performing a separate type
test on porcelain-housed arresters, as it will not give any additional information.

A moisture ingress test is included in IEC60099-4 which applies to polymer arresters only, and
demonstrates the ability of the arrester to resist ingress of moisture after being subjected to
specified mechanical stresses.

The test includes subjecting the arrester to both thermal as well as mechanical cycling, as
depicted in Figure 30. After the cycling, the arrester is placed in boiling salt water for 42 hours,
and thereafter moisture is given time to possibility penetrate the arrester (Fig. 31). Electrical
measurements are made both before and after the test sequences to verify that the specimen has
not absorbed any moisture. If the electrical characteristic of the arrester has changed during the
tests, the most likely conclusion is that moisture has penetrated inside the housing, which would
imply that the arrester no longer fulfils the original requirements.

Temperature
Temperature

+60 ºC
Boiling water
+45 ºC

24 h 48h 72 h 96 h 50 ºC
Time

-25 ºC
Ambient temperature

-40 ºC Time
Load direction: Load direction: Load direction: Load direction: 0h Water 42 h Time as Cooling Within 8 h:
0º 180º 270º 9 0º
immersion long as verification
test necessary tests

Fig. 30 Thermo -mechanical preconditioning Fig. 31 Water immersion test

(Source IEC60099-4) (Source IEC60099-4)

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7.1.13 Mechanical tests
Surge arresters are normally self-supported, and consequently will be subjected to a bending
moment when mechanical forces are applied. These forces can originate from various sources,
including:
• line connectors
• wind, ice and snow
• seismic accelerations (earthquake)
• arrester’s own weight

These forces will cause a bending moment, which typically has its maximum at the base of the
arrester. The arrester must withstand this moment. In the case of multi-unit arresters, individual
units must also withstand the moment at their length resulting from the applied forces. Bending
moment tests are performed by fixing the housing to the floor and subjecting it to a horizontal
force at the top. The force is then slowly increased until the housing breaks, or in the case of
verification, that the declared value is reached. The test may be performed on complete
arresters or arrester units.

According to IEC60099-4, several Mean value of

breaking load > 120%
sample tests should be performed on (several sample tests)
Damage limit > 100%
porcelain-unit housings to determine the
Guaranteed mean 120%
mean value of breaking load. It is then value of breaking load

possible to assign the housing a Maximum permissible Maximum permissible 100%

100%
maximum permissible dynamic service dynamic service load service load
(MPDSL) (MPSL)
load (MPDSL), i.e. the 100% value in
Figure 32, which can be considered its
maximum withstand moment against
dynamic loads such as short circuit Permissible static
40%
service load
forces, gust winds, earthquake, etc. (PSSL)
This should not be confused with the
breaking limit proven during testing, 0% 0%
which is an average of 20% above this
value. The permissible static service Porcelain housings Polymer housings
load (PSSL), which is the maximum
static (continuous) moment, should Fig. 32 Definition of mechanical loads according to IEC60099-4
be limited to 40% of the MPDSL.

Polymer arresters lack rules for the definition of dynamic and static service loads, which strongly
depend on the arrester design. Nevertheless, a damage limit is defined as the “lowest value of a
force perpendicular to the longitudinal axis of a polymer housed arrester leading to mechanical
failure of the arrester housing”. Similarly, the maximum permissible service load (MPSL) is
defined as the “greatest force perpendicular to the longitudinal axis of a polymer housed arrester,
allowed to be applied during service without causing any mechanical damage to the arrester”.

However, for polymer arresters with enclosed gas volume (i.e. tubular design), an alternative
definition may be found in IEC61462 “Composite Insulators”, which instead gives criteria for
specified mechanical load (SML) and maximum mechanical load (MML).

Of at least equal interest is the performance of polymer arresters under static loading of a cyclic
nature. Due to their construction, polymer arresters may flex under mechanical load and, when
this is repeated cyclically (as would occur over their service lifetime), may be the factor which
determines the true limit of loading which is able to be applied continuously, i.e. declared
permissible static service load (PSSL). However, suitable definitions and testing for this criteria
remain as yet undefined in the Standards.

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7.2 Routine Tests
The ambition with the routine tests is to ensure that the produced arresters meet the design
specification. The routine tests are consequently an integrated part of the quality control during
manufacture.

As a minimum requirement for routine tests, IEC specifies the following to be performed on each
arrester or arrester unit:
• Reference voltage measurement
• Lightning impulse residual voltage test
• Internal partial discharge check
• Leakage check of the sealed housing (for arresters with a sealed housing)
• Current distribution check on multi-column arresters

Minimum requirements for routine tests are similarly specified by IEEE

• Power-frequency test
• Discharge-voltage test
• Ionization voltage test
• Seal test
• Current sharing test

In addition, manufacturers may choose to perform additional quality checks. For example, all
ABB type EXLIM and PEXLIM arresters are subjected to the above tests (as applicable), plus a
measurement of power losses and grading current at Uc (MCOV). All test results have to be
within preset limits to qualify the arresters for delivery.

7.3 Acceptance Tests

An acceptance test, as per IEC vocabulary, means a complete surge arrester should be tested,
i.e. preferably with all individual units connected in series as a fully operational arrester.

Acceptance tests according to IEC incorporate:

• Measurement of power frequency voltage of the complete arrester at reference current
• Lightning impulse residual voltage test at nominal current or lower on the complete
arrester or units
• Internal partial discharge measurement on the complete arrester

Additionally, a Special thermal stability test is given as an option, which has to be specially
agreed upon. This is, in principle, a shortened version of the Operating Duty test, performed on
blocks from the same batch (or similar) as those used in the arresters from the delivery lot.

The number of arresters to be tested is the nearest lower whole number to the cube root of the
number of arresters in the delivery lot.

IEEE calls these conformance tests, and specifies the following:

• Discharge voltage test, on the complete arrester or individual units
• Internal ionization voltage (IIV) and Radio-influence voltage (RIV) on the complete arrester

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7.3.1 Value of acceptance tests
There exists a degree of confusion as to the meaning, and thereby the value, of acceptance
tests on surge arresters. Unlike some other high voltage apparatus, acceptance tests on surge
arresters are not the same as repeated routine tests, particularly in the case of multi-unit
arresters.

That said, the routine tests made after assembly of a single-unit arrester could perhaps be
regarded as an acceptance test, since the routine tests are then performed on a “complete
arrester”. However, to fulfil the requirements of the Standard, an additional lightning residual
impulse voltage test on the unit may be required if this is not performed as routine. It is
permitted, for example, as a routine test to measure residual voltages on individual ZnO blocks
for a specific applied current and then sum up the values to give the total residual voltage for the
unit. Because of the lower voltage required at the block level, this permits testing with high
lightning impulse currents (e.g. 10kA) and good measuring accuracy. Conversely, testing a
complete unit (or complete multi-unit arrester) at higher voltages can present problems
regarding circuit capacity to achieve high lightning impulse currents, as well as potential loss of
accuracy in the measured values compared with performing the test on individual blocks.

For a multi-unit arrester, consisting of several individual units, the units may be regarded as
impedances connected in series, where each individual unit has a specific voltage drop (or
residual voltage) for a specific applied current. Thus, measured values on units when summed
up can be regarded as valid for the complete arrester.

Similarly, in the case of reference voltage measurement at reference current, provided that the
current is high enough to not be affected by stray capacitances during the measurement, then
the summed values on individual units can also be regarded as valid for the complete arrester.

For the internal partial discharge test, provided the pro-rata voltage used during the routine test
on individual units is equal to or higher than the required test voltage during the acceptance
test, then assembling the units together will not influence the result with respect to internal PD
measurement.

In conclusion, acceptance tests need not be considered necessary, provided already performed
routine, batch and sample tests are sufficient to ensure that the acceptance test criteria are
fulfilled. If this is the case, acceptance tests will then not give any additional information about
the surge arrester characteristics than obtained during the other tests, nor add value or security
to the arresters from a delivery lot.

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8. ARRESTER CLASSIFICATION AS PER STANDARDS

According to the IEC standard, surge arresters are classified by their nominal discharge
currents. The test requirements and rated voltages related to different classes are given in
Table 6 below.

Standard nominal discharge current

20 000 A 10 000 A 5 000 A 2 500 A 1 500 A
Rated voltage, U r (kV) 360 < U r < 756 3 < U r < 360 Ur < 132 Ur < 36 Under consideration

Insulation withstand Lightning and If U r > 200 kV Lightning impulse Lightning impulse Lightning impulse
tests on the arrester switching impulse Lightning and and power and power and power
housing voltage test switching impulse frequency voltage frequency voltage frequency voltage
voltage test. test test test
If U r < 200 kV
Lightning impulse
and power
frequency voltage
test
Residual voltage test
a) Steep current X X X X X
impulse residual
voltage test

b) Lightning impulse X X X X X
residual voltage test

c) Switching impulse X X Not required Not required Not required

residual voltage test
Long duration current
impulse test LDC 4 or 5 LDC 1, 2 or 3 75 A, 1 ms 50 A, 0.5 ms Not required
Operating duty test
a) High current impulse Not required LDC 1 With 65 kA With 25 kA With 10 kA
operating duty test with 100 kA

b) Switching surge X LDC 2 and 3 Not required Not required Not required
operating duty test
Power frequency
voltage versus time X X X X X
curve (TOV test)
Pressure relief test
(Short circuit test) X X X Not required Not required
Arrester disconnector
(when fitted) X X X X X
Polluted housing test X X X X X
Internal partial X X X X ---
discharge test
Mechanical tests X X X X X

Table 6 Arrester classification and test requirements as per IEC60099-4

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The classification as per the IEEE standard is shown below (Table 7).

Arrester classification Max. system voltage (kV) Impulse value crest (kA)

Station 800 20
Station 550 15
Station < 550 10
Intermediate All 5
Distribution, Heavy Duty All 10
Distribution, Normal Duty All 5
Distribution, Light Duty All 5

Table 7 Lightning impulse classifying current as per IEEE standard

The switching surge protection level is defined at a current impulse with virtual front time of 30
to 100 µs (IEC) or 45 to 60 µs for time to actual crest (IEEE). The current amplitudes are given
in Table 8 below.

Arrester classification Max. system voltage (kV) Peak current (A)

IEC, 20kA, LDC 4 and 5 -- 500 and 2000

IEC, 10kA, LDC 3 -- 250 and 1000
IEC, 10kA, LDC 1 and 2 -- 125 and 500

IEEE, Station 326 – 900 2000

IEEE, Station 151 – 325 1000
IEEE, Station 3 – 150 500
IEEE, Intermediate 3 – 150 500

Table 8 Current peaks for switching surge protection level.

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9. ARRESTER SELECTION

This section contains only a brief guidance for selection of the most important parameters of
surge arresters used for standard applications such as transformer protection For a more
comprehensive guide, reference is made to IEC 60099-5 and ABB Selection Guides. For
specialized applications, a more detailed system analysis or insulation co-ordination study may
be necessary to permit selection.

The basic selection is carried out in two major steps:

• Matching the electrical characteristics of the arrester to the system’s electrical demands

• Matching the mechanical characteristics of the arrester to the system’s mechanical and
environmental requirements

9.1 Matching the electrical characteristics

The simplified process for selection of the electrical characteristics is depicted in the following
flowchart (Fig. 33).

Fig. 33 Flowchart for simplified electrical selection of surge arresters

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9.1.1 Selection of continuous operating voltage and rated voltage

a) Obtain System Parameters

The maximum highest system voltage (Um ) should be known. But if not, it may be estimated as
5 to 10% higher than the nominal system voltage.

The most commonly occurring TOV is that at a single line-earth fault. The amplitude is given by
multiplying Um /√3 by the earth-fault factor ke, which in turn is determined from the earthing
conditions. The below Figure 34 gives the value for ke based on the system sequence
reactances and resistances for the most unfavourable fault resistance. Should these system
parameters be unknown, ke is usually estimated to be 1.4 for directly earthed systems and 1.73
for resonant earthed or isolated neutral systems.

Fig. 34
Curves showing relationship between
R0 /X1 and X0 /X1 for constant earth fault
factor k e and zero fault resistance
(Source: IEC)

R0 = zero sequence resistance

X0 = zero sequence reactance
X1 = positive sequence reactance

The duration of the applied TOV during earthfault depends on the fault-clearance time. If this is
not known, it may usually be estimated to be in the range of 1 to 3 seconds for directly earthed
HV systems and 3 to 10 seconds for directly earthed distribution systems. For isolated neutral
or resonant earthed systems, the duration is important to determine more specifically, as it may
vary from a few seconds to some hours or even days; depending on whether fault-clearing is
used or not. For an anticipated fault duration over 2 hours, the TOV should generally be
considered (in most cases) as continuous, and the arrester rating chosen accordingly.

Generally, only the TOV during earth-faults and at load rejection are of interest. Certain network
configurations can however give resonance overvoltages. These may also arise during non-
simultaneous operation of breaker poles. Nevertheless, resonance overvoltages should be
avoided by proper system design (especially for normal AC transmission and distribution
systems) and usually should not need to be the basis for selection of the arrester TOV
capability.

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In some cases, efforts are made to reduce the earth-fault current by selectively earthing the
neutrals of only a few transformers, yet maintaining an effectively-earthed system overall. In
such cases, there is a possibility that some parts of the system may become non-effectively
earthed (i.e. increase in value of ke) for certain periods of time when one or more of the earthed-
neutral transformers are out of service. An earth fault during this time may lead to higher TOV
and subsequent arrester failure if this contingency is not taken into account. Since such
occurrences are rare, it may be justified to accept a risk of arrester failure instead of selecting
an arrester with higher TOV capability and thus a higher protective level.

b) Select the Continuous Operating Voltage

In a 3-phase system with arresters connected phase-ground, the actual continuous operating
voltage, Uca, is not higher than Um /√3. If the system does not have any abnormal service
conditions, Uc should therefore be equal to or higher than Um /√3.

Should a considerable amount of harmonics (> 10%) be present in the system, a safety factor of
1.05 (i.e. 5%) is recommended (IEC60099-5) to account for the increase in peak value of Uca.
However, in systems with short automatic fault-clearance times, a safety factor of 1.0 is often
sufficient. Similarly for non-effectively earthed systems.

It should be noted that any arresters with Uc > Uca are generally equally suitable, with regards
solely to continuous operating voltage.

The manufacturer should be consulted if abnormal service conditions exist which are outside of
those specified by the Standards: such as ambient temperature below –40 °C or above +40 °C,
frequencies under 48Hz or above 62Hz, presence of heat sources (e.g. furnaces) near the
arrester, etc. Such abnormal service conditions may lead to the need for selection of higher Uc
and/or rated voltage (Ur), unless the arrester has been designed and verified to withstand the
specified service conditions. All ABB arresters, for example, can withstand wider ranges of
temperature (–50 °C to +45 °C) and frequency (15 Hz to 62 Hz), without the need for special
consideration.

c) Select a sufficiently high Rated Voltage to meet TOV demands

In general, surge arresters are not used to protect equipment against TOV as this would require
an enormous number of parallel columns of blocks. Such applications may be considered only
in cases of limitation or elimination of resonance TOV, and carefully detailed studies are then
required to select arresters with suitable energy capability.

Factors affecting the TOV capability of an arrester are pre-energy absorbed (i.e. the initial
temperature of the blocks) prior to the application of TOV and the applied voltage following the
TOV.

For a given arrester type, the rated voltage (Ur), defined as per IEC, is a measure of its
overvoltage capability. Hence, the additional TOV capability of the arrester can be specified as
a multiple of Ur. A different philosophy adopted by some manufacturers is to give the TOV
capability in multiples of Uc .

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The following procedure is suggested to select an arrester with sufficient TOV capability:
• Select a preliminary rated voltage (Ur 0) based on Uc , with Ur 0 = Uc /0.8
where 0.8 is the “design factor” for ZnO arresters

• Determine the TOV amplitude and duration at earth fault as

TOVe = ke * Um /√3
ke < 1.4 normally for directly earthed systems (effectively earthed)
ke = 1.73 normally for resonant earthed and isolated systems (non-effectively earthed)
For specific cases, determine the actual ke factor.

• Determine other temporary overvoltages TOV1, TOV2 , … TOVn with amplitude and
duration as calculated or estimated.

• Consider the possible energy absorption W (in kJ) prior to the TOV and calculate W/Ur0.
For each TOV, determine the minimum required rated voltages Ure, Ur1, Ur2, … Ur n by
dividing the determined TOV amplitude by the temporary overvoltage strength factor Tr
for a selected type of arrester for the actual duration of the TOV and the calculated
energy absorption W/Ur0. If the calculated specific energy absorption W/Ur0 is higher
than the value given for the first choice of arrester type, then increase Ur0 or select an
arrester type with a higher energy capability.

Thus Ure = TOVe/Tre, Ur1 = TOV1/Tr 1, Ur 2 = TOV2/Tr2, etc

• Select a final rated voltage, Ur, which is the highest of the values Uro, Ure, Ur1, Ur2, etc.
If this is a non-standard rating, choose the next higher rating.

9.1.2 Selection of nominal current

It is often difficult to calculate the arrester current, especially those caused by lightning.
Therefore, rough estimations are mostly used. The relatively small variations in discharge
voltage with current waveshape and amplitude makes this estimation less critical with ZnO
arresters.

Important parameters affecting the selection of the nominal current are:

• the importance of the protected object
• number of lines connected to the station
• the line insulation
• ground flash density in the area
• line performance with respect to backflashes and shielding failures some spans out from
the station

As a general guidance, nominal currents as given in the following Table 9 are proposed.

Maximum system voltage (kV) Nominal current (kA)

800 20
550 20 (or 15 as per IEEE)
245 < Um < 420 10 or 20
36 < Um < 245 10
< 36 5 or 10
Table 9 Recommended selection of nominal currents

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9.1.3 Selection of Energy Capability
The case generally considered to be decisive for energy capability is fast reclosing against a
trapped charge on a transmission line with the arrester installed at the open far end.
Z1

Transmission line with:

Surge
~ 1 p.u
Surge impedance Z
Travel time T arrester
Initial voltage -1 p.u

Fig. 35 Simple single-phase model of energy decisive case

If the surge travel time of the line is short compared with one cycle of power frequency and Z1
presents a low impedance, the current through the arrester will have a rectangular shape with a
duration equal to twice the travel time T of the wave on the line. The current through the
arrester and its residual voltage at this current are given by the intersection of the relevant
switching surge characteristics and the load line, and can be determined by plotting a load
diagram, as depicted in Figure 36
.

Fig. 36
Load Diagram
UL

Ups UL Prospective overvoltage

Z Line surge impedance
Ia Surge arrester current
Ups Surge arrester switching surge
protection level (residual voltage)

In reality, the arrester current does not have a purely rectangular waveform. The source
impedance, Z1, will affect the voltage imposed on the line at breaker closing, the voltage wave
will be distorted during its travel on the line, return waves will cause reflections at the sending
end and, for multi-phase systems, the phases will interact.

However, this simple single-phase model is useful in many cases. To avoid expensive
computer studies, the simplified method can be applied as a first attempt to estimate the
arrester stresses caused by switching. If these calculations reveal higher energies and the
need for more qualified studies than had been considered initially, a more accurate study would
be justified.

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Different types and makes of arresters could also be easily compared when high absolute
accuracy in calculated stresses is not required.

In order to use the simplified method, the parameters in the above figure must be determined in
some way. Typical values for different system voltages are given in the following Table 10
.

System voltage (kV) Surge Impedance, Z (ohm) Prospective overvoltage

without arresters, U L (p.u)

Under 145 450 3.0

145 to 345 400 3.0
362 to 550 350 2.6
765 (800) 300 2.2

Table 10 Proposed system parameters

The base for the per-unit values is the peak value of the highest system voltage phase-to-earth

The prospective overvoltage (UL) depends on a number of parameters such as location of

arresters, type of switching operation, presence or absence of pre-insertion resistors, the
feeding network and the parallel compensation.

The wave propagation time (T) depends on the line length and the velocity of wave propagation.
For aerial lines and GIS bus ducts the propagation velocity (v) is approximately equal to the
velocity of light (0.3 km/µs). For cables, the velocity is much lower (around 0.15 km/µs).

The energy (W), given in J, absorbed by the arrester is given by the equation:

W = [(UL – Ups)/Z] * Ups * 2T * n

where
UL prospective overvoltage or line-charging voltage (kV)
Ups switching surge protection level (residual voltage) of the arrester (kV)
Z surge impedance (ohm)
T wave propagation time (µs) = l/v, where
l = length of line (km)
v = velocity of propagation (km/µs)
n number of consecutive discharges

It can be seen that the energy absorbed also depends on the protection level. Thus, a higher
protection level reduces the demands in kJ/kV.

The energy absorption capability of ZnO arresters has to be proven in the so called line
discharge tests. The energy absorbed by an arrester in a line discharge test is a function of
both the line discharge class and the switching impulse protection level of the arrester.

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For a given arrester, an estimate of the energy absorbed in the relevant line discharge test
could be obtained by using the arrester’s switching surge protective level from the catalogues
and checking for absorbed energy from the Line Discharge Class characteristic curves (e.g. as
per IEC; Fig. 37 below). This value is then compared with the required discharge energy (W)
calculated from the above equation.

The IEC and IEEE line discharge tests 7

CLASS 5
comprise repeated discharges and the

SPECIFIC ENERGY, kJ/kV (Ur)

6
thermal stability of the arrester has to be
verified for two consecutive discharges 5
CLASS 4

with 50 to 60 seconds between them. 4

CLASS 3
For single operations, many arrester
3
types could be stressed with a higher CLASS 2
energy, equal to the single impulse 2
CLASS 1
energy capability. 1

Usually, the design case has a very low 0

1.0 1.4 1.8 2.2 2.6 3.0
probability of occurrence, and it may
therefore be sufficient to design for one Ups/Ur
RELATIVE PROTECTIVE RATIO, U a/Ur
single operation and not for two Fig. 37 IEC Line Discharge Class
consecutive discharges. characteristic curves

If the chosen energy capability is not sufficient, the most economical solution is to increase the
arrester rated voltage. If this leads to an unacceptable protection level, then select another type
with a higher energy capability. For very high demands, parallel ZnO columns and/or arresters
may be needed to meet the energy requirements. In these cases, proper and careful matching
must be undertaken to ensure sufficiently equal current sharing, as full current sharing is not
necessarily assured with standard arresters.

At lower system voltages (below 245 kV), the energy due to switching will generally be low. At
the same time, less attention is often paid to effective grounding and shielding of such systems.
Hence the design capability will be determined by lightning stresses.

A conservative estimate for the arrester energy capability for lightning surges is obtained in the
high current test using a 4/10µs impulse. This wave subjects the arrester to high energy during a
very short time and hence to a thermal shock as well. It is worth noting that discharges of the
amplitudes stipulated in the tests rarely occur in reality, and the real impulse durations seen in
service may be longer than the stipulated test impulse duration.

An arrester with blocks of larger diameter will withstand the lightning stresses better for two
reasons:
• the current density will be lower
• the residual voltage will be lower and consequently also the energy discharged

Hence, it is advantageous to choose an arrester with larger diameter blocks (and consequently
higher discharge capability) for
• areas with high lightning activity
• important installations and apparatus
• lines and stations where grounding or shielding conditions are inadequate

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9.1.4 Check of protection levels (U pl and U ps )
For insulation co-ordination purposes, consider the lightning impulse protection level (Upl) at the
selected nominal current (5, 10, 15 or 20kA according to Table 9). Similarly, the switching
impulse protection level (Ups ) for co-ordination purposes is taken at a current amplitude ranging
from 0.5kA to 2kA, depending on the system voltage. Refer Table 11.

Maximum system voltage (kV) Maximum current (kA)

420 – 800 2
145 – 362 1
< 145 0.5

Table 11 Recommended switching surge co-ordinating currents

9.1.5 Protection margins

Protection margins (in %) calculated at co-coordinating impulse currents, are defined as follows:

• Margin for lightning impulses = ((LIWL/Upl) -1) *100

• Margin for switching impulses = ((SIWL/Ups ) -1) *100

Note! IEEE standards refer to LIWL as BIL and SIWL as BSL

Margins are normally excellent for ABB arresters due to the low Upl and Ups , and also the fact
that most equipment at present has a high LIWL and SIWL. However, depending on the
electrical distance between the arrester and the protected equipment, the margin for lightning
impulses can become reduced, and thus arresters fail to protect equipment that is not in close
vicinity (i.e. within their protection zone).
BIL %
The flexible erection alternatives for polymer
arresters may be of benefit in reducing the
distance effect. Additional line-entrance
arresters may also help.

It is recommended that the protection margins

(after taking into account the ”distance effect”)
should be in the order of 15 - 20% or more to
account for uncertainties and possible reduction
in the withstand values of the protected
equipment with age. Should the selected
arrester type not give the desired protection
margins, the selection should be changed to an
arrester of a higher line discharge class, which Years
automatically leads to a lower protection level.
Fig. 38
Note! It is not recommended to use a lower Insulation withstand with time for paper and oil
than selected rated voltage (Ur) to improve the insulated power transformers. Ageing reduces
margins, as this may lead to an unacceptably insulation withstand of equipment and thus the
low TOV capability. protection margin.

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9.1.6 Consideration of distance effect
One may well wonder why it should be necessary to have a protection margin at all, when it
would seem sufficient that the protection level of the arrester was equal to the insulation
withstand of the equipment (after consideration of possible ageing effects on the insulation).
The reason is that the calculated protection levels and margins are only valid across the arrester
itself, i.e. if the arrester is mounted directly on the protected object. When there are connection
leads and a distance between arrester and object, then the protected object will be subjected to
a higher overvoltage. This is illustrated in the following Figure 39.

Fast-fronted overvoltages spread out along a line in the form of travelling waves. When a
travelling wave reaches a point where the surge impedance changes, reflections and refractions
take place. If the surge impedance is considered infinite – for example a transformer winding or
an open circuit breaker – then a total reflection will occur. The positive instantaneous sum of
the resultant oscillations cause the voltage at the remote end to increase step-wise to as much
as double the value of the initial incoming voltage.

When surge arresters are connected in front of the protected object, complex interactions and
oscillations will take place between the two with their different surge impedances. Via the
travelling wave process, the value of the voltage seen by the protected object can be
considerably higher than at the arrester itself. How much higher depends to a large extent on
the electrical distance between the arrester and the protected object and the front-steepness of
the incoming wave.

v L

a Upl U

h Surge arrester

Fig. 39 Voltage increase due to distance effect (simplified method)

The generally-used formula to estimate the voltage increase due to distance effect is:

U = Upl + (2 * S * L) / v
where
U voltage at the protected object (kV)
Upl lightning impulse protection level of the arrester (kV)
S steepness of the incoming voltage wave (kV/ µs)
L electrical distance between arrester and protected object
including connection leads (a + b) and arrester height (h)
v velocity of wave propagation (m/µs); approximately equal to the velocity
of light 300m/µs, except for c ables for which 150m/µs may be used

Note! The distance effect reduction does not apply to the Ups margin since the front-time
of a switching surge impulse is longer.

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The protection margin will therefore dramatically reduce with increased separation distances, as
well as with increased steepness of the incoming wave. The latter is a function of how close to
the substation the lightning strikes the transmission line and risk for backflash or shielding
penetration. Steepnesses of 1200 kV/µs and 2000 kV/µs have been well established in
Standards and practical insulation co-ordination studies for HV sub-stations, and are often used
as reference surge steepnesses. Nevertheless, the determined strike rate leads to the choice of
actual steepness for a given application.

This simplified method must be used with caution as it is only an approximation. It does not
take into account any capacitance of the protected object, nor inductance effects nor the initial
voltage at the instant of surge. This simple method may not be sufficient in the case of small
margins between the arrester protection level and the object’s LIWL; whereby more complex
computer modeling may then be necessary.

In all cases, the importance of short distances and connection lead lengths cannot be over-
emphasized.

9.1.7 Neutral-ground arresters

In those cases where efforts are made to reduce the local earth-fault currents by not earthing
the neutral of the transformer, each such neutral brought out through a bushing should be
protected against lightning and switching overvoltages by an arrester.

For neutral-ground arresters, the recommended rated voltage is approximately the maximum
system voltage (Um ) divided by √3, assuming a relatively long fault duration. Short or very long
fault durations may warrant selection of a different rated voltage, after taking into account the
specific TOV requirements. In addition, special considerations must be taken for resonant-
earthed systems with long radial lines, as a higher rated voltage may be necessary.

The neutral-ground arresters should preferably be of the same type (Line Discharge Class) as
the phase-ground arresters on the same transformer. The electrical characteristics are then
usually identical to standard catalogue arresters with the corresponding rated voltage. However,
for arresters connected neutral-ground, Uc is usually zero, as they are not subjected to any
continuous voltage stress during normal service conditions. Consequently, demands for
creepage distance and voltage grading do not normally apply to these arresters.

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9.2 Matching the mechanical characteristics
It is equally important that the arresters are mechanically designed so that they can withstand
normal, as well as specified abnormal, service conditions.

9.2.1 Selection of external creepage distance

IEC 60815 defines four levels of pollution (from light to very heavy) and stipulates the required
minimum creepage for porcelain housings as indicated in the following Table 12.

Pollution level Specific creepage ( mm/kV Um )

Light (L) 16
Medium (M) 20
Heavy (H) 25
Very Heavy (V) 31

Table 12 Pollution levels according to IEC600815

For porcelain-housed arresters, select the housing to give the desired creepage - generally the
same as for the other equipment in the same location. If the creepage demand exceeds
31 mm/kV, a special design may be required.

Silicone-housed arresters, being highly hydrophobic, are better suited for extremely polluted
areas than porcelain- or EPDM-housed arresters. One step lower specific creepage may be
justified in many cases for silicone-housings.

9.2.2 Selection of short circuit capability

The arrester’s short-circuit (pressure relief) capability is chosen on the basis of the prospective
symmetrical short circuit in the system at the arrester location or calculated from the formula:

I = Sk / (√3 * Um )
where
I prospective symmetrical short–circuit current (kA)
Sk 3-phase short-circuit power in MVA
at the point where the arrester is to be installed
Um maximum system voltage (kV)

If Sk is not known, the breaking capacity of the associated breaker can be used as a guide for
the short-circuit current.

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9.2.3 Selection of mechanical strength
The cantilever strength (bending moment) of the arrester must be sufficient to withstand
specified mechanical loads. These loads will cause a bending moment, which typically has its
maximum at the base of the arrester - except perhaps in the case of a multi-unit arrester utilizing
different strength porcelains for individual units.

Mechanical loads on surge arresters can be divided into either static or dynamic loads. Static
loads are those which are applied continuously (e.g. weight of line conductors, normal wind,
etc), whereas dynamic loads are often higher in magnitude, but need only be withstood for short
periods (e.g. short-circuit current forces, gust winds, earthquake, etc). Consideration should
also be given to the fact that some loads may act alone or in combination.

The maximum permissible horizontal load for individual forces is calculated as the maximum
moment which the arrester can withstand, divided by the distance between the base of the
arrester and point of the applied force. Loads at the line terminal connections can be
considered to act at the centre of the terminal, whilst wind loads are assumed to act generally
about the arrester’s centre of gravity. For areas with high seismic risk, different specifications
and verification methods exist, and the manufacturer should be consulted to verify the arrester’s
withstand capability.

In the case of multiple loads acting in combination, the horizontal loads from individual forces
should be used to calculate the vector sum of the bending moments acting about the base, to
determine if the arrester housing can withstand them when applied simultaneously.

Loads resulting from tensile and compression forces are not usually of concern, as these are
normally limited for standard applications and arrester housings are also typically strong in
these directions. Torsional loading on the arrester is also considered an abnormal service
condition, but may need closer consideration should it exist.

In general, the continuous total current through an arrester is of the order of only a few
milli-Amps. Hence, connecting the arresters to the line by light, vertical and slack tee-offs, can
considerably reduce the demand for mechanical strength.

Due to their flexible construction, there may be a visible deflection at the line-end of polymer
arresters under mechanical load. This may ultimately determine the limit of loading which is able
to be applied. However, since polymer arresters are light compared to equivalent porcelain-
housed arresters, they permit innovative erection alternatives which could reduce the loading;
for example suspended or under-hung erection or special bracing.

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10. INSTALLATION GUIDELINES

Upon arrival to site, the contents of all packages shall be checked against the respective
packing lists, and shortages identified. During unpacking, a visual inspection shall be made for
any obvious signs of transport damage.

Reference shall be made to the assembly and special instructions provided for details of correct
installation, and these shall always be followed and take precedence.

Since ABB undertakes such extensive routine, batch and sample tests on the
ZnO blocks, surge arresters and counters/monitors (in excess of the requirements of the
applicable standards), additional testing or commissioning checks are not considered
warranted or necessary at installation or before taking EXLIM or PEXLIM arresters into service.

10.1 Conductor dimensioning

Under normal operating voltages the arrester represents a high impedance and hence only
milliamps of current are typically flowing constantly through the connecting conductors. Even
under surge conditions, although the current can be significant (10's of thousands of Amps) it is
only present for a very short time (microseconds). Such currents will have a negligible heating
effect on the conductor. Consequently, the question of conductor size and cross-sectional area
is perhaps not as important for surge arresters as it is for other high voltage apparatus.

The true criteria comes when the arrester has overloaded and the system short-circuit current is
thereafter flowing through the arrester and its conductors. If the cross-section is thermally
insufficient for this condition, the connection may act as a fuse and melt before the protection
has operated to clear the fault. However, this may be able to be accepted, since the arrester
has to be replaced anyway. If this is not acceptable, the cross-sectional area for the conductors
must be based on the system short-circuit current and duration.

For the line conductors, the simple practical solution is often to use the same conductor as for
current-carrying equipment connected to the same line. However, as noted above, this is often
unnecessarily large and may result in undue mechanical loading on the arrester. Lighter
droppers may be preferable (and even recommended) for this reason.

The earth conductor cross section shall be overridingly chosen in accordance with local
regulations and earth fault current requirements.

a) Connections between arrester earth terminal and surge counter

When a surge counter is mounted on an earthed pedestal structure, it is necessary to insulate
the cable/busbar connecting the arrester to the counter, both to avoid parallel current paths and
the risk for flashover during surges. Otherwise the counter will not register as it should.

The required insulation level for this connector is based on foreseen lightning levels. The
voltage drop due to the internal resistance and inductance in the cable itself will be negligible in
the case of lighting impulses, and what is important is the circuit inductance. In the general
case, the lightning surge current generates a magnetic flux in the circuit comprising the
insulated base, the support pedestal and the insulated conductor. The voltage induced is
proportional to the magnetic flux in the closed loop, and is little affected by the size of the
conductor. For this reason, the same insulation level is usually required for all earth connectors,
regardless of their cross sectional area.

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The following general guidelines are recommended:

• The earth conductor between the arrester and counter should be insulated for at least
5 x L kV (LIWL), where L is the conductor length in metres between the arrester earth
terminal and the surge counter terminal. Note that the maximum permissible length L of
the earth conductor between arrester and surge counter is determined by the LIWL of
the insulated base which the arrester is mounted on.

• The LIWL of the insulating base must also withstand this induced voltage; otherwise it
will flashover and the impulse will be earthed through the structure without passing
through the counter.

• Even if the LIWL of the insulated cable and the insulating base are sufficient, this lead
must be kept as short as practicable since its inductance drop adds to the protection
level of the arrester.

b) Connection between surge counter and earth

The conductor between the counter and earth should be the same as other earthing conductors
in the station. The cross-section is generally based on the system short-circuit current and
duration or as per local regulations. However, whether or not this conductor is insulated has no
account with regards to the registration of surges by the counter.

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11. MAINTENANCE AND MONITORING

A surge arrester does not contain any moving parts, which is why there is normally no need to
perform any form of periodical checking or monitoring. In general, a correctly chosen and installed
arrester is regarded as maintenance free during its entire lifetime. A correctly chosen arrester in
this context means that its electrical and mechanical characteristics are matched to actual service
conditions.

Periodical cleaning of porcelain-housed arresters is usually only necessary after periods of heavy
marine or industrial pollution. Surge arresters may be washed under voltage (live-washing),
following the sam e safety regulations as for any other high voltage equipment, plus with the
following additional precautions:

• surge arresters normally employ shorter flashover distance compared to other insulators,
leading to an increased risk for external flashover during the washing

• surge arresters with series connected units must have all units washed simultaneously to
avoid overheating of any unit

Arresters with silicone-housing should, in general, not need to be washed at all. Nevertheless, it
is acknowledged that silicone insulators exposed to heavy pollution for long periods may
become discoloured and appear dirty over time. This is as a result of low molecular-weight
silicone oils diffusing to the surface, ultimately encapsulating the pollution layer and making the
housing appear dirty and difficult to clean. This function permits the housing to ultimately
recover its hydrophobicity, even after a temporary loss; a unique feature amongst insulators. Of
importance is that, unlike other types of insulators, this discolouration does not necessarily
mean that a silicone insulator's in-service pollution performance is affected. In fact, cleaning of
a silicone insulator can actually have the disadvantage of washing away the silicone oils
deposited on the surface, thereby reducing its hydrophobicity for a period.

Should washing be undertaken on a silicone-insulator in any case – to remove large amounts of

solid layer deposits, for example - then only plain water at low to moderate pressure should be
used to prevent damage to the soft housing. If a cleaner housing than can be achieved by live-
washing is desired, then hand-washing with plain water and a soft cloth may be necessary. No
form of detergents, cleaning agents, abrasive cloth or hard brush should be used, unless
approved by the arrester manufacturer. WARNING! The arrester must be de-energized and
out of service before any work requiring handling is undertaken.

Regardless of how dirty the insulator appears, what is of interest is whether or not the surface of
the housing is hydrophobic or not. A class scale exists for measuring the degree of
hydrophobicity, and tests can be undertaken for determining the extent to which the surface of
the arrester has become hydrophilic. Seven classes of the hydrophobicity (HC 1-7) have been
defined, whereby HC 1 corresponds to a completely hydrophobic (water-repellent) surface and
HC 7 to a completely hydrophilic (easily wetted) surface. Typically, silicone-housings exhibit HC
1 – 2 when new. In contrast, a porcelain insulator exhibits HC > 5 when clean and new and HC
7 after a time in service, i.e. completely hydrophilic, without the ability to recover. If desired, this
class scale provides a coarse value of the wetting status and is particularly suitable for a fast
and easy check of insulators in the field. Refer STRI Guide for further details (see Note).
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Note. The information for hydrophobicity classification is, with minor changes, taken from STRI AB,
Guide 1, 92/1 Hydrophobicity Classification Guide. STRI AB is an independent company for development
and testing in the field of electrical power transmission and distribution.

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In areas with extreme pollution, a silicone insulator’s hydrophobicity may become temporarily
reduced from its original level. However, even under such extreme pollution conditions, the
hydrophobicity transfer mechanism of the silicone results in the silicone-housing performing
better than porcelain-housings with equivalent creepage distance and shed profile. Unlike a
porcelain insulator, a silicone-housed insulator is not necessarily at risk for flashover just
because the surface is covered with pollution. Of importance is the extent to which the
hydrophobicity recovers via transfer of low molecular weight silicone oils through the pollution
layer to the surface. This is denoted as Hydrophobicity Transfer (HT) and is the relationship
between the ESDD (equivalent salt deposit density, being the total amount of salts on the
surface) and ASDD (apparent salt deposit density; being the portion of the pollution not covered
by the low molecular weight silicone oils). The difference between the values of ESDD and
ASDD represents the part of the pollution layer that does not conduct any current.
Note! This is not a simple test to perform in the field. However, it can be undertaken on an
individual insulator removed from service as a means to evaluate the pollution performance of
silicone insulators under specific site conditions.

11.1 Condition monitoring

Despite being “maintenance free”, as with all HV apparatus, external factors can place stress
on surge arresters, leading to a risk for their deterioration over time and potential failure. As
businesses strive to remain competitive, unplanned outages are increasingly unacceptable, and
it can therefore be of advantage to regularly check and/or monitor the condition of HV surge
arresters connected to the network, so that they can be taken out of service before the situation
becomes acute.

Periodical external visual inspection can be undertaken to detect obvious evidence of

deterioration which could affect the arrester’s in-service performance, eg. physical damage,
connections, flashover, tracking, erosion, puncture, etc. However, since arresters are delivered
as sealed units from the factory, they cannot be disassembled for any internal inspection or
tests, as doing so would be the same as destroying the arrester.

For system voltages above approximately 100 kV, surge counters are often installed in series with
the arresters. The main reason for the use of surge counters is to check if a particular
transmission line or phase suffers from an exceptionally high number of overvoltages leading to
arrester operation - lightning faults on a line, for example. If this is the case, some preventative
counter-measures may be necessary to limit the number of surges.

A sudden increase in the counting rate may also indicate an internal arrester fault. Conversely,
a steady high counting rate from the beginning may indicate an unsuitable choice of arrester
rating. In either case, the arrester should be investigated further.

If a surge counter is used, the surge arrester must be equipped with an insulating base; thus
ensuring that the discharge current is passing exclusively through the surge counter and not
discharged directly to earth.

However, surge counters tell only part of the story, as they simply register the number of surges
according to their operating characteristic. The user therefore has no way of telling the
magnitude of the surge and if it was significant, nor when it occurred and if it was coincident with
a system event.

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A complete check of an arrester can only be made by measurements under laboratory conditions.
There is no simple way to check an arrester during service, and normally there is no such need
either. If, however, it is decided to perform a check on an arrester, it is desirable that the
measurements can be made without disturbing the normal service, i.e. without disconnecting the
arrester from the phase conductor.

Many measuring methods have been employed over the

years for gapless ZnO arresters, with the simplest method
utilised being the connection of a standard mA-meter in series It
with the arrester to measure leakage current.

The AC leakage current through the arrester can be divided

Ic U Ir
into a capacitive and a resistive part. At continuous
operating voltage (Uc ), a ZnO surge arrester acts as a
capacitor, leading to a predominantly capacitive component
of current and a significantly smaller resistive part.

The specific capacitance of a ZnO varistor results in typical

values of the capacitive current ranging from 0.5 to 3mApeak ,
depending on the varistor diameter. For a complete surge Fig. 40
Principle diagram for a gapless
arrester, the capacitive current is further dependent on stray
ZnO arrester, where U is the
capacitances, pollution currents on the insulator surface,
voltage across the arrester, It is
number of varistor columns in parallel and the actual total leakage current and Ic and Ir
operating voltage. Meanwhile, the resistive component of are the capacitive and resistive
the leakage current of a varistor is at the same time in the components, respectively, of the
range 50 to 250µApeak , and is temperature and voltage leakage current
dependant.

Since the capacitive component of the current dominates so greatly, the total leakage current
measured on a simple mA-meter will be very sensitive to the installation; making interpretation
of the readings difficult. Further, there is no evidence that the capacitive current would change
significantly due to deterioration of the voltage-current characteristic of the surge arrester.
Consequently, measurement of capacitive current cannot reliably indicate the condition of ZnO
arresters. Although increasing values may be of some use in indicating that cleaning of the
insulators is necessary.

Instead, it is generally recognised (IEC60099-5) that the only reliable indicator for the condition
of a gapless arrester which can be assessed during normal service is to measure the resistive
component of the leakage current (or estimate it from the 3rd harmonic), and compare it with the
maximum allowable resistive current, as given by the manufacturer, under prevailing service
conditions i.e. temperature and applied voltage. Ageing of the ZnO varistors will generally cause
a gradual increase of the resistive leakage current with time.

Because of the order of magnitude difference (µA vs. mA), a significant change in the resistive
current would be required before it could be noticed on a milliamp meter. Therefore, special
measuring apparatus are necessary to separate out the two components, and give a reliable
detection method for the analysis of the leakage current through gapless ZnO surge arresters.
Two such devices are ABB Surge Arrester Monitor EXCOUNT-II and TransiNor Leakage
Current Monitor LCM-II.

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11.2 Replacement of gapped surge arresters
Since the statistical chance of a malfunction is greater for very old arresters, these should be
identified and removed from service as soon as possible as the first step in any replacement
program. In general, aged insulation has a lower withstand level from its original capability. This
means that the margin of protection is reduced and the possibility of equipment failure increases
with age. Hence, a replacement program should also identify older equipment, and replace the
arresters protecting the most valuable equipment first.

Further, when systems expand, there may be a need to upgrade the arresters connected to
them; a fact that is often overlooked. The result is heavier-than-designed operating duty and
increased failure risk. Arresters manufactured around 1960 to 1970 may not be provided with
any suitable pressure-relief mechanism for safe operation during internal short circuit. Even
where such mechanism exists, it may not function satisfactorily if the short-circuit capacity of the
line has been increased after the original installation and is now higher than the arrester
capability. Such arresters will fail violently in the event of their malfunction, and may damage the
equipment nearby as well as pose a serious risk of injury to any personnel in the vicinity.

As there are still many gapped silicon-carbide (SiC) surge arresters in service worldwide, it is
worthwhile mentioning what can be done to assess their condition, since aged gapped arresters
can malfunction due to a number of reasons, including:
• sealing failures
• arc erosion
• grading component failures

Monitoring may be undertaken on-line as a first step by scanning the arrester with an infrared
camera to reveal any unusual hot spots.

After the arrester is disconnected from the supply source, additional information can be gained
off-line by the following tests:
• Physically examine the arrester units externally to see if the gaskets have deteriorated or
there is any sign of moisture ingress.
• Megger each unit separately to detect any shorted units. However, when grading
components are present, the readings should not tend to infinity, otherwise a
discontinuity may be suspected in the unit.
• If the grading current of an arrester is known at the time of its manufacture or installation,
this figure can be used to compare with the value after it has been in service for some
time. Considerable deviations from the original recording should motivate further
investigation or replacement of the arrester.
• Perform a spark-over test at power frequency (50 or 60 Hz) and compare the results with
the values obtained during routine tests. If the results are more than +10% from that
given in the data sheet, the unit should be replaced. The sparkover voltage for an
arrester must be measured in a high voltage laboratory to obtain the necessary
sensitivity and control, and thus this test cannot be performed on site.

With consideration to the age and residual life of most gapped arresters, versus the time and
cost to remove them from site, install replacements, perform tests in a HV lab, interpret the
results and then possibly reinstate them in service, many users decide it is better to simply
undertake a replacement program of all installed gapped arresters of a certain age without
further analysis.

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12. SPECIAL APPLICATIONS

With increased focus on system reliability, together with ongoing developments occurring in the
field of overvoltage protection, new and innovative applications are continuing to be found for
the use of surge arresters.

Many of these are, however, quite specialised and require more in depth discussion than is
considered possible within the scope of this Guide. Nevertheless, this section briefly discusses
a number of these topics, and refers the reader to other ABB technical information for further
reading should they be of interest. See References.

12.1 Reduced clearance distances

In order to reduce the risk of insulation failure to an economically and operationally acceptable
level, the insulation withstand of substation equipment is selected with regard to expected
overvoltages, taking into account the protective characteristics of the surge arresters.

The insulation withstand of the surge arrester itself has to be co-ordinated with its own
protective characteristics. The arrester has to be positioned with respect to grounded objects
and surge arresters in adjacent phases, without increasing the total risk for insulation failure.
The insulation withstand properties of surge arresters in a substation can be divided into:
• insulation withstand of the surge arrester itself, including the insulation between flanges
and grading rings, etc.
• insulation withstand between the surge arrester and grounded objects
• insulation withstand between the surge arrester and other equipment connected to the
same phase, e.g. bushings
• insulation withstand between surge arresters in adjacent phases

The insulation withstand should be the only constraint when selecting suitable clearances for
properly dimensioned surge arresters. Any effects which various phase-to-ground and phase-to-
phase clearances may have on the voltage distribution along the ZnO block column should have
already been accounted for in a well-made design.

The insulation withstand of the surge arrester itself should also have been thoroughly
considered at the design stage. Spacings between metal flanges, as well as spacings between
flanges and grading rings, should be designed to be sufficiently large to withstand overvoltages
appearing during current discharges; at least up to the design altitude (and perhaps more).

a) Phase-to-ground clearance
The phase-to-ground clearance in substations is usually based on the selected standard rated
lightning and switching impulse withstand voltages. International Standard IEC 60071-2, for
example, recommends minimum clearances.

In general, the clearance between a grounded object and a surge arrester should be the same
as the phase-to-ground clearance selected for other high voltage equipment in a substation. If it
is not possible to use the normal phase-to-ground clearance in special applications, a smaller
clearance may be chosen, considering the protective characteristics of the arrester, and after
correction for altitude. However, this is generally only possible if there is a fairly big margin
between the standard rated withstand voltage for a substation and the protective level of the
arresters.

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b) Other equipment in the same phase
The clearance between a surge arrester and other high-voltage equipment connected to the
same phase, e.g. bushings or post insulators, is usually not of importance during normal
operating conditions. In polluted conditions, however, the transient voltage distribution on the
insulator surfaces may become extremely uneven. This creates high voltage stresses between
the surge arrester housing and any high-voltage insulator positioned nearby. It is recommended
therefore to choose half the phase-to-ground clearance as the minimum metal-to-metal
clearance between the upper (energized) end of the surge arrester and the top (energized) end
of other high-voltage equipment. Furthermore it is recommended to use the phase-to-ground
clearance also for the spacing between the lower (grounded) end of the surge arrester and the
bottom (grounded) end of other high-voltage equipment.

c) Phase-to-phase clearance
The phase-to-phase clearance for high-voltage equipment in a substation is normally based on
the selected standard rated lightning and switching impulse phase-to-phase withstand voltages.
International Standard IEC 60071-2, for example, recommends minimum phase-to-phase
clearances. Note that the normal selection of surge arrester protective levels does not directly
protect the phase-to-phase insulation.

In general, the clearance between surge arresters in adjacent phases should be the same as
the phase-to-phase clearance selected for other high-voltage equipment in the substation. If it is
not possible to use the normal phase-to-phase clearance in a special application of surge
arresters, the minimum clearance with regard to lightning overvoltages can be derived, and
should include altitude correction.

Similarly, the minimum phase-to-phase clearance for arresters with respect to switching
overvoltages should also, if possible, always be based on the selected standard rated switching
impulse phase-to-phase withstand voltage for the substation. If a special application requires a
minimized phase spacing, a favourable electrode configuration established by the grading rings
may permit a reduction of the phase-to-phase clearance in certain cases.

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Insulation withstand and clearances with EXLIM and
PEXLIM surge arresters”.

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12.2 Station protection
When lightning surges enter a station, reflections occur and oscillations are set up due to the
capacitance of the station apparatus and inductance of busbars and connection leads. For
steep incoming surges, the difference in voltage shape and amplitude at different locations in
the station will be significant. A station should be designed for a low probability of failure, and
thus the protection against lightning surges is not only a question of which arrester to choose,
but even more important, is to determine the number and location of arresters needed in order
to obtain an adequate protection.

Two examples of this application include:

a) Line entrance arrester

If, due to any reason, it is impossible to install an arrester in a sub-station as close to important
equipment as ideally necessary, the protective distance of the station arrester may be improved
by installing an additional arrester at the entrance into the station of the incoming line. This
arrester also fulfils a second function as protection for an open line breaker.

b) Protection of open breaker

In over half the ground flash cases, the first lightning stroke will statistically be followed by one
or more successive strokes. The first stroke may lead to a single-phase or multi-phase ground
fault on a line, causing the relay protection to operate and to open the line breakers. If a rapidly
following successive stroke hits the line, the lightning surge may reach the breaker in open
position before the breaker has fully recovered its dielectric strength across the contacts.
A restrike and possible breaker damage may occur.

The normal arresters in the station cannot protect the breaker against this event, and instead a
separate set of arresters on the line side of the breaker are required. Such additional breaker
arresters give the additional benefit of improving the overall overvoltage protection of the station.

For a full treatment of the problem of station protection, many parameters must be considered
concerning probability-distribution of lightning currents, station layout, power frequency voltage,
grounding, shield wires, protection levels and connection leads of surge arrester, insulation
levels, etc. In view of the complexity of the problem, practical engineering has long been based
mainly on “rules of thumb” or on simplified formulas, which often disregard many of the
important parameters.

The final choice is always left to the system engineer to decide upon an acceptable level of risk;
taking into account additional parameters such as the importance of the station and the cost of a
failure compared with the cost of improved protection.

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Application guidelines for station protection”.

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12.3 Lightning protection of transmission lines
Transmission lines in the lower system voltage range, 72 kV - 245 kV, are often sensitive to
lightning overvoltages for the simple reasons that:
• the insulation withstand is relatively low
• the transmission line often lacks shielding wires
• the footing impedance of the towers is high
• the transmission line lacks a continuous counterpoise (shield earth wire)

Despite this, meshed networks with rapid re-connection of faulted lines for the most part give
satisfactory operational continuity. Short-time disturbances (around 0.5 seconds) must be
tolerated in radial networks, as well as the voltage drop during the fault time (around 0.1
second) occurring in the meshed networks.

There are, however, some types of loads where even brief disturbances can have a severe
impact for the on-going process - e.g. steel mills, paper mills, refineries, etc. The cost for such
an interruption, both in terms of value of lost production and the costs to re-start the production,
are unacceptable. In today’s deregulated energy market, such costs will be more visible to the
network operator than before, since the buyer can set high demands on delivery security.

The traditional methods to reduce the number of faults caused by lightning have been:
• installation of shield wires
• improvement of the earthing impedance of the towers
• increasing the insulation level

Unfortunately, implementing these methods gives only marginal improvements of the delivery
security, especially if the earthing conditions are difficult due to a high earth resistivity.

A better alternative to reduce the number of line faults caused by lightning is to install ZnO
arresters with polymeric insulators in parallel with the line insulators. These transmission line
arresters (TLA) normally consist of standard polymer-housed arresters together with a
disconnecting device and fastening equipment for installation on the line itself or on the tower.

TLA’s give complete protection against lightning flashovers for the actual line insulator.
Insulators in adjacent phases and in other towers, however, are not protected; which is why
TLA’s are mainly installed on all phases on the towers that are intended to be protected. In
reality, TLA’s are seldom installed throughout an entire line length, but instead only in areas
where lightning gives most problems due to exposed position, bad earthing conditions etc.
Modern localisation systems for lightning-storms in combination with traditional fault statistics
are excellent tools to identify towers where TLA’s should be installed to be of most effective use.

The dimensioning of a TLA generally follows the same criteria as for an arrester in a substation.
However, it is of particular importance that the TLA is designed correctly with respect to energy
and TOV capability, since the stresses on the arrester at lightning are highly dependent on the
earthing conditions, presence of shield wires, etc.

More information on the selection of surge arresters for this application is available in the ABB
brochure “PEXLINK: Transmission-line protection for disturbance-free system operation”.

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12.4 Switching surge control in EHV systems
In any complex electromagnetic system, a sudden change in state gives rise to transient
oscillations which, in turn, can cause high overvoltages unless suitably damped. For EHV
systems it has been common practice for many years to equip circuit-breakers with closing
resistors, as a means of controlling such system transient interactions during closing or
re-closing operations. The closing resistors are inserted in series with the load circuit being
switched for a short period of time before closing the main contacts of the breaker – thereby
damping the transient overvoltages. Without any form of control, switching overvoltages during
reclosing of a fault-cleared line could, under certain circumstances, rise as high as 3 – 4 p.u of
the phase-ground peak voltage. Pre-insertion resistors typically function to limit this overvoltage
to in the order of 1.5 – 2.0 p.u.

Optimum overvoltage control requires correct choice of the resistor value in relation to the
source impedance level, the line length and the line parameters. Although a well-proven
technology, pre-insertion resistors can lead to a number of problems in mechanical design and
operation; with adverse impact on overall system reliability. As robust and efficient alternatives,
used either alone or in combination, the microprocessor-based ABB relay type Switchsync and
PEXLINK Transmission Line Arresters (TLA) could be substituted instead.

The "intelligent" Switchsync relay makes it

possible to connect the load to the network Fig. 41
at a predetermined instant, which gives Switchsync and CAT
optimum transient suppression. relays in conjunction
with PEXLINK TLA
PEXLIM silicone-housed surge arresters
(forming part of the PEXLINK concept),
located at line ends and along the line at
selected points, function to limit switching
surge overvoltages and thus line insulation
requirements. To locate arresters along the
line has previously not been a practical
solution due to the fact that only porcelain-
housed arresters with high discharge
energy capability have been available. Now
with lightweight polymer-housed arresters
available for use even on EHV systems, a
very efficient overvoltage control along long
transmission lines is possible.

Different line and switching configurations

lend themselves to one or more stand alone
solution, or a combination. Switchsync
and/or PEXLINK will, in most cases, provide
a cost effective, more reliable and efficient
method of controlling line switching
transients than pre-insertion resistors.

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Application guidelines for transmission line switching
overvoltage control” and ABB brochure “PEXLINK: Transmission-line protection for
disturbance-free system operation”.

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12.5 Shunt capacitor banks
Shunt capacitor banks are used to an increasing extent at all voltage levels. Three-phase
capacitor bank sizes vary from a few tenths of MVAr to several hundreds of MVAr, with both
ungrounded wye and grounded wye banks in use.

It is common practice today to use ”restrike-free” breakers. However, since many banks are
switched on a daily basis, the probability of obtaining high transients associated with capacitor
switching increases. Furthermore, the standardized procedure to verify that the breaker is
restrike-free includes only a limited number of tests. The use of arresters in this application not
only gives protection if a restrike does occur, but also decreases the probability of multiple
restrikes since the trapped charge on the capacitors is reduced.

Generally speaking, capacitor protection by surge arresters has been a difficult task before ZnO
arresters became available. The high discharge currents and possible energies associated with
an arrester operation at a capacitor bank heavily stressed the spark gaps in a SiC gapped
arrester. The possible high energies could also result in overstressed SiC blocks. Once a
sparkover occurred, the arrester which sparked-over had to discharge the whole energy stored
in the capacitor bank and also carry a power-frequency follow current before a resealing at the
next voltage zero was possible.

With the introduction of ZnO surge arresters, it is possible to meet any energy demand by
simply paralleling the necessary number of blocks, even if the procedure to ensure current
sharing is quite sophisticated.

Many capacitor banks are operated without surge arresters. However, there are a variety of
beneficial reasons to install arresters:

• To prevent capacitor failures at a breaker restrike or failure

• To limit the risk of repeated breaker restrikes
• To prolong the service life of the capacitors by limiting high overvoltages
• To serve as an ”insurance” against unforeseen resonance conditions which otherwise
would lead to capacitor failures
• For overall limitation of transients related to capacitor bank switching which can be
transferred further in the system and cause disturbances in sensitive equipment
• For upgrading of capacitors by preventing high overvoltages and/or for increasing the
service voltage
• To serve as protection against lightning for capacitor banks and filters connected to lines

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Guidelines for selection of surge arresters for shunt
capacitor banks”.

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12.6 Series capacitors
Series capacitors have been applied for more than 50 years on EHV transmission lines in order
to increase the possible power transfer and improve the transient and steady state stability of
the power transmission system. The ever-growing need for electrical power, high costs and
difficulties to obtain ”right-of-way” for new lines, together with the availability of ZnO varistors as
highly effective overvoltage protection, have resulted in a boom for series compensation in the
last decade.

In addition to the old, but still valid, arguments for series compensation, the possibilities to use
adjustable capacitors for load-flow control and balanced loading between parallel lines make
series compensation even more interesting for the future.

An extremely vital component for the series compensation scheme is its overvoltage protection.
Historically, it comprised a single spark gap (for moderate demands on capacitor reinsertion
speed) or a dual spark gap protective scheme (for faster reinsertion or other tougher
requirements). With the availability of ZnO varistors, the protective schemes have been further
improved by using the varistors in parallel with the spark gaps, and ultimately even without the
spark gaps. This has led to simple and robust protection with ultra-fast re-insertion speeds, low
re-insertion transients and low protection levels.

Modern all-film capacitors have low losses, but their overvoltage withstand capability is less
than that for the old type of paper-film capacitors. This leads to requirements of low protection
levels to obtain an economical capacitor design. Low protection levels, however, may be difficult
to achieve with spark gaps alone, since reinsertion transients can give unwanted gap
operations. With ZnO varistors, this problem is easily solved and, in addition, the reinsertion of
the capacitor will be instantaneous as soon as the voltage across the capacitor decreases
below the conduction ”knee-point” of the ZnO varistor. The spark gap is used as overload
protection for the varistor and is also usually used to quickly by-pass the capacitor/varistor for
internal faults in order to limit the necessary design energy capability for the varistor. For higher
protection levels, it may be necessary to use two gaps in series. With further improvements in
varistor energy capability and faster by-pass breakers, it is possible in most cases to dispense
completely with the spark gaps.

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Overvoltage protection of series capacitors”.

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12.7 HVDC arresters
Surge arresters are applied in many different locations within a HVDC scheme, where the
normal service voltage waveforms differ widely - from pure power frequency and DC voltages to
mixed wave-shapes with commutation overshoots.

The introduction of ZnO-technology had a great impact on the insulation coordination for HVDC-
converter stations. With gapless ZnO arresters, it has been possible to reduce drastically the
protective levels, especially as the coordinating cases originate from internal faults and/or
switching events. These result in rather low discharge currents (some kA) compared to the
usually considered lightning currents (tens of kA) for general AC applications.

ABB pioneered the world’s first gapless ZnO DC arrester, with a DC-line arrester installed in the
Skagerrak HVDC transmission between Norway and Denmark, and has subsequently built on
this success to gain extensive unique experience in this extremely specialized field.

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Overvoltage protection of HVDC-Converter stations”.

12.8 Current sharing considerations

To meet very high energy requirements, parallel columns of ZnO blocks have to be used in
surge arresters, and/or several arresters in parallel, so as to share the current and thus the
energy. Typical high energy applications are protection of series capacitors and arresters used
in HVDC schemes, with as many as 400 parallel columns of high energy varistors having been
commissioned. However, even more traditional applications sometimes warrant the use of
parallel columns, where the energy demands are beyond the capability of a single column
arrester.

With ZnO arresters, the energy capability can be increased to meet any possible energy
requirements by simply adding sufficiently many parallel columns; provided that no series or
parallel spark gaps are used. To make full use of this benefit of ZnO arresters, however, it is
necessary to ensure a good current, and thereby energy, sharing between the parallel columns.
The columns can be mounted all in the same housing or in separate housings, depending on
the necessary number of block columns. For special cases it can also be necessary to ensure
that several different arresters share the energy in order to avoid overloading of the arresters.
Such matched arresters have to be specially requested, since standard arresters may not
necessarily achieve full current sharing.

Whenever multi-columns are supplied, additional routine testing is required to verify adequate
current and energy sharing between each column.

More information on the selection of surge arresters for this application is available in the ABB
Technical Information document “Current sharing considerations”.

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13. REFERENCES

[1] International Standards and Reports

IEC 60060-1 High-voltage test techniques. Part 1: General definitions and test requirements
IEC 60068-2-11 Environmental testing - Part 2: Tests. Test Ka: Salt mist
IEC 60068-2-14 Environmental testing - Part 2: Tests. Test N: Change of temperature
IEC 60068-2-42 Environmental testing - Part 2: Tests. Test Kc: Sulphur dioxide test for contacts
and connections
IEC 60071-1 Insulation co-ordination - Part 1: Definitions, principles and rules
IEC 60071-2 Insulation co-ordination - Part 2: Application guide
IEC 60099-1 Surge arresters - Part 1: Non-linear resistor type gapped surge arresters for
a.c. systems
IEC 60099-4 Surge arresters - Part 4: Metal-oxide surge arresters without gaps for
a.c. systems
IEC 60099-5 Surge arresters - Part 5: Selection and application recommendations
IEC 60815 Guide for the selection of insulators in respect of polluted conditions
IEC 61166 High-voltage alternating current circuit-breakers - Guide for seismic qualification
of high-voltage alternating current circuit-breakers
IEC 61462 Composite insulators - Hollow insulators for use in outdoor and indoor electrical
equipment - Definitions, test methods, acceptance criteria and design
recommendations

[2] American National Standards

IEEE C62.11 IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV)
IEEE C62.22 IEEE Guide for the Application of Metal-Oxide Surge Arresters for
Alternating-Current Systems
IEEE 693 Recommended Practices for Seismic Design of Substations

[3] Metal Oxide Surge Arresters in AC Systems, CIGRE Technical Brochure No. 66 by
Working Group 06 of Study Committee 33

[4] ABB Technical Information and brochures

2200en High Voltage Surge Arresters Buyer’s Guide
2300E Selection Guide for ABB HV Surge Arresters
2310E Application guidelines for station protection
2311E Application Guidelines for Transmission Line Switching Overvoltage Control
2312E Guidelines for selection of surge arresters for shunt capacitor banks
2350en Physical properties of zinc oxide varistors
2353en Voltage grading of EXLIM and PEXLIM surge arresters
2354E Insulation withstand and clearances with EXLIM and PEXLIM surge arresters
2364E Current sharing considerations
2380E Overvoltage protection of HVDC-Converter stations
2381E Overvoltage protection of series capacitors
9100en Silicone rubber in outdoor insulators
1HSA 954321-10en PEXLINK

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[5] STRI AB, Guide 1, 92/1, Hydrophobicity Classification Guide

[6] A.R. Hileman, Insulation Coordination for Power Systems, Marcel Dekker, Inc. 1999

[7] M. Mobedjina, L. Stenström, “Improved Transmission Line Performance using Polymer-

housed Surge Arresters”, presented at CEPSI Seminar, Manila, October 23-27, 2000

[8] M. Mobedjina, L. Stenström, “Limitation of Switching Overvoltages by use of Transmission

Line Surge Arresters”, CIGRE SC-33 International Conference, Zagreb, 1998, Technical
Paper P.30

[9] L. Stenström, J. Lundquist, ”Selection, Dimensioning and Testing of Line Surge Arresters”,
presented at the CIGRÉ International Workshop on Line Surge Arresters and Lightning,
Rio de Janeiro, Brazil, April 24 -26, 1996

[10] L. Stenström, J. Lundquist, ”Energy Stress on Transmission Line Arresters Considering

the Total Lightning Charge Distribution”, presented at the IEEE/PES Transmission and
Distribution Conference and Exposition, Los Angeles, September 15-20, 1996

[11] M. Mobedjina, B. Johnnerfelt, L. Stenström, “Design and Testing of Polymer-housed

Surge Arresters”, presented at GCC CIGRE 9th Symposium Abu Dhabi, October 28-29,
1998

[12] L. Stenström, J. Lundquist. ”New Polymer-housed ZnO Arrester for High Energy
Applications”. CIGRÉ 1994 Session August 28 to September 3, Technical Paper 33-202

[13] S. Vitet, L. Stenström, J. Lundquist. ”Thermal Stress on ZnO Surge Arresters in Polluted
Conditions – Part I: Laboratory test methods”, presented IEEE, PES 1991 T&D
Conference and Exposition, Dallas, Texas September 22-27, 1991

[14] S. Vitet, A. Schei, L. Stenström, J. Lundquist. ”Thermal Behaviour of ZnO Surge Arresters
in Polluted Conditions – Part II: Field test results”. presented IEEE, PES 1991 T&D
Conference and Exposition, Dallas, Texas September 22-27, 1991

[15] S. Vitet, M. Louis, A. Schei, L. Stenström, J. Lundquist. ”Thermal Behaviour of ZnO Surge
Arresters in Polluted Conditions”. CIGRÉ 1994 Session August 30 to September 5,
Technical Paper 33-208

[16] J. Lundquist, L. Stenström, A. Schei, B. Hansen, ”New method for measurement of the
resistive leakage currents of metal-oxide surge arresters in service”, presented at IEEE
SM, Long Beach, California, July 9-14, 1989

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 Product Guide 2004 edition

ABB
Publication SWG/AK 97-50en

ABB Power Technologies AB

High Voltage Products
Surge Arresters

SE-771 80 LUDVIKA, Sweden

Tel. +46 (0)240 78 20 00
Fax. +46 (0)240 179 83
E-mail: arresters.div@se.abb.com
Internet: http://www.abb.com/arrestersonline
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ABB HV Surge Arrester Product Guide

Edition 2004en Page 96 of 96
ABB