Ikrima Alfi, S.T., M.Eng.

• The main task of an arrester is to protect the equipment from the

effects of over-voltages.
• During normal operation, it should have no negative effect on the
power system.
• Moreover, the arrester must be able to withstand typical surges
without incurring any damage.
• Non-linear resistors with the following properties fulfil these
requirements:
 Low resistance during surges so that overvoltages are limited
 High resistance during normal operation in order to avoid
negative effects on the power system and
 Sufficient energy absorption capability for stable operation
 MO (Metal Oxide) arresters are used in medium-, high-and extra-
high-voltage power systems
• Switching Impulse
• Lightning Impulse
• FOW (Fast Front of Wave)
• Power Frequency
• Typically defined as:
 Voltage wavefront 250/2500 micro sec (IEC)
 Current wavefront 36/90 micro sec
• Caused by:
 Circuit breaker operation
 Protective Switching
 Equipment Energisation
Load Rejections
 Convertor switching transients
• Definition
 Voltage wavefront: 1.2/50 micro sec (IEC60071)
 Current wavefront: 8/20 micro sec (IEC600990-1)
• Origins
 Direct lightning stroke to line - rare on shielded systems
 Back flashover - lightning strikes pylon sheild wires
• Definition
 Current wave front: 1/2 micro sec
• Origin
 Bushing flashovers within valve hall, causing discharge
stray capacitances through relatively short lengths of
conductor
• Lightning Impulses
 Switchyard shielding limits direct strikes <20kA
 Back flashover on first tower outside switchyard
 Filter bus flashover may result in 8/20 micro seconds
wave fronts of between 30 and 100kA within filter
• Switching Impulses
 Load Rejection / Re-energisation 1kA<Ic<3kA
• Lightning Impulses
 Not significant in Valve Hall ; Shielding by Convertor
Transformer
 An arbitrary 1kA nominal LIPL is however considered for
determining arrester characteristics
• Switching Impulses
 Generally in the range 1kA to 3kA
 Result from
 AC system switching events transferred through convertor
transformer
 Convertor commutation overshoots during TOV conditions
• FOW Impulses
 Typically of the order of 1kA
• The starting point of the insulation co-ordination process within
the valve hall is the rating of the valve surge arrester (V) by
balancing continuous energy absorption against protective level.
• Once the valve arrester Switching Impulse Protective Level (SIPL)
has been determined, all other switching impulse protective
levels throughout the system may be established.
• Lightning Impulse Protective Levels (LIPL) may then be
established by applying a typical manufacturer ratio of LIPL to
SIPL
Protect critical piece of equipment against transient overvoltage
 Convertor transformer
 Thyristor valves
 Reactor
 Filter components
 Circuit breakers
AC arresters are designed to
• Limit overvoltages on the AC system arising from convertor load
rejections
• Protect AC filter components against overvoltages during filter
switching
• Protect all AC system side components (particularly wound
components) against lightning strikes on overhead lines and
capacitor back flashovers
DC arresters are designed to Protect the DC convertor equipment
from switching voltage transients arising from
 Convertor blocks
 TOV conditions
 Switching events transferred from AC system
 Bushing flashovers within the DC area
On DC side Over Voltages can be reduced by-
• Shielding the converter station and Transmission lines
• Suitable design of the converter control equipment.
• Selecting system parameters to try and avoid resonance under
fault conditions.
On the AC side….
• Voltage support equipment is essential to provide voltage
control during transient and dynamic system disturbances.
• Compensating equipments requires detailed studies to
determine the relationship between DC system recovery and
voltage support equipment and response time
• AC System consists of parallel connected circuits and
apart from some special cases the requirement is to
establish the insulation level bet. phase to earth and phase
to phase level.
• HVDC converter stations on the other hand consist of series
connected bridges, each bridge requiring a different
insulation strength to earth and within each bridge the
electric strength is different for the various components.
• Arresters on AC side are • On DC side rated voltage is
usually specified by their not defined and continuous
rated voltage and maximum operating voltage is defined
continuous operating voltage differently.
• Rated Voltage for Arresters
For DC Application are
Specified As:
 PCOV (Peak Continuous
Operating Voltage)
 CCOV (Crest Continuous
Operating Voltage)
 ECOV (Equivalent
Continuous Operating
Voltage
• On AC side > Peak value of voltage between phase
conductor & earth or between phase conductors having highest
system voltage peak.
• On DC side> PCOV (Peak Continuous Operating Voltage)
There are basically six types of surge arresters, which are commonly denominated
by the letters “A” through “F
• Type A: AC bus arresters which are located close to termination of incoming AC
lines and close to transformers to give protection against lightning surges.
• Type B: Valve arresters to protect the thyristor valves from excessive
overvoltages. The protective level shall be as low as possible since the costs of
the valves are roughly direct proportional to the insulation level across the
valves.
• Type C: Converter unit arrester for protection against overvoltages at the
converter DC bus
• Type D: DC bus or DC line arrester to protect the DC switchyard equipment
connected to the DC pole.
• Type E: Neutral bus arrester to protect the neutral bus and the equipment
connected to it. Neutral bus arresters may be subjected to very large energy
discharges in case of ground faults.
• Type F: AC and DC filter arrester to protect the AC and DC filter reactors and
capacitors
• Insulation withstand is the capability of a given item of
equipment to withstand applied transient overvoltages
(switching, lightning and fast front) and power frequency
overvoltages.
• In terms of transient overvoltage withstand the key parameters
are:
 BIL: Basic Insulation Level: The proven ability of an item of
equipment to withstand the lightning impulse voltage
transient (typically defined as 1.2/50µs wavefront).
 BSL: Basic Switching Level: The proven ability of an item of
equipment to withstand the switching impulse voltage
transient (typically defined as a 250/2500µs wavefront).
• Housing types of MO arresters :
o porcelain housing
o Polymer housing
• Puncak (crest) gelombang, E (kV), yaitu • Polaritas, yaitu polaritas dari gelombang,
amplitude maksimum dari gelombang. positif atau negative.
• Muka gelombang, t1 (mikrodetik), yaitu waktu • Suatu gelombang berjalan (surja) dinyatakan
dari permulaan sampai puncak. sebagai: E, t1x t2
• Dalam praktek ini diambil dari 10 % E sampai • Jadi suatu gelombang dengan polaritas
90% E, lihat gambar b. positif, puncak 1000 kV, muka 1,2
• Ekor gelombang, yaitu bagian di belakang mikrodetik dan panjang 50 mikrodetik
puncak. dinyatakan sebagai: + 1000, 1,2 x 50
• Panjang gelombang, t2 (mikrodetik), yaitu
waktu dari permulaan sampai titik 50% E
pada ekor
• Untuk bentuk gelombang surja standard IEC 1.2/50 µs
koefisien a = 1,426 x 104 /s, dan b = 4,877 x 106 /s
• Continuous AC - Normal operation
Dry
 clean conditions Rain
 contamination Snow & Ice

 other Bird
Fires
• Temporary overvoltage
Vegetation
 Abnormal system conditions
• Switching overvoltage
 System switching operations (lines, loads and equipment)
• Lightning overvoltage
 Lightning flashes to and around lines
Ambient stresses can be very different in the different regions of the
world.
• Very cold climates with ice and snow load have to be considered as
well as climates with high temperature and high relative humidity.
• Mechanical stresses like seismic loads influence strongly the structure
and materials used for the design of the MO arresters.
• Vibrations as well as static loads have to be considered and
appropriate test procedures have been developed accordingly.
• Observations of biological growth on the surface of polymer
insulation have been made worldwide.
• Three types of organic growth have been identified: Algae, Fungi
and Lichen. Despite all the reports of biological growth on the
insulation in some areas of the world there are no known failures of
MO arresters caused by it
Primary objective is:
• Establish maximum steady state, temporary and transient
overvoltages on equipment
• Select
 insulation strength and characteristics of equipment and
protective devices

To ensure a safe, economic and reliable installation

Exemple of minimum Insulation margins (based on IEC 60071-5):
 VALVES
• 20 % for steep front impulses
• 15 % for lightning impulses
• 15 % for switching impulses
 DC EQUIPMENT
• 20 % for lightning impulses
• 15 % for switching impulses
 Converter transformer, valve side
• 20 % for lightning impulses
• 15 % for switching impulses
 AC equipment, line side
• 25% for lightning impulses
• 20 % for switching impulses
• Continuous operating voltage
• Climate (ambient temp., rain, sunshine)
• Mechanical stresses
• Temporary overvoltages, TOV
• Transient overvoltages
 Protection function
 Energy- and current strength
 Outer insulation
• High outer pollution
• Short circuit proof
• Current amplitude
• Duration
• Time between current pulses
• The number of pulses before cooling
• Total energy
AC
• Much higher current through arresters (coordinating current) for
lightning than for switching overvoltages. That leads to higher
BIL than SIL
• The same maximum operating voltage in both ends of a
transmission line
DC
• About the same current through arresters (coordinating current)
for lightning as for switching overvoltages. That leads to about
the same BIL as SIL
• Lower maximum operating voltage in the receiving end of a
transmission line