Types of Surge Absorber:

The travelling waves set up on the transmission lines by the surges may reach the terminals
apparatus and cause damage to it. The amount of damage caused not only depends upon the
amplitude of the surge but also upon the steepness of its wave front. The steeper the wave front of
the surge, the more the damage caused to the equipment. In order to reduce the steepness of the
wave front of a surge, we generally use Different types of Types of Surge Absorber.

A surge absorber is a protec ve device which reduces the steepness of wave front of a surge by ab-
sorbing surge energy.

Although both surge diverter and surge absorber eliminate the surge, the manner in which it is done
is different in the two devices. The surge diverter diverts the surge to earth but the surge absorber
absorbs the surge energy.

Different Types of Surge Absorber are

1. Condenser or Capacitor Surge Absorber

2. Inductor and Resistance Surge Absorber

3. Ferran Surge Absorber

1. Condenser or Capacitor Surge Absorber:

A condenser connected between the line and earth can act as a surge absorber. Fig. 24.14 shows
how a capacitor acts as surge absorber to protect the transformer winding. Since the reactance of a
condenser is inversely propor onal to frequency, it Will be low at high frequency and high at low
frequency. Since the surges are of high frequency, the capacitor acts as a short circuit and passes
them directly to earth. However, for power frequency, the reactance of the capacitor is very high and
prac cally no current flows to the ground.

2. Inductor and Resistance Surge Absorber:

Another Types of Surge Absorber consists of a parallel combina on of choke and resistance
connected in series with the line as shown in Fig. 24.15. The choke offers high reactance to surge
frequencies (XL=2πfL). The surges are, therefore, forced to flow through the resistance R where they
are dissipated.
3. Ferran Surge Absorber:

Fig 24.16 shows the Types of Surge Absorber. It is called Ferran surge absorber. It consists of an air
cored inductor connected in series with the line. The inductor is surrounded by but insulated from an
earthed metallic sheet called dissipator. This arrangement is equivalent to a transformer with short-
circuited secondary. The inductor forms the primary whereas the dissipator forms the short-circuited
secondary. The energy of the surge is used up in the form of heat generated in the dissipator due to
transformer ac on. This type of surge absorber is mainly used for the protec on of transformers.

Fig. 24.17 (i) shows the schema c diagram of 66 kV Ferran surge absorber while Fig. 24.17 (ii)
shows its equivalent circuit.
Types of Surge Arresters
Learn about the most common types of surge arresters used to protect against
transient overvoltages and lightning strikes.

TECHNICAL ARTICLEDec 18, 2020 by Lorenzo Mari

Surge arresters introduce shunting resistance to the ground when a surge appears,
absorbing energy from the surge without the voltage becoming excessive. They then
extinguish the power follow current after dissipating the surge. The most common
arrester types in power systems are silicon carbide (SiC) and zinc oxide (ZnO). This
article describes these arrester types in more detail.

Characteristics of Different Surge Arrester Types

The first surge arresters provided lightning protection utilizing an air gap connected
between the line and the ground. Their main drawback was the requirement of a
series linear resistance and a fuse to break the power follow current. Additionally,
when the gap sparks over, it creates a fault in the circuit – and an unpleasant outage
when cleared by a circuit breaker.

A device able to limit the voltage without producing a power outage is more
appealing.

After several generations of surge arresters, the introduction of valve-type silicon-

carbide arresters in 1954 was a significant technological advance. The valve element
(or valve block) consisted of a non-linear resistor – commonly silicon carbide (SiC) –
whose value decreases abruptly as the voltage rises. The name valve block comes
from the valving action to the flow of the current.

Silicon carbide arresters allowed for a reduction to the basic lightning impulse
insulation level (BIL) of substation equipment, high fault current withstand, and
smaller size, with significant economic savings.

Introduced around 1976, modern metal-oxide arresters – typically zinc oxide (ZnO) –
do not need gaps and exhibit better handling characteristics for switching surges,
reduced current under steady-state conditions, and reduced lead lengths.
Although silicon carbide arresters provided good service for many years, the better
performance and improved power system availability make metal-oxide devices a
better choice.

There are arresters with different voltage and power levels to best suit the protected
equipment’s needs.

Silicon Carbide (SiC) Valve-Type Surge Arresters

SiC valve-type surge arresters employ a non-linear valve element (resistor) made of
silicon carbide and inorganic binders. Silicon carbide is a compound of silicon and
carbon.

Some arrester applications require that the valve element have a low resistance value
during steady-state conditions to deal with particular surge and power system
characteristics, creating excessive power losses. Valve-type surge arresters have spark
gaps in series with the valve elements to manage this difficulty.

Series spark gaps keep the valve element isolated under steady-state conditions, in
order to reduce losses, and they introduce the valve element when a surge emerges
from the gap’s sparkover. There is no leakage current flow between the line and
earth, allowing the valve design to deal with its voltage-limiting role and energy
dissipation capacity only under surge conditions.

The total voltage across the arrester is the gaps’ sparkover level plus the voltage
across the valve element. The lower the total voltage, the better the protection level.

SiC arresters also contain current limiting gaps to limit the system follow current.
These gaps reduce the energy absorbed during operation, allowing for fewer valve
elements, shorter arrester length, and reduced voltage levels. The arrester gaps
exhibit drawbacks, like producing transients during the sparkover to engage the
valve elements.

Another crucial matter is the arc-quenching ability of the arrester. Arrester design
provides creative ways for quenching the arcs created in the gaps, protecting the
valve element against the continuous flow of current – the follow current – after the
surge is rerouted and steady-state conditions resume.

Figure 1 shows a volt-ampere characteristic for a gapped silicon-carbide arrester.

 Figure 1. V-I characteristic of a gapped silicon-carbide surge arrester. Image courtesy of Industrial-
electronics.

Figure 2 shows a diagram of a typical 6kV silicon-carbide surge arrester with its
components: main gap units, magnetic coil, valve elements, bypass gap, and
shunting resistors.
 Figure 2. Schematic diagram of a gapped silicon-carbide surge arrester. Image courtesy of General
Electric.

The pre-ionizing tips help to initiate the gap’s breakdown when an overvoltage
develops. The bypass gap short-circuits the magnetic coil during the surge current
transit, placing the surge voltage across the valve element, which presents low
resistance at high voltage, and the surge current goes to the ground. The magnetic
coil helps to quench the arcs into the main gaps after the surge current passes. The
shunting resistors regulate the power frequency voltage across the main gap
elements.

Figure 3 shows silicon carbide surge arresters for various voltages.

 Figure 3. Silicon-carbide surge arresters. Image courtesy of General Electric.

Metal-Oxide Surge Arresters (MOSA)

A metal-oxide surge arrester contains non-linear metal–oxide resistive disc elements
with excellent thermal energy withstand capabilities. Each disc includes powdered
zinc oxide material mixed with other metal oxides. This type of surge arrester works
like a high-speed electronic switch – opened at steady-state voltages and closed at
overvoltages.

Zinc oxide surge arresters are highly non-linear – their non-linear characteristic is
much more pronounced than that of silicon carbide – and have low losses under
steady-state conditions.

There are three types of metal-oxide arresters:

1. Gapless
2. Series-gapped
3. Shunt-gapped
As with silicon-carbide surge arresters, the first metal-oxide arresters had a gap in series with non-
linear resistors. At that me, the resistors’ thermal duty was rela vely small and they could not
withstand the thermal energy of the leakage current under steady-state condi ons, requiring the
gap. Gapless arresters appeared around 1980, and their resistors tolerate the constant small leakage
current.

Zinc oxide arresters are easy to manufacture, have low cost, and absorb or dissipate large amounts of
energy. Nowadays, most arresters employed in new systems or revamps are gapless zinc oxide
devices.

Figure 4 shows a gapless zinc oxide surge arrester’s cutaway, containing a single column of ZnO
blocks.

Figure 4. Parts of a porcelain-housed gapless zinc-oxide surge arrester. Image courtesy of ABB.

1 Porcelain insulator 6 Sealing cover

2 Ven ng duct 7 Sealing ring

3 Spring 8 Indica on plates

4 Desiccant bag 9 ZnO-blocks

5 Copper sheet 10 Flange cover

Figure 5 shows a high voltage zinc oxide surge arrester for areas with very high lightning intensity.
Note the external grading rings that long arresters regularly require to maintain constant voltage
stress along their length.

Figure 5. Zinc-oxide surge arrester. Image courtesy of ABB.

Surge Arrester Classifica on and Applica on

Based on voltage ra ng, protec ve characteris cs, and durability in pressure-relief or fault-withstand
characteris cs, the classifica on of surge arresters used in power systems is as follows:

 Sta on arresters: Provide the best protec ve levels – lower discharge voltages, higher energy
absorp on, and more significant pressure relief. Typical applica ons are large substa ons
and sites with strong surges.

 Intermediate arresters: Have inferior protec ve characteris cs and energy discharge

capability. Typical applica ons are small substa ons, underground cable protec on, and dry-
type transformers.

 Distribu on arresters: Provide the lowest protec ve levels and energy discharge capability.
They are used in medium voltage networks.

Insula on Coordina on
The system and equipment insula on’s voltage withstand ability depends on the surge’s rise me. In
this instance, insula on capability is a func on of me.

A surge arrester’s protec ve characteris cs are also a func on of me; hence, the need for
coordina ng the insula on and arrester volt- me characteris cs to get adequate protec on – the
insula on coordina on procedure.

Insula on coordina on compares the system or equipment insula on’s impulse withstand ability
with the voltage across the arrester for the selected discharge current, in accordance with the
preferred protec on level. The choice of insula on levels and coordina on prac ces affects costs
considerably. A drop of one level in BIL can reduce major electrical equipment costs by thousands of
dollars.

As an example, Figure 6 shows the en re V-I withstand curve for an oil-filled power transformer and
the protec ve characteris cs of a surge arrester – front-of-wave sparkover and discharge voltage.

Figure 6. Oil-filled transformer insula on withstand and arrester protec ve characteris cs. Image
courtesy of Cooper.

The arrester’s sparkover crest voltage should be below the transformer’s chopped wave withstand. It
is safer to compare the arrester’s sparkover with the transformer’s front-of-wave test when the la er
is available.

Another comparison is the arrester’s discharge voltage and the 1.2/50 µs impulse sparkover with the
transformer’s full-wave test (BIL).
A Review of Surge Arrester Types and Characteris cs

The first surge protec ve devices were the rod gaps. Rod gaps are cheap but have several
disadvantages: they may not protect for fast fronts, produce steep surges during sparkover, and
generate a fault on every opera on – they do not reseal.

Silicon-carbide valve-type surge arresters employ a silicon carbide non-linear valve element and
series spark gaps. The spark gaps keep the valve element isolated under steady-state condi ons –
reducing losses – and ac vates it when a surge emerges, but they create transients during the
sparkover.

Zinc-oxide arresters were introduced around 1976. Zinc oxide is a subs tute for silicon carbide. ZnO
arresters have a more pronounced non-linear characteris c than SiC and can be used without series
gaps due to their small current at nominal voltage. Yet, they are extremely effec ve at limi ng surge
voltages.

Most arresters employed today in new systems or revamps are gapless zinc-oxide devices.

There are three classes of power system surge arresters: sta on-, intermediate-, and distribu on-
class. Sta on arresters provide the best protec ve levels but are more expensive.

Insula on coordina on is essen al. This coordina on compares the system or equipment insula on’s
impulse withstand ability with the voltage across the arrester while surge current is being discharged.