Surge Arrester
Surge arrester is a protective device that limits voltage on equipment by discharging
or bypassing surge current which caused by lightning or power surge.
Standards for surge arresters are defined by IEEE standard C62.11 and IEC standard
60099-4.
How does a surge arrester work?
A metal oxide varistor (MOV) surge arrester contains a series of metal oxide varistor
blocks. These MOV blocks are like a voltage-controlled switch, which acts as an
insulator with line voltage. Once the voltage the arrester experiences rises above the
reference voltage of the arrester, the MOV blocks go into conduction. Since the
MOV blocks are highly non-linear, once the voltage drops below the reference
voltage, the conduction ends.
Types of surge arresters
The first surge protective 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 operation – they do not reseal.
Silicon-carbide valve-type surge arresters employ a silicon carbide non-linear valve
element and series spark gaps
Zinc oxide is a substitute for silicon carbide. ZnO arresters have a more pronounced
non-linear characteristic than SiC and can be used without series gaps due to their
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small current at nominal voltage. Yet, they are extremely effective at limiting surge
voltages.
There are three classes of power system surge arresters: secondary, Distribution,
intermediate and Station class arresters.
Secondary arresters
Secondary arresters are arresters rated under 1000V.
Secondary arresters are used to protect against secondary
surges.
Distribution arresters
Distribution arresters are 1kV to 36kV rated.
Within the distribution class, there are light duty,
normal duty and heavy duty arresters. Normal duty
arresters are used in low lightning applications, heavy
duty arresters are used in high lightning applications.
Intermediate arresters
Intermediate arresters offer better discharge voltages, have a
high fault current withstand capability and are available in
ratings from 3kV to 120 kV.
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Station class arresters
Station class arresters offer the best discharge
voltages of all arresters, provide high energy
handling capabilities, have the highest fault current
withstand capability and are available in ratings
from 3 to 684 kV. Station class arresters have
varying cantilever strengths for the most demanding
applications.
Characteristics of Surge Arrester
The electrical characteristics of this include the following.
§ The voltage across this arrester where the current flow is interrupted after
spark over, this is known as resealing voltage.
§ It has the highest power frequency which ranges 50 Hz/60 Hz
§ Maximum constant operating voltage
§ Rated short circuit current
§ Nominal discharge current and the values are 5 kA, 10 kA & 20 kA.
§ These are connected among the life conductors as well as the earth.
§ While installing the arresters with above 52 kV, surge arresters may be
supplied by discharge operation counters.
