For purposes of practical arrester application, switching
surge voltages can be classiÞed according to required arrester reseal voltage
capability, which depends upon the number and severity of possible surge
currents discharged by the arrester and the duration of ensuing temporary
overvoltages.
- Transients Where Several Sparkovers Are Permitted
These transients are usually initiated by inherent switch
action and stress the surge arrester through multiple operations. The
transients are repetitive in nature and may be of sufÞcient magnitude to
produce sparkover many times.
The severe duty on the arrester results from the repeated
follow current rather than from the energy in the surge. Heating of the gap and
valve block assemblies can be excessive and hence cause arrester failure.
Satisfactory performance of the surge arrester under these conditions is dominated
by the capability of the switching device.
Arrester problems are best avoided through the use of
switching devices that do not prestrike, restrike, or chop excessively.
Alternatively, surges may be controlled by performing switching by disconnects
only on de-energized systems. Some arresters may perform satisfactorily, but
performance is difficult to predict.
-Transients Where Only One or Two Sparkovers Are
Permitted
Operation of circuit breakers or load switches may be
accompanied by rapidly decaying switching surges. Transients of this type
generally stress the surge arrester through high energy dissipation
requirements.
This duty can result from discharge of high-magnitude
switching surges or multiple discharges of high or intermediate-level surges.
The stress on the arresters is severe because, in addition to discharge of the
system surge, the arrester must reseal against subsequent temporary
overvoltages or normal power-frequency voltage.
In some severe applications, the surge arrester must have
the ability to reseal above arrester rating
Satisfactory performance of the arrester in these conditions
is typically dominated by system conditions; that is, reclosing surges are
generally higher than energizing surges, and energy dissipation from long lines
is greater than from short lines.
In addition, surge suppression by power circuit breaker
preinsertion resistors is commonly used at EHV and generally not at HV.
Switching of a transmission line and a transformer may produce more severe
temporary overvoltages following the initial surge than switching without a
transformer connected.
Restriking of circuit
breakers on capacitor switching or line dropping will generally produce severe
duty on surge arresters.
-Transients Where Temporary Overvoltages Approach Reseal
Capability of the Surge Arrester
Temporary overvoltages that are sustained for more than a
few cycles are an important consideration in surge arrester application. These
overvoltages should not exceed the arrester’s ability to reseal so that
multiple operations of the arrester will not occur if the arrester is sparked
over by an impulse or spontaneous sparkover of the arrester.
Failure to reseal would result in multiple operations with
power-follow current and failure of the arrester. Successful performance of the
arresters is based on assurance that the temporary overvoltages will be less
than the reseal capability of the surge arrester.
Overvoltages can be controlled or influenced by system
grounding, system configuration, as well as generator excitation controls in
the case of load rejection. Resonance or ferroresonance conditions should be
prevented by avoiding system conditions that produce the overvoltage.
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