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.

Related post

No comments: