SF6 Breaker Voltage Transient Failures 2

[ghostbuster7]

We recently turned back on an indoor 27.6 kv. SF6 breaker feeding a transformer with no connected secondary load.At the instant this breaker closed(connected on the same 27.6 kv.bus) an indoor nearby 27.6 kv. loaded dry type transformer flashed over ( high voltage winding terminal to ground).

Doing some research, it would appear this is a common problem with using SF6 breakers and dry high voltage transformers.One group is recommending installing snubbers ,claiming they will attenuate this high frequency voltage transient better then the installed lightning arrestors.

The transformer now needs to be repaired.We do not want to repeat the same type of electrical failure again.Do we need to install snubbers everytime when there are SF6 breakers and high voltage dry type transformers?

 

[PHovnanian]

Snubbers will do a better job of attenuating ferroresonant conditions than relying on lightning arrestors. The margin between the normal system voltage and the LA's operating point is too great and its possible that some overvoltage condition that lands within this margin can damage the transformer and still not be caught by the arrestor.

As to whether you'll have to install them every time: That depends on your ability to do an analysis of each system to evaluate the probability of such a condition occurring. If you feel that this is within your capability, then you might be able to rule out some snubber installations.

 

[prc]

Problem faced by ghostbuster7 is known to industry from 70's and suitable solutions are offered by breaker manufacturers/system planners. Problem is more with vaccum breakers.A snubber ( Capacitor and resistor in series to line and earth) is the usual solution. You can get more details from IEEE draft standard,
PC57.145/D5.2 ,Jan 2009, " Guide to describe the occurrrence and mitigation of switching transients induced by transformer,breaker and system interaction" and also breaker amnufacturers like ABB,S&C Electric.