Re: Flash Testing of High vltage switch Gears
Posted by Bert Hickman on 5/11/2008, 1:24 pm, in reply to "Flash Testing of High vltage switch Gears"
--Previous Message-- |
: Dear Sirs,
: I am a mechanical engineer and interested to learn about electical issues
: and in particular High voltage testing.
: When our new high voltage 11KV switch gear was tested they told me they
: first tested to 28KV at their factory and when installed it can not be
: tested to the same volatege as the first test would have stressed the
: switch gear. It can only be tested to 22Kv 80% of the first test and any
: further tests will be reduced to 80% of previous tests.
: What is this stress, is it because when the Cu busbrs are subjected to
: 28KV the charge it is subjected to impose such a electical field on the
: insulators so the covelant bonds in the insualtaion mateiral minily
: polimers get damaged of the conductor material Cu when subjectd to
: excessive electircal field with no current passing enegrise the electorsn
: and attoms so they reposition themsleves and get stressd and get brittle.
: Thank you
When the switchgear was initially tested at the factory, it was not connected to anything else, so it could be tested to to its particular overvoltage specification (apparently 100% overvoltage). However, when installed, the overvoltage limitations of other equipment connected to the switchgear was apparently less than than that of the switchgear itself, hence the 80% limitation.
When testing interconnected equipment, it is the "weakest link" in the entire system that will limit the maximum voltage. High voltage systems are designed with significant safety margins that allow lightning or switching-induced transient overvoltages to be safely handled without damaging the equipment. High voltage power designers intentional add "weak links" (such as surge arrestors, spark gaps, etc.) that safely break down at lower voltages than more expensive equipment (transformers, circuit breakers). This technique (called "insulation coordination" is is used extensively in power system design to prevent expensive equipment from being overvolted, since weaker links will break down first. From your description, this sounds like the case with your switchgear and associated equipment. Google "insulation coordination" for more information. Here's some information that may help:
As long as your substation bus bar insulators stay clean and physically undamaged, they will withstand the same high overvoltage transients, independent of past transient history. However, over time, the cumulative effects of dust buildup, salt spray, fog, and corona can eventually degrade ceramic, glass and polymer insulators through the formation of partially conductive paths. Ozone and acids can contribute to chemical and physical breakdown of insulator material and corrosion of the associated metal hardware. Google the terms "dry banding" or "insulation tracking" for more information. However, I think these are different phenomena than what you have described above.