Bushings can fail, and they can often fail violently--sometimes violently enough to take the transformer with it. Mike Horning explains how power factor testing can detect problems before that damage is done.
There's really no other practical way to test a bushing other than a power factor test. Most bushings rated 15 KvA or higher offer access to the insulation for testing purposes.
If you find a problem? Get it out of service, or replaced, before it fails.
Transcription:
Why power factor your bushings, most bushings rated 15 KV and higher have what's called a test tap or maybe a potential tap. And they're there for access to the insulation internally in that bushing. Insulation layers can fail. They're capacitor graded bushings, and there's a series of capacitors in there. They can fail layer to layer. You could have some other problems that have bushings such as moisture contamination from a leak. And, there's really no other way to tell if you have a bushing problem other than power factor. You don't oil test the bushing like you do the transformer. You could, but it's not practical. You have to de-energize it to do it and it's just something that's not a general practice that's done. You might find a problem with infrared. If you find a bushing that's a little bit warmer than its mate that's there.
So temperature differentials may indicate some internal problem, like a capacitive layer that is a shorted out, which would increase in terms of internal temperature a little bit. But really other than power factor, you don't have a lot to go with. So a bushing, if it's in trouble and it fails, a lot of times, it will fail pretty violently. It could possibly take the transformer with it - fail the transformer. Even if it doesn't fail the transformer, you'll leave debris in the tank. It is very hard to clean out. We may think we have it clean and everything may test good, but a piece of debris may get in an area that in six months to a year or later could cause that transformer to fail. So it's really important to test the bushings. Know when you have a bushing problem, get it out of service or get it replaced before it does fail. And power factor testing will certainly help us do that.
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