Substation Transformer Voltage Sag

Well 8 milliseconds is almost exactly one half cycle of a 60 Hz signal. Unless you really did mean 8 mile seconds? Then with the electricity traveling at a nominal 65% of the speed of light in a vacuum that puts it at about 121,000 miles/second.... Nah that's not making sense. You must mean that you see a power dip for one half of a power cycle. Frankly, I can think of some power distribution arrangements that this would be normal. I can also think of ones that this would be a sign of instability. It all depends on your configuration and your method of testing. Sorry but I don't have enough information to go on.

Or maybe I've just confused myself again.

Hi redfred,

not mile seconds, but it is milliseconds. Thanks for the correction. However the exact sister unit feeding the same load doesn't exhibit the voltage sag problem during the tap change transition.

any thought?

Isn't this dip normal for this type tap changer.

Talk to tap changer manufacture.

I agree with Carl Pugh, check with the LTC manufacturer.

However, it could be a problem with the contacts of the tap changer. How old is it? How many operations does it show on the op counter? If the contacts are very burned, there could be arcing taking place, or poor contact making/breaking. This could cause voltage excursions. Maybe a tap changer internal inspection and service is required. Contact a reputable, experienced HV testing & maintenance company for help.

Mechanical contacts rarely move in the single Millisec range. 10 msec or above is good. Now, improvements are made with make before break arrangement.

BUT, wherever real power is in play, contacts normally come in pairs. One makes and breaks, suffers from arcing. And for all this abuse, its contact characteristics are not great. The other of the pair is not abused. It only makes contact after the first, to make a low Ohm safe connection.

IF, you cannot measure precise enough to distinguish, and your abused first contact is flaky or already failed, you described your situation quite correctly.

The slight power sag or delay may have been caused by a contact bounce or some sort of dirt caught in between the contact points?

Hi there

Check the Tapchanger contacts...maybe they are burnt

Cheers