Transformer Shorted Turns - instant failure or detectable by TTR? 3

[electricpete]

I was under the impression that when a shorted turn occurs on a 2-winding transformer, the fault will progress to failure (ground-fault or phase-to-phase fault resulting in transforme trip) very quickly. If that is true, then it seems there is not much usefulness for doing TTR test as a routine predictive test, is there? (if the transformer was operating normally without trips at the time we took it out of service, then it is very unlikely we will find a shorted turn during testing, right?).

The theory I have heard mentioned: a shorted turn (on either primary or secondary) acts like it's own teriary winding with a shorted output. There is present an induced voltage of volts-per-turn times number of shorted turns. The impedance in the shorted-turn loop is just the resistance of the wire and resistance of the short-circuit contact. (virtually no leakage inductance since this is mutually-linked flux). So unless the short-circuit is a high-resistance type, there will be very high current and rapid failure.

Any opinion? Any comments on the theory? Any experience in finding shorted turns on otherwise healthy transformer during predictive test?

Thanks in advance.

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[DougMSOE]

electricpete,
Almost an instant failure i.e. a few cycles at most.
The only time that I will require a TTR test is when a transformer has been moved or jared in any way. I'll require a TTR prior to its move and after it has been set into place.

 

[PWR]

Your inquiry came on the day myself and another service engineer asked each other during some transformer maintenance testing had either of us ever "caught" something during such a test. The answer was no.

The TTR test is very appropriate (on all taps) during acceptance testing. Unless you have moved the transformer or done something else that might cause a problem, there is little to be gained in repeating the test again.

In my 20 plus years in the testing business, I've not only not found it useful during maintenance testing, I have been repeatedly surprised at how obviously failed units can often "pass" the test.

 

[stevenal]

Inconsistent ratios from phase to phase could mean mechanical problems in a tap changer.

 

[Borti]

We recently tested a delta-delta transformer to find the reason for the voltage unbalance (about 4%). We did a TTR test and got perfect results. Then, when performing a winding resistance test, we found one primary winding "missing". TTR test would not show too much on a delta-delta transformer.

 

[Deansharafi]

TTR plus other tests can confirm internal problems, but I agree that it should be done as acceptance test and if we suspect something is wrong with the transformer. However not finding a problem in 20 years also does not guarantee that there won't be a problem. There might be inter-turn shorts that can not be picked up unless you carry out a thorough testing of TTR.