Megger Testing on Distribution Transformers

[Dante Heater]

In Megger testing distribution transformers, how much importance is there in including the LV/HV bushings in the test. If you disconnect and have high readings independent of the bushing, and then put the bushings back into the circuit and the readings fall dramatically, what is that saying about the bushings? Thanks...

 

[Bernie G]

In Megger testing distribution transformers, how much importance is there in including the LV/HV bushings in the test. If you disconnect and have high readings independent of the bushing, and then put the bushings back into the circuit and the readings fall dramatically, what is that saying about the bushings? Thanks...

There are at least 4 bushings. You should connect one at a time and see if any individual bushing causes readings to fall dramatically.

 

[Windadct]

I am a little confused by your question - are you referring to the PF Tap on a HV Bushing (not really a distribution transformer) - or a basic bushing like on a pole mount or pad type transformer I am assuming the basic bushing - say 15KV and below?

I do not recall ever disconnecting the bushings unless we had a bad reading for the whole unit - so you are opening the transformer and disconnecting the bushings internally? - or remioving the busing form the outside, pulling out the leads, and disconnecting? IMO- the whole act of opening these transformers and working in them presents far more risk than the benefit of testing individual components.

Then what do you mean by High readings ( high leakage =bad or High M Ohms = Good).

 

[Bernie G]

"I am a little confused by your question"

Me too. Dante, is your question hypothetical of did you actually disconnect the bushings from the transformer?

 

[Dante Heater]

Actually measured the transformer with the bushings cut out (yes we were inside the 75 kVa pole mount transformer). The entire unit measured less than 500 Mega ohms on the Megger handheld. When we opened unit and disconnected the bushings we were in the 1.5 Giga ohm reading range. It is obvious that the bushings play a role, but by independantly replacing each of the three bushings, the numbers stayed below 500 Mega ohms until the third bushing was replaced. At that time the unit jumped up to 150 Giga ohms. In measuring the bushings independently to ground/tank they read 200 Mega ohms. We believe that there must have been some issue in the bushing. But what it tells us that a Megger measurement doesn't only take into account the coil/windings for potential life, but also anything else in the circuit. Due to the parallel hookups, it seems that if any of the touch points are less than 500 Mega ohms, the unit will not meet the standard.

 

[Windadct]

The way you posted in #1 - is not the procedure in #5. Yes - the point of testing the whole thing is any weak segment will be "seen" by the megger - you then disconnect individual elements - to identify the problem. The megger is is (IMO) relatively poor in determining potential life - but will help to identify trans that need further investigation. If for example you disconnected each element and found one of the windings was the weak link -- the transformer could be processed, but they are often not worth the effort.