Megging Transformer

[augie47]

An E/C asked me today about a problem with a transformer. A 480/208Y120 transformer tripped the primary breaker.
With the primary leads disconnected the E/C checked it with a standard ohmmeter and found no problem,. He then checked it with a megger and stated he got a low reading (he could not give me the exact reading but it was less than 10k to ground).
The transformer was installed in an area with presence of some metallic dust but the E/C could located no sign of carbon trail at the connections.
I voted internal transformer short. Is there another possibility ?

 

[jim dungar]

I voted internal transformer short. Is there another possibility ?

It is very hard to have a transformer coil short to ground. It is much easier to have a turn-turn or a winding to winding short.

I vote for an improperly chosen primary protective device. Many "new" energy efficient 75kVA transformers have some of the highest inrush currents you have ever seen.

 

[templdl]

While as a sales and applications engineer for a dry type power transformer manufacture this question has come up often, that is what is an acceptable megger reading. A megger reading is not part of a factory NEMA/ANSI/IEEE factory test so I asked of testing department to do a megger of one of the transformers and the results were essentially infinite. As such I would strongly be suspect with a megger reading of less than 10K .
As always a lot of information has been left out:

When did the transformer trip the breaker? While energized and supplying a load? Or upon energization?
Did the breaker trip instantaneously (magnetically) or thermally? Breakers that trip magnetically can often be reset and closed immediately after they trip where a breaker that trips thermally has to be left to cool off before it can be reset and closed.

Has the transformer been left setting unenergized for a while in a humid atmosphere before it was energized?

 

[zog]

An E/C asked me today about a problem with a transformer. A 480/208Y120 transformer tripped the primary breaker.
With the primary leads disconnected the E/C checked it with a standard ohmmeter and found no problem,. He then checked it with a megger and stated he got a low reading (he could not give me the exact reading but it was less than 10k to ground).
The transformer was installed in an area with presence of some metallic dust but the E/C could located no sign of carbon trail at the connections.
I voted internal transformer short. Is there another possibility ?

Get a real testing company out there to look at it. It sure does not sound like this guy knows what he is doing.

They should do:
TTR
Insulation Resistance
DAR/PI
Winding resistance
Several more tests if it is >500kVA

 

[zog]

While as a sales and applications engineer for a dry type power transformer manufacture this question has come up often, that is what is an acceptable megger reading. A megger reading is not part of a factory NEMA/ANSI/IEEE factory test so I asked of testing department to do a megger of one of the transformers and the results were essentially infinite. As such I would strongly be suspect with a megger reading of less than 10K .

ANSI/NETA does have a spec for dry type transformers, for <600V rated a minimum of 1000VDC is applied and the minimum insulation resistance acceptable is 500M (tempature corrected to 20C)

Did the breaker trip instantaneously (magnetically) or thermally? Breakers that trip magnetically can often be reset and closed immediately after they trip .

That would be an OSHA violation

1910.334(b)(2)"Reclosing circuits after protective device operation." After a circuit is deenergized by a circuit protective device, the circuit protective device, the circuit may not be manually reenergized until it has been determined that the equipment and circuit can be safely energized. The repetitive manual reclosing of circuit breakers or reenergizing circuits through replaced fuses is prohibited.

Note: When it can be determined from the design of the circuit and the overcurrent devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.