Hipot testing of transformers 2

[Baboso]

Does repeated hipot testing of a transformer during the production process significantly degrade the isulation of the magnet wire?

There are times when it would seem logical to test a unit following a particular step during the prodution process to verify that there is no shorting to other windings or to the core/case before proceeding to the next step in production. Typically this test is done 2 to 3 times but it can be as many as 4 or 5 times for a particular unit. Some have argued that hipot testing is a destructive test and should be done as little as possible.

I would greatly appreciate any knowledge or wisdom on this subject.

 

[uWaveFDL]

http://www.ce-mag.com/archive/01/Spring/Ahmadi.html

This article from Compliance Engineering may help.

 

[buzzp]

I read that article and it is informative and should answer the question. Good link.

 

[electricpete]

I honestly don't know the answer.

But maybe there is some insight to be gained by comparing the accepted approach for field testing of motors and transformers:

Hi-pot testing of large electrical machines (motors and generators) in the field is performed by quite a few folks (recommended by EPRI, performed by Bureau of Reclamation, and lots of others). Test voltage is reduced to approx 60-80% of factory value.

Hi-pot testing of large transformers in the field is never done (to the best of my knowledge).

I don't know the exact reason for this difference. It is certainly plausible that the reason we don't hipot transformers in the field is out of concern for insulation degradation. On the other hand, if you have been repeating it in the factory and haven't seen failures during your final testing, that would seem to be a pretty good argument that the hi-pot isn't hurting.

Lots of info on hi-pot testing at the following link:

http://www.usbr.gov/power/data/fist/fist3~1/3~1_cont.htm

 

[busbar]

Although the OP sounds like the application may concern lower-voltage dry-types, a link [related to the usbr/fist info] is: usace.army.mil/inet/usace-docs/armytm/tm5-686 describes potential danger with DC overpotential testing of liquid-filled transformers. There is an interesting description of voltage division in DC testing. It sounds like there may be problems with unequal (hence catastrophic) distribution of electric charge between oil and paper. Relying on online boilerplate for procedures on testing of a $100,000 component can be misguided and costly.

NETA specs list 0.5-5kVDC hipot for power transformers as a minimum, but give no further details. Paul Gill’s text [Electrical Power Equipment Maintenance and Testing] warns of non-reversible polarization of carbon bonds in mineral oil as a reason to limit test voltage to 5kV, regardless of the voltage class of the DUT. There is always (fine-print) caution in instructions for high-voltage testing of apparatus; that all of the required ‘spare parts’ be immediately available if unanticipated breakdown should occur.

 

[electricpete]

busbar - I vote you a star for a good answer to my question.