Doble Testing on Dry Type Transformers 1

[hidalgoe]

Hello:

We recently specified Doble testing (tip-up) on a 750/1000kVA dry-type transformer (20+ years old) as part of a comprehensive test regimen. Our NETA certified test company said that they would do it, but would not be able to get any useful information because the transformer is a dry-type.

Any comments, folks?

Thank you,

hidalgoe

 

[oldfieldguy]

The Doble literature lists historic data for many different manufacturers and models of dry transformer for comparison purposes. Evaluation of a single test of your transformer can possibly be made by comparing to this data if your testing company has access to the data and knows how to read it.

In the absence of this data, interpretation of a single test is more difficult because dry transformers employ several different construction methods giving varying dielectric constants within the transformer. What is an acceptable reading for one make and model is not acceptable for another.

A power factor tip-up test might be useful in detecting voltage-related issues. Also, testing a "good" transformer today will provide data for comparison with future tests for better evaluation.

Some transformer users do not bother to test transformers in this size range.

old field guy

 

[ScottyUK]

Partial discharge monitoring will tell you far more about the condition of your transformer than a Doble test. PD is usually present at some level from new but, as with oil sampling, a trend is more useful than a spot value.


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If we learn from our mistakes I'm getting a great education!

 

[CanadianKid]

This is an often asked question in the electrical maintenance field. As a provider of test equipment (we are a competitor to Doble) and with 30 years of testing experience, "Doble" (PF Insulation) does not give consistent results on dry type transformers. I have seen insulation PF of 5% on a new dry type transformer right out of the drying oven and yet the transformer passed every test conducted on it. If it is specified, it typically because no history exists for the transformer and this is a common test on HV transformers.

One of the problems with the dry type transformer is the surface of the transformer (typically an expoxy or resin) becomes partially conductive which skews readings all over the place.

The most consistent test I have seen for dry type transformers is Polarisation index where we look for readings above 2 (10 minute result/1 minute result) The test voltage should be below the operating voltage of the transformer (L-G)

 

[ANDY197]

Doble power factor is not the test for the dry type transformer does not proof anything.

 

[Zogzog]

All good comments so far, just wanted to add that a PF test is optional per NETA MTS, and that is for the reasons OFG stated.