Power Transformer Testing (Help!!!!)

[hidalgoe]

Hello:

We need to test a group of transformer ranging in size from 3750kVA to 20mVA. We are in a budgetery crunch at the present time (what else is new...?); of all the tests: doble, insulation and winding resistance, turns-ratio, etc, which ones do I need to do at an absolute bare minimum.

Thank you,

Gene Hidalgo, PE

 

[DougMSOE]

I would and have recommended the Doble test for the insulation test for phase to phase, phase to ground, high side to low, etc. The TTR test is almost manditory if a transformer is moved i.e. before move and after the move.
Please remember that a shorted turn will be found very quickly (read ka boom).
Don't forget to test the oil DGA recommended with oil acidity.

 

[busbar]

Are you buyer, seller, testing firm or inspection agency? It may seem trivial at this point, but a prime aspect of the process is likely spelled out in the contractual/procurement documents.

 

[hidalgoe]

I work for a Federal Government Agency and Engineering Branch is directing our Maintenance Contractor to give us a proposal.

Gene Hidalgo, PE

 

[DanDel]

Since the transformers have to be shut down for any testing, usually megger, TTR, and Doble testing should be performed. The time for each test is minimal compared to the time taken to power down and disconnect the transformers for testing.

 

[Deansharafi]

If you want to do the minimum and find out what is the status of the transformers, normally a simple insulation resistance test will reveal many things. If the result of the insulation resistance is not necessary and/or you suspect that the transformer has a problem you can proceed with, turns ratio test, DGA on oil and Tan delta test. But as per my experience, if there is a problem with the transformer a simple megger test and confirmation with Polarization Index test will tell you. Hope it helps.
Dean

 

[busbar]

The listed sizes are not exactly cookie-cutter/“toss out” transformer sizes. Consider the potential cost of “production” loss.

Start with §7.2 of NETA Maintenance Testing Specifications, 2001.

 

[jbartos]

Suggestion: The absolute testing minimum should not be the criterion. It would be better to let the testing to the Contractor's judgment. Also, the transformer overall evaluation will be needed, e.g. remaining life-expectancy, necessary maintenance, etc. The financial crises and cutbacks might create a lot of problems in the future for these sizes of transformers. Ask the Contractor to give you a good price for the transformer testing in exchange for the guaranteed future service to that group of transformers. Some more fortunate contractor would welcome such an arrangement. This is a situation "pay now or pay later."

 

[Alwaysexciter]

I work for a testing contractor so naturally my first response is "Do It ALL" because it's all good. However, since sometimes budgetary conserns out way what I personally would like to see done, Here is what I would consider the order of priority for tranformers over 3000 KVA.

First: Sample the oil and perform "screening test" and "Gas Analisys" Cost about $110.00 each. No Shut down required. (This should be done ahead of time if you plan on shutting down so that any problems detected can be addressed.)

Second: Power factor and excitation current. Depending on the difficulty in isolation the time for this can vary. On average this testing will take two technicians about 8 hours. Estimated cost about $1500.00

Third: Megger, Turns Ratio and Winding Resistance testing are usually done together since they require relativly little time each. If done in conjunction with the power factor testing then this would usually take an additional 8 manhours assuming the connections have already been removed. Estimated cost = about $750.00

Some savings could be gained if you have site electrician to do the connections and cleaning of the bushings. If you were doing several transformers in a day that would also reduce the final cost.

One last note. Make sure that you are fully aware of what you want and how you want it done. For example, some contractors may give you a price based on not disconnecting the transformers even though this is not the recommended method. A clear scope of work is vital!

 

[jbartos]

Suggestion: Reference:
2003
Acceptance Testing Specifications
for
Electrical Power Distribution
Equipment and Systems

7. INSPECTION AND TEST PROCEDURES
7.2.1.2 Transformers, Dry Type, Air-Cooled, Large
and
7. INSPECTION AND TEST PROCEDURES
7.2.2 Transformers, Liquid-Filled

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by
InterNational Electrical Testing Association
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