Transformer Oil Power Factor Test

brupp · Jul 24, 2003

We have a 10/12.5MVA sealed transformer operating at 13.8 - 4.16kV and our recent oil tests gave "unacceptable" results for the power factor. The transformer consistently has a positive pressure nitrogen blanket on it, approximately 3-5 psig. I think it is very unlikely that water is the culprit and I don't believe the unit has been ridden hard. Normal load is well under 50% of nameplate.

The Liquid Power Factor history on this transformer is as follows:

Date 25C 100c
4/18/00 .072% Acceptable 1.916% Acceptable
6/6/01 .046% Acceptable 3.182% Questionable
6/29/02 .154% Questionable 3.410% Questionable
6/4/03 .239% Questionable 5.336% Unacceptable

As I am not familiar with the liquid power factor, I cannot think of what is causing the problem and what to do about it. Perhaps this isn't a serious problem. I have not asked the oil testing company about it but their written report says that this test is meaningful in determining the aging and life expectancy. Any thoughts? Suggestions? Warnings?

electricpete · Jul 24, 2003

http://www.usbr.gov/power/data/fist/fist3_5/3_5_ae.htm

Excerpt:

"Good, new oil has a power factor of 0.05 percent or less at 20 ºC. Higher power factors indicate deterioration and/or contamination with moisture, carbon or other conducting matter, varnish, Glyptal, sodium soaps, asphalt compounds, or deterioration products. Carbon or asphalt in oil can cause discoloration. Carbon in oil will not necessarily increase the power factor of the oil unless moisture is also present. It is suggested that the following serve as guides for grading oil by power factor tests.

a. Oil having a power factor of less than 0.5 percent at 20 ºC is usually considered satisfactory for service.

b. Oil having a power factor between 0.5 and 2 percent at 20 ºC should be considered as being in doubtful condition, and at least some type of investigation should be made.

c. Oil having a power factor of over 2 percent at 20 ºC should be investigated and either reconditioned or replaced.

The above-mentioned guides may be elaborated on by saying that good, new oil has a power factor of approximately 0.05 percent or less at 20 ºC and that the power factor can gradually increase in service to a value as high as 0.5 percent at 20 ºC without, in most cases, indicating deterioration to warrant investigation. When the power factor exceeds 0.5 percent, an investigation is indicated. The question of what decision to make regarding disposition of the oil depends on what is causing the high power factor. Dielectric strength tests should be made to determine the presence of moisture. The necessity for further tests will depend to a large extent on the magnitude of the power factor, the importance of the apparatus in which the oil is used, its rating and the quantity of oil involved."

Assuming that that 20C limit equates roughly to your 25C reading, you have not even entered the questionable range.

It's been awhile so I don't know whether I would put blind faith in those limits above. My memory is that power factor is a quick and dirty test which can be done using equipment typically availabel to transformer maintenance guys (their doble test set). There certainly are more direct ways of measuring oil quality. If you don't see any problems on moisture in oil by Karl Fischer than moisture is certainly not an issue. Of course there is a wide variety of tests available and if it's a crtical transformer it is wise to supplement your power factor test periodically with complete dga and oil screen: moisture, dielectric, neut number, (I'm sure I left something out).