Liquid Power Factor - Part 3

Testing for Liquid Power Factor in Mineral Insulating Oil

Part 3 – Applications

In Part 1 and Part 2, we introduced liquid power factor testing and discussed acceptable, questionable, and unacceptable values for in-service transformer oil. Here, we want to look at when we recommend running the test. We will also discuss acceptable values for new oil from a supplier and for newly installed oil in transformers.

For in-service insulating liquids, we recommend running liquid power factor at both 25°C and 100°C on all mineral oil-filled transformers except, perhaps, for very small distribution class transformers such as small pad mounted or pole mounted units. Liquid power factor is particularly important when D1816 dielectric breakdown voltage is also performed, since the results from the power factor may indicate the cause(s) of poor D1816 results. Equipment types where testing generally incorporates transformer testing packages, such as three phase regulators, should also be subjected to liquid power factor testing at both recommended temperatures.

Liquid power factor is an extremely valuable new oil test. Testing liquid power factor during all phases of purchasing, installing, and energizing new transformers is listed, together with appropriate criteria for continued operation, in IEEE C57.106-2006, Guide for Acceptance and Maintenance of Insulating Oil in Equipment. Values for new oil from a supplier are 0.05% maximum @ 25°C and 0.3% maximum @100°C. Values for new oil received in new equipment and for new oil processed for filling into new equipment are 0.05% maximum @ 25°C and 0.4% maximum @100°C.

IEEE Standard 637 lists an acceptable maximum value of 1% for the liquid power factor @ 100°C for oil that has been reclaimed.

One of the most valuable applications of liquid power factor testing in the past few years has been to run the test on oil from newly installed transformers. The test is run before energizing the unit for the first time, and then retested at 30 days and at three months or six months, after energizing has proven to be effective at picking up contamination. There have been a number of cases where abnormally high 100°C liquid power factors were discovered during this initial testing. A diagnosis of contamination arose from construction materials which had leached into a new transformer. Liquid power factor testing in these cases has proven to be a valuable diagnostic tool.

Liquid power factor is less valuable for fluids that are not transformer oil, or for equipment types other than transformers. Usually, the difficulties in these other applications are associated with either very high "normal" liquid power factor reducing the diagnostic value of the test, or with insufficient history concerning power factor changes in alternate fluids. On some occasions, liquid power factor testing of some types of oil-filled equipment other than transformers may be done for purposes of trouble-shooting. Some transformer owners also have their own monitoring needs regarding non-mineral oil applications. In this last event, we may limit the testing to the 25°C value only.