Liquid Power Factor - Part 2

Testing for Liquid Power Factor in Mineral Insulating Oil

Part 2 – Acceptable, Questionable, and Unacceptable Values

In Part 1, we introduced testing of the liquid power factor of insulating liquids as a tool for evaluating oil-filled transformers. Here, we are going to discuss the values we obtain from liquid power factor tests and what these values indicate. We are going to introduce typical and specification values for new, unused oil and newly installed oil in transformers, as well as acceptable, questionable, and unacceptable values for In-service oil.

When liquid power factor determinations are run on transformer oil, they are usually run at two temperatures: 25°C and 100°C. This is done because the two readings, and how they change over time, can be useful in diagnosing which condition (moisture, oil oxidation, or contamination) is causing a high power factor. Further, the 100°C value is many times more sensitive to small changes in oil characteristics.

Liquid power factor values for new oil from a supplier or for newly installed oil are usually very small. For example, consider new oil installed in a new transformer of primary voltage class less than 230 kV. The 25°C liquid power of that oil should be no more than 0.05% – this is the recommended test limit for this value in ANSI/IEEE C57.106-2006, the current edition of the Guide for Acceptance and Maintenance of Mineral Insulating Oil in Equipment. This an upper limit – in actual new installations, the measured liquid power factor at 25°C is frequently much lower.

The values we use to classify liquid power factor results for in-service oil are the same for all primary voltage classes of equipment. For oil filled transformers, those values are:

(100 DEGREES C) 0-2.99 AC
(25 DEGREES C) 0-0.1 AC

When we evaluate the results for purposes of establishing a service recommendation, the 100°C value is always considered more carefully than the 25°C value. QU or UN values for liquid power factor should be investigated and the cause should be diagnosed. Reclaiming the oil or hot oil cleaning the transformer will reduce the liquid power factor. Drying out the oil may also improve the liquid power factor, particularly the 25°C reading.

based on a high liquid power factor reading. Chemical changes in the oil may occasionally cause elevated liquid power factor values. These high readings are usually temporary and are not of concern. If elevated values persist, and they continue to appear in the next annual testing, then the cause should be identified and corrected, if appropriate.

Extremely elevated liquid power factors at 100°C in transformer oil usually indicate contamination. Care must be taken when dealing with fluids other than transformer oil because some of these either have naturally high liquid power factor values when new, or develop high values very soon after being put in-service. For these fluids, liquid power factor may not always be an appropriate test for monitoring in-service degradation of the insulating liquid.

In Part 3, we will talk about when we recommend running liquid power factor and talk about what are acceptable values for new oil from a supplier and for newly installed oil in transformers.