What Do Liquid Power Factor Test Values Indicate?

This article is Part 2 of a three-part series on liquid power factor testing in mineral insulating oil. Part 1 covered the fundamentals of the test method and the dielectric loss principle. Here we will address classification thresholds and acceptable values for new, newly installed, and in-service oil. Part 3 will address testing frequency recommendations and acceptance criteria for new and newly installed oil.

What Is Liquid Power Factor?

Liquid power factor is tested at two temperatures — 25°C and 100°C — because each reading provides distinct diagnostic information, and the way both values shift over time helps identify whether moisture, oxidation, or contamination is driving an elevated result. The 100°C value is considerably more sensitive to small changes in oil characteristics and is therefore weighted more heavily when establishing a service recommendation.

Acceptable Values for New Oil

For new oil from a supplier or newly installed oil, liquid power factor values are expected to be very low. As a reference point, ANSI/IEEE C57.106-2006 — the Guide for Acceptance and Maintenance of Mineral Insulating Oil in Equipment — specifies a maximum 25°C liquid power factor of 0.05% for new oil installed in transformers with a primary voltage class below 230 kV.¹ In practice, measured values in new installations are frequently well below that threshold.

Classification Thresholds for In-Service Oil

For in-service oil in oil-filled transformers, the same classification thresholds apply across all primary voltage classes:

When evaluating results, the 100°C value carries greater diagnostic weight. Any values outside these acceptable ranges warrant investigation to identify and diagnose the underlying cause.

Responding to Elevated Values

Remediation options

When liquid power factor exceeds acceptable thresholds, several corrective actions are available:

• Oil reclamation will reduce elevated liquid power factor values.
• Hot oil cleaning of the transformer is another effective remediation method.
• Drying out the oil can improve liquid power factor — particularly the 25°C reading — when moisture is a contributing factor.

Temporary elevations from chemical changes

Chemical changes in the oil can occasionally produce elevated liquid power factor readings that are transient in nature and not diagnostically significant. If elevated values resolve on their own, no intervention is typically required. However, if elevated values persist into subsequent annual testing cycles, the cause should be identified and corrected as appropriate.

Extremely Elevated 100°C Values

Extremely elevated liquid power factor values at 100°C in transformer oil are a strong indicator of contamination. When working with fluids other than conventional transformer oil, additional caution is warranted: some alternative insulating fluids exhibit naturally high liquid power factor values when new, or develop elevated values shortly after entering service. For these fluids, liquid power factor may not be an appropriate condition-monitoring parameter for tracking in-service degradation.

Summary:

• Testing at 25°C and 100°C provides complementary diagnostic information; the 100°C value is more sensitive and carries greater weight in service evaluations.
• ANSI/IEEE C57.106-2006 sets a maximum 25°C liquid power factor of 0.05% for new oil in transformers below 230 kV primary voltage class — actual values in new installations are often significantly lower.
• Classification thresholds for in-service oil are uniform across all primary voltage classes; QU or UN results at either temperature require investigation.
• Remediation options include oil reclamation, hot oil cleaning, and drying — with drying being most impactful on the 25°C reading.
• Transient elevations from chemical changes in the oil are generally not cause for concern unless they persist across successive testing intervals.
• Extremely high 100°C values typically indicate contamination; for non-mineral insulating fluids, liquid power factor may not be a reliable degradation indicator.

References:

¹“IEEE Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment,” in IEEE Std C57.106-2015 (Revision of IEEE Std C57.106-2006), vol., no., pp.1-38, 23 March 2016, doi: 10.1109/IEEESTD.2016.7442048.

In This Series:

• Part 1: How Is Liquid Power Factor Used to Evaluate Transformer Oil?
• Part 2: What Do Liquid Power Factor Test Values Indicate?
• Part 3: When and How Should Liquid Power Factor Testing Be Applied?