When and How Should Liquid Power Factor Testing Be Applied?

This article is Part 3 of a three-part series on liquid power factor testing in mineral insulating oil. Part 2 addressed classification thresholds and acceptable values for new, newly installed, and in-service oil. Here we will cover testing frequency recommendations and acceptance criteria for new and newly installed oil.

Recommended Testing Scope

For in-service mineral oil-filled transformers, liquid power factor should be run at both 25°C and 100°C across all equipment classes, with the possible exception of very small distribution-class units such as small pad-mounted or pole-mounted transformers. Three-phase regulators and other equipment types that fall under standard transformer testing packages should also be included in routine liquid power factor testing at both temperatures.

Liquid power factor is especially important when ASTM D1816 dielectric breakdown voltage is also being performed, as power factor results can help identify the cause or causes of poor D1816 outcomes.

Acceptance Criteria for New Oil

IEEE C57.106-2006, Guide for Acceptance and Maintenance of Insulating Oil in Equipment, provides acceptance criteria for liquid power factor across all phases of purchasing, installing, and energizing new transformers.¹ The applicable limits are:

For reclaimed oil, IEEE Standard 637 specifies an acceptable maximum of 1.0% at 100°C.²

Testing Newly Installed Transformers

Pre-energization and early in-service testing

One of the most diagnostically valuable applications of liquid power factor testing in recent years has been systematic testing on newly installed transformers. The recommended protocol is:

• Before first energization — establish a baseline reading
• At 30 days post-energization — initial in-service check
• At 3 or 6 months post-energization — follow-up confirmation

This sequence has proven effective at detecting contamination that would not be apparent from a single pre-energization sample.

Contamination from construction materials

A number of confirmed cases have involved abnormally elevated 100°C liquid power factor values discovered during this initial testing window. In these instances, diagnosis pointed to contamination from construction materials that had leached into the transformer during manufacturing or installation. Early-stage liquid power factor testing has been instrumental in identifying these conditions before they develop into more serious equipment issues.

Limitations in Non-Transformer Applications

Liquid power factor is a less reliable diagnostic tool for fluids other than conventional transformer oil and for equipment types outside the transformer category. The primary limitations are:

• Elevated baseline values — some alternative insulating fluids have naturally high liquid power factor values when new, which compresses the diagnostic range and reduces the test’s sensitivity to degradation.
• Insufficient service history — there is limited data on how liquid power factor evolves over time in many alternative fluids, making trend interpretation less reliable.

Liquid power factor testing on non-transformer oil-filled equipment is occasionally performed for troubleshooting purposes. Where transformer owners have specific monitoring requirements for non-mineral oil applications, testing may be limited to the 25°C value only.

Summary:

• Routine testing at 25°C and 100°C is recommended for all mineral oil-filled transformers except very small distribution-class units; three-phase regulators and similar equipment should be included.
• Liquid power factor and D1816 dielectric breakdown voltage are complementary tests — power factor results can help diagnose the cause of poor D1816 performance.
• IEEE C57.106-2006 governs acceptance criteria for new oil; the 100°C limit is slightly more permissive for oil received in or processed for new equipment (0.40%) than for oil supplied in bulk (0.30%).
• IEEE Standard 637 sets a 1.0% maximum at 100°C for reclaimed oil.
• Pre-energization and early in-service testing — at baseline, 30 days, and 3–6 months — has proven effective at detecting contamination from construction materials in newly installed transformers.
• Non-mineral oil applications present diagnostic limitations due to elevated baseline values and limited service history; testing in these cases may be restricted to the 25°C reading only.

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.

² “IEEE Guide for the Reclamation of Mineral Insulating Oil and Criteria for Its Use,” in IEEE Std C57.637-2015 (Revision of IEEE Std 637-1985), vol., no., pp.1-38, 20 Nov. 2015, doi: 10.1109/IEEESTD.2015.7328235.

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?