Power factor testing is the most widely used method for evaluating transformer insulation condition in the field. The test takes an hour or two. The result is a number most people accept without fully understanding what it measures, what drives it up, and when it means something is actually wrong.
A perfect insulator passes no current. Real insulation passes a small amount, and that current has two components: capacitive current that leads voltage by 90 degrees, and resistive current that is in phase with voltage. The resistive component represents losses in the insulation itself, whether from moisture, contamination, thermal degradation, or physical damage.
Power factor is the ratio of resistive losses to total current drawn. Expressed as a percentage, new dry transformer insulation comes in below 0.5%. As insulation ages or picks up moisture, the resistive component grows and power factor rises. A reading of 2% or above on a winding test is a signal worth investigating. Above 5% is serious.
Dissipation factor and tan delta measure the same thing expressed differently. The terms are sometimes used interchangeably depending on the test equipment and the lab doing the analysis.
A two-winding transformer has insulation between the high-voltage winding and the low-voltage winding, between each winding and the core/tank, and in the bushings. To isolate where a problem lives, the test is run in three configurations.
CHL (capacitance H-to-L) energizes the high-voltage winding with the low-voltage winding shorted to ground and the tank grounded. This measures the insulation between the two windings. It is the most sensitive reading for inter-winding problems.
CH (capacitance H-to-ground) energizes the high-voltage winding with the low-voltage winding floating and the tank grounded. This picks up everything between the H winding and ground, including the major insulation structure and the HV bushings.
CL (capacitance L-to-ground) energizes the low-voltage winding with the high-voltage winding floating and grounded. This tests the L-winding insulation to ground, including the LV bushings.
Running all three lets you do the math to isolate which component is pulling the overall reading up. If CHL is clean but CH is elevated, the problem is in the H winding-to-ground path, not between windings. Bushing power factor tests, done separately with the bushing tap, further narrow it down.
Power factor is temperature-dependent. A transformer tested at 40°C will show a meaningfully higher number than the same transformer tested at 20°C, even if nothing has changed in the insulation. Without correcting to a standard temperature, you cannot compare results from one year to the next if ambient conditions were different.
The standard correction reference is 20°C. Correction factors depend on insulation type and are published by the test equipment manufacturers. Record both the corrected and uncorrected values. A technician who only logs the corrected number is throwing away information.
Oil temperature is the right temperature to record, not ambient air. The core and windings are equilibrated to oil temperature, not to whatever the air is doing at the time of the test.
IEEE C57.12.90 and NETA acceptance tables give you absolute thresholds, typically 0.5% to 1.0% for new transformers and 2.0% as a caution level for in-service units. Those numbers are a starting point, not the whole answer.
Some older transformers with kraft paper insulation have always run above the modern thresholds. A unit that tested at 1.8% fifteen years ago and still tests at 1.9% today is in stable condition. A unit that was 0.4% three years ago and is now 1.2% is telling you something, even though 1.2% is still technically within acceptable range by most tables.
Trending over time is more useful than a single absolute reading. A baseline established at commissioning or the first maintenance outage gives you a reference point that no published table can provide for that specific transformer.
Moisture is the most common cause. Kraft paper insulation is hygroscopic. It absorbs moisture from the oil, from the atmosphere during maintenance, and over decades of thermal cycling. Moisture raises power factor and, more importantly, accelerates the thermal breakdown of the paper itself. A transformer with 3% moisture content in the paper is not the same machine it was at 0.5%.
Contamination is the next most common cause. Carbon from LTC arcing can migrate into the main tank if the barrier between compartments is compromised. Oil contaminated with carbon or other conductive material shows elevated readings even when the solid insulation is fine. A dissolved gas analysis (DGA) run at the same time often separates these two causes.
Thermal degradation is harder to see in power factor results until it is advanced. Aged paper loses mechanical strength before it loses dielectric strength. A transformer with brittle winding paper can look acceptable on a power factor test right up until a fault loads the windings and they cannot absorb the impulse.
A single elevated reading rarely justifies an immediate outage. It justifies a closer look. Cross-reference with DGA, oil sample results, visual inspection, and the history of the unit. A rising trend across two or three test cycles is more actionable than a single number above a threshold.
If moisture is the driver, vacuum oil processing or vapor-phase drying can pull the reading back down without removing the transformer from service. If the problem is in the bushings, replacing the bushing is a straightforward repair compared to a transformer rewind. Isolating the source before deciding on remediation saves money and avoids unnecessary outages.
What to avoid: treating a power factor result as a checkbox. The number has meaning, but only when you know the transformer's history, the test conditions, and what the other diagnostic data shows. A technician who understands why the number is what it is will always outperform one who just compares it to a table and moves on.
Our field crews run power factor, DGA, and insulation resistance testing on power transformers across Florida and the Southeast. Send us your location and equipment and we’ll get back to you within one business day.