Power Factor Testing

power factor testingPower Factor Testing is a critical step in determining the overall health of the winding in the transformer. Other tests give you some insight into the transformers winding condition however the PF Test is key when looking at capacitance and dielectric loss as they relate to Insulation Integrity over time.

Why Perform Power Factor Test on Transformers?

  • Detect moisture, carbon tracking, contamination, or aging of insulation

  • Identify interwinding insulation failure

  • Check condition of bushings and windings without opening transformer

  • Prevent failures and schedule maintenance before breakdown

Typical Power Factor Testing Results

Condition Power Factor
New Insulation < 0.5%
Slightly Aged 0.5% – 1.0%
Aged/Deteriorated > 1.0%
Critical/Failing > 2.0%

How the Test is Done

Test Setup:

  • Transformer is de-energized and grounded

  • Test instrument: e.g., Doble M4000/M4100, Megger Delta Series

  • Conducted at 60 Hz (or 50 Hz) using low AC voltage (typically 2–10 kV)

  • Transformer must be dry (i.e., no internal condensation)

Test Modes:

  1. CHL (Capacitance High-Low): HV to LV with ground guarded

  2. CHG (HV to Ground): HV winding to ground

  3. CLG (LV to Ground): LV winding to ground

  4. CH / CL: Interwinding capacitance measurements

  5. UST (Ungrounded Specimen Test): Isolates specific winding from ground during test

Interpreting Results

  • Compare power factor values over time (baseline vs current)

  • Sudden increase may indicate moisture ingress

  • Gradual rise can indicate insulation aging

  • Temperature correction to 20°C is essential for valid comparisons

Test Report Should Include:

  • Capacitance (in pF or nF)

  • Power factor (%) or tan delta

  • Test voltage

  • Temperature and corrected values

  • Test configuration (CHL, CHG, etc.)

Common Issues Detected

  • Moisture in oil or windings

  • Bushing degradation

  • Insulation delamination

  • Contamination due to sludge or carbon tracking

  • Thermal aging or hotspots

 When to Perform?

  • During commissioning

  • Regular preventive maintenance (1–5 years)

  • After faults or suspected insulation issues

  • After repairs or oil replacement

Contact Infra-red to find out more.