Partial Discharge (PD) Testing and Diagnostics in Cast Resin Transformers

Dielectric breakdown mechanisms in epoxy casting, apparent charge (pC) calibration per IEC 60270, PD inception (PDIV) and extinction (PDEV) voltages, and Phase-

1. Physics of Partial Discharge in Solid Dielectrics

Partial discharge (PD) is a localized electrical breakdown within micro-voids, delaminations, or cast impurities in the solid epoxy resin matrix. Under high electric field gradient, electron avalanches gradually erode the surrounding insulation.

2. Routine Factory Acceptance Criteria (IEC 60076-11)

Per IEC 60076-11, dry-type transformers must undergo mandatory PD measurement via coupling capacitor circuits calibrated per IEC 60270:

1. Pre-Stress Voltage: Voltage is ramped to 1.8 × Um / √(3) and held for 30 seconds to initiate any potential PD activity.

2. Measurement Voltage: Voltage is decreased to 1.3 × Um / √(3) and held for 3 minutes while measuring apparent charge.

3. Acceptance Threshold:

q ≤ 10 pC

3. Phase-Resolved Partial Discharge (PRPD) Pattern Diagnostics

Defect TypePRPD Phase Distribution PatternRoot Cause Mechanism
Internal Gas Cavity / VoidDischarges centered around 45° and 225° with equal positive/negative polarityTrapped micro-bubbles during vacuum resin casting
Surface TrackingDischarges centered around voltage zero-crossings (0° and 180°)Surface dust, moisture, or resin contamination
Corona into AirSharp discharge cluster at the negative peak (270°)Sharp metallic burr or unshielded connection point

4. Field Diagnostics & Inception Voltage (PDIV)

In field commissioning, ensuring that the Partial Discharge Inception Voltage (PDIV) is at least 1.30 × higher than nominal system voltage guarantees zero degradation over 30+ years of operation.

*Reference: IEC 60270:2000+AMD1:2015; IEC 60076-11:2018; CIGRE TB 676.*