# 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-Resolved Partial Discharge (PRPD) analysis.

- **Category:** Dielectrics & Diagnostics
- **Author:** Voltformer High Voltage Diagnostics Laboratory
- **Publication Date:** 2026-04-18
- **Reading Time:** 8 min read
- **Key Tags:** #Partial Discharge, #IEC 60270, #Cast Resin, #PRPD Analysis, #Dielectric Testing
- **Canonical URL:** https://voltformer.com/articles/partial-discharge-diagnostics-in-cast-resin-dry-type-transformers
- **Markdown Source:** https://voltformer.com/articles/partial-discharge-diagnostics-in-cast-resin-dry-type-transformers.md

### 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 $1.8 \times U_m / \sqrt{3}$ and held for 30 seconds to initiate any potential PD activity.
2. **Measurement Voltage:** Voltage is decreased to $1.3 \times U_m / \sqrt{3}$ and held for 3 minutes while measuring apparent charge.
3. **Acceptance Threshold:**

$$ q \le 10\text{ pC} $$

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

| Defect Type | PRPD Phase Distribution Pattern | Root Cause Mechanism |
|---|---|---|
| **Internal Gas Cavity / Void** | Discharges centered around $45^\circ$ and $225^\circ$ with equal positive/negative polarity | Trapped micro-bubbles during vacuum resin casting |
| **Surface Tracking** | Discharges centered around voltage zero-crossings ($0^\circ$ and $180^\circ$) | Surface dust, moisture, or resin contamination |
| **Corona into Air** | Sharp discharge cluster at the negative peak ($270^\circ$) | 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 **$1.30 \times$ 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.*

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