{
  "language": "en",
  "interfaceLanguage": "en",
  "url": "https://voltformer.com/articles/partial-discharge-diagnostics-in-cast-resin-dry-type-transformers",
  "id": "art-6",
  "slug": "partial-discharge-diagnostics-in-cast-resin-dry-type-transformers",
  "title": "Partial Discharge (PD) Testing and Diagnostics in Cast Resin Transformers",
  "summary": "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.",
  "readTime": "8 min read",
  "date": "2026-04-18",
  "author": "Voltformer High Voltage Diagnostics Laboratory",
  "category": "Dielectrics & Diagnostics",
  "tags": [
    "Partial Discharge",
    "IEC 60270",
    "Cast Resin",
    "PRPD Analysis",
    "Dielectric Testing"
  ],
  "contentMarkdown": "### 1. Physics of Partial Discharge in Solid Dielectrics\n\nPartial 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.\n\n### 2. Routine Factory Acceptance Criteria (IEC 60076-11)\n\nPer **IEC 60076-11**, dry-type transformers must undergo mandatory PD measurement via coupling capacitor circuits calibrated per **IEC 60270**:\n\n1. **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.\n2. **Measurement Voltage:** Voltage is decreased to $1.3 \\times U_m / \\sqrt{3}$ and held for 3 minutes while measuring apparent charge.\n3. **Acceptance Threshold:**\n\n$$ q \\le 10\\text{ pC} $$\n\n### 3. Phase-Resolved Partial Discharge (PRPD) Pattern Diagnostics\n\n| Defect Type | PRPD Phase Distribution Pattern | Root Cause Mechanism |\n|---|---|---|\n| **Internal Gas Cavity / Void** | Discharges centered around $45^\\circ$ and $225^\\circ$ with equal positive/negative polarity | Trapped micro-bubbles during vacuum resin casting |\n| **Surface Tracking** | Discharges centered around voltage zero-crossings ($0^\\circ$ and $180^\\circ$) | Surface dust, moisture, or resin contamination |\n| **Corona into Air** | Sharp discharge cluster at the negative peak ($270^\\circ$) | Sharp metallic burr or unshielded connection point |\n\n### 4. Field Diagnostics & Inception Voltage (PDIV)\n\nIn 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.\n\n*Reference: IEC 60270:2000+AMD1:2015; IEC 60076-11:2018; CIGRE TB 676.*",
  "contentLanguage": "en",
  "sources": []
}