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Practical Guide to Partial Discharge (PD) Pattern Recognition

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🔹 1. What is PRPD?

PRPD (Phase-Resolved Partial Discharge) patterns show the relation between:

  • Phase angle (0°–360°) of the AC cycle,
  • Discharge magnitude (q, in pC),
  • Number of occurrences (repetition rate).

Each type of PD creates a unique fingerprint in the PRPD plot. By analyzing the location, symmetry, and density of the discharge pulses, the source of PD can be identified.

🔹 2. Typical Types of PD and Their Patterns

Type of PD PRPD Pattern Characteristics Likely Cause / Defect
Corona Discharge • Pulses appear in a narrow phase window (usually 60–120° and 240–300°)
• Asymmetric between positive and negative half-cycles
• Magnitude: low to medium (few pC to 100s of pC)		 Sharp electrode tips, air gaps, loose connectors, poor design in HV terminals
Surface Discharge • Pulses in both half-cycles, mostly near voltage peaks (±90° and ±270°)
• Symmetrical distribution
• Magnitude: medium to high		 Pollution, moisture, cracks on insulator surface, insufficient creepage distance
Internal Discharge (Voids) • Discharges distributed over most of the AC cycle
• Symmetric but irregular amplitude
• Magnitude: few to hundreds of pC		 Voids, cavities, or gas bubbles inside solid insulation (epoxy, XLPE, paper-oil, etc.)
Tracking Discharge • Similar to surface PD, but increasing rapidly with voltage
• High repetition rate, strong clustering
• Magnitude: high		 Conductive path formation on insulation surface (moisture + contamination)
Floating Electrode (Loose Connection) • Random distribution, not tied to a clear phase window
• Erratic pulses, sometimes only one polarity
• Magnitude: unstable		 Unconnected metal part, poor grounding, loose connections

🔹 3. Step-by-Step Method to Identify PD Source

  1. Check symmetry:
    • Symmetrical in both half cycles → Surface or Internal PD.
    • Present only in one polarity → Corona or floating electrode.
  2. Check phase position:
    • Clustered around 90° / 270° → Surface discharge.
    • Spread across most of the cycle → Internal discharge.
    • Localized in a small window → Corona.
  3. Check magnitude and density:
    • Low magnitude (<50 pC) and sparse → Corona.
    • Medium to high magnitude, dense clustering → Surface / Tracking.
    • Irregular magnitude, distributed → Internal.
  4. Check behavior with voltage increase:
    • Corona: magnitude grows slowly, mostly at higher voltages.
    • Surface: grows faster with voltage, sensitive to humidity.
    • Internal: appears early and grows significantly with applied voltage.

🔹 4. Practical Examples (Reference Patterns)

  • Corona → Looks like “two small islands” at fixed phase angles.
  • Surface → Dense clouds near ±90° and ±270°.
  • Internal → Spread-out clouds across entire cycle.
  • Tracking → Like surface PD but stronger, with rapid growth.
  • Floating electrode → Scattered dots without clear phase correlation.

🔹 5. References

  1. IEC 60270 – High-voltage test techniques – Partial discharge measurements
  2. F.H. Kreuger – Partial Discharge Detection in High-Voltage Equipment
  3. IEEE Std 1434-2014 – Guide for Measurement of Partial Discharges in AC Electric Machinery
  4. OMICRON Application Notes – “How to Analyze Partial Discharge”
  5. EA Technology – Blog: What is a Partial Discharge Phase Plot telling you?

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