Technical Articles

Practical Guide to Partial Discharge (PD) Pattern Recognition

🔹 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

Check symmetry:
- Symmetrical in both half cycles → Surface or Internal PD.
- Present only in one polarity → Corona or floating electrode.
Check phase position:
- Clustered around 90° / 270° → Surface discharge.
- Spread across most of the cycle → Internal discharge.
- Localized in a small window → Corona.
Check magnitude and density:
- Low magnitude (<50 pC) and sparse → Corona.
- Medium to high magnitude, dense clustering → Surface / Tracking.
- Irregular magnitude, distributed → Internal.
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

EA Technology – Blog: What is a Partial Discharge Phase Plot telling you?

IEC 60270 – High-voltage test techniques – Partial discharge measurements

F.H. Kreuger – Partial Discharge Detection in High-Voltage Equipment

IEEE Std 1434-2014 – Guide for Measurement of Partial Discharges in AC Electric Machinery

OMICRON Application Notes – “How to Analyze Partial Discharge”