What Is PRPD Pattern Analysis?

Phase-Resolved Partial Discharge (PRPD) analysis is a technique that displays PD pulses in relation to the AC voltage phase angle. In a PRPD pattern, each PD pulse is plotted as a point on a graph with the AC phase angle (0 to 360 degrees) on the horizontal axis and the discharge magnitude (in pC or dB) on the vertical axis. The density of points at each phase-magnitude coordinate creates a "fingerprint" pattern that characterizes the type of PD source.

The principle is simple: different PD mechanisms discharge at different points in the AC cycle. For example:

• Internal voids discharge symmetrically around the voltage zero crossings (0 and 180 degrees)
• Surface discharge typically occurs on the rising and falling edges of the voltage waveform (45-90 and 225-270 degrees)
• Corona in air discharges near the voltage peak of the negative half-cycle (270 degrees)
• Corona in SF6 gas shows a different pattern with pulses near both positive and negative peaks

The Unitech Tools HPD500 Advanced PD Analyzer includes built-in PRPD display capability, showing real-time PD patterns on its 5-inch color screen. Engineers can immediately classify PD types without needing to transfer data to a PC for analysis.

Common PD Patterns and Their Meanings

PD Type	PRPD Pattern	Phase Position	Typical Cause	Severity
Internal void discharge	Symmetrical, rabbit-like, centered at 0 and 180 deg	0-30 and 180-210 deg	Voids in epoxy, XLPE, or cast resin	Moderate-High
Surface discharge	Asymmetrical, spreading pattern on rising/falling edges	45-90 and 225-270 deg	Contamination tracking on insulator surfaces	Moderate
Corona in air	Sharp pulses, negative half-cycle dominant	250-290 deg	Sharp points in air-insulated equipment	Low-Moderate
Corona in SF6	Pulses near both positive and negative peaks	80-100 and 260-280 deg	Sharp protrusions in GIS compartments	Moderate-High
Floating electrode	Very large pulses, stable phase position, often above 1000 pC	Variable, but consistent	Loose metallic shields or floating components	High-Critical
Noise / interference	Random, fills entire phase range uniformly	0-360 deg (uniform)	External EMI, switching transients	N/A (noise)

The ability to distinguish real PD from electrical noise is one of the most valuable aspects of PRPD analysis. An experienced operator can immediately recognize the uniform, random pattern of EMI interference and avoid unnecessary maintenance actions on false positives.

PRPD vs PRPS: What’s the Difference?

While PRPD is the most commonly used pattern analysis technique, PRPS (Phase-Resolved Pulse Sequence) provides additional diagnostic information. Here's how they differ:

• PRPD aggregates many AC cycles into a single density plot. It shows the statistical distribution of PD magnitude and phase angle over time. This is excellent for identifying PD type but loses information about pulse-to-pulse timing.
• PRPS displays each individual PD pulse in sequence across consecutive AC cycles. Each pulse is plotted as a point in 3D (cycle number, phase angle, magnitude). This reveals pulse-to-pulse behavior, such as alternating patterns or pulse bursts that indicate specific discharge mechanisms like electrical tree propagation in cables.

Most modern PD analyzers, including the HPD500, offer both PRPD and PRPS displays. For routine screening, PRPD is sufficient. For detailed diagnostics — especially on cables where tracking the evolution of individual PD pulses over time is important — PRPS provides deeper insight.

"PRPD pattern analysis is the single most important skill for a PD testing engineer. A well-trained operator with a quality PRPD instrument can diagnose the PD source type and estimate its severity with 90% accuracy without any additional measurements."

— Unitech Tools Technical Training Manual

How Pattern Analysis Improves Diagnostic Accuracy

Pattern analysis goes beyond simple yes/no PD detection. The key benefits include:

1. PD type classification — Recognising the pattern tells you what kind of defect is present (void, surface, corona, floating electrode), guiding the repair approach.
2. Severity assessment — A PRPD pattern with widely spread, high-magnitude pulses (>500 pC) indicates more severe insulation damage than a tight cluster of low-magnitude pulses (<50 pC).
3. Noise rejection — Uniform, random patterns across all phase angles are almost always noise. Engineers can confidently ignore readings that would otherwise trigger false alarms on threshold-based PD detectors.
4. Trend analysis — Comparing PRPD patterns from the same asset over time shows whether the PD source is stable, growing, or accelerating. A pattern that spreads wider in phase angle and increases in magnitude over successive tests indicates developing failure.
5. Cross-referencing with TEV/ultrasonic — Combining PRPD analysis with TEV or ultrasonic measurements provides multi-sensor confirmation. For example, a PRPD pattern consistent with surface discharge confirmed by audible ultrasonic signal gives high confidence in the diagnosis.

Using PRPD Analysis with Unitech Tools PD Detectors

Unitech Tools PD detection instruments are designed with PRPD/PRPS analysis at the core of their diagnostic capability:

• HPD500 Advanced PD Analyzer – Full PRPD and PRPS display with four sensor inputs (HFCT, TEV, ultrasonic, UHF). Features auto-classification algorithms that suggest the most likely PD type based on the measured pattern. Stores PRPD snapshots with GPS location data for asset-level trending.
• SC Series PD Detector – Basic PRPD display on a handheld instrument. While the SC Series does not offer the full PRPS analysis of the HPD500, it provides real-time PRPD patterns that allow engineers to distinguish PD types during routine surveys.
• Report generator software – Included with both instruments, enabling PRPD screenshots and trend graphs to be incorporated into inspection reports.

Field Case: Pattern Analysis in Action

A Colombian hydropower operator detected elevated TEV readings (18-22 dB) on their 34.5 kV generator circuit breaker panel during a routine survey. The SC Series PD Detector showed a PRPD pattern with asymmetrical pulses concentrated at 50-80 degrees on the positive half-cycle and 230-260 degrees on the negative half-cycle — the classic surface discharge fingerprint.

Based on the PRPD pattern, the team identified surface tracking on the bushing insulator as the likely cause. An inspection during the next planned outage confirmed contamination tracking on the epoxy bushing surface. The bushing was cleaned and coated with anti-tracking silicone. Follow-up PRPD measurements showed a clean pattern with no PD activity. The pattern-based diagnosis avoided unnecessary switchgear disassembly and saved approximately 8 hours of maintenance labor.

Read more about PD detection methods for switchgear in our detailed comparison guide.