Online & Offline Partial Discharge (PD) Diagnostics in High Voltage Power Systems

1. What Partial Discharge Is and Why It Is the Best Early Warning

Partial Discharge (PD) is a localized breakdown inside or on the surface of insulation while the insulation as a whole still holds. It occurs where the local electric field at a void, contaminant, or sharp metallic edge exceeds the local dielectric strength. Each discharge releases only a small amount of energy, but the cumulative erosion of insulation material grows into an electrical tree and ends in flashover or complete puncture.

What makes PD so valuable to maintenance engineering is timing: it begins long before actual failure, often months or years, and radiates several detectable signatures — high-frequency electrical pulses, ultrasonic acoustic waves, electromagnetic emissions, and ultraviolet light. Measuring PD is effectively listening to insulation warn of its own impending failure, and no other test provides such localized early warning on energized high-voltage equipment.

2. Measurement Standards and Sensor Technologies

Electrical PD measurement in laboratories and acceptance tests follows IEC 60270, which defines apparent charge measurement in picocoulombs via a coupling capacitor circuit. Online field surveys instead favor unconventional, non-intrusive methods: Transient Earth Voltage (TEV) sensors on metal cladding, airborne ultrasonic sensors, High-Frequency Current Transformers (HFCT) clamped on earth conductors, and UHF sensors for GIS.

Each sensor type suits different defects — ultrasonics excel at surface PD and corona with an air path, TEV suits internal PD in metal-enclosed switchgear, and HFCT suits cables and accessories. A good field survey therefore combines several sensors and analyzes the Phase-Resolved PD pattern of pulses against the voltage phase angle to classify the defect type and separate genuine PD from electrical noise.

• IEC 60270 — electrical PD measurement in picocoulombs
• TEV — internal PD in metal-enclosed switchgear
• Ultrasonic — surface PD and corona
• HFCT — cables and accessories via earth conductors
• Phase-resolved patterns classify the defect type

3. Conducting an Online PD Survey in a Facility

A survey begins by recording background noise away from the equipment as a comparison baseline. Each switchgear panel is then scanned with TEV at multiple points on the cladding while ultrasonic sensors probe door seams, ventilation openings, and bushings. Any location reading significantly above neighboring panels or the noise baseline is marked and re-measured several times to confirm the signal is persistent rather than transient interference.

Suspect locations are then examined with a phase-resolved pattern analyzer, and severity is judged from signal magnitude, pulse repetition rate, and trend against previous surveys. The assessment drives prioritization — increased monitoring frequency, a planned internal inspection outage, or immediate correction. Every survey should systematically record locations, readings, and ambient conditions, because the real value of PD work lies in long-term trend comparison.

4. Limitations, False Signals, and Making PD Programs Effective

The main field challenge is interference from inverters, motor drives, certain lighting, and radio communications, all of which can mimic PD. Inexperienced surveyors can err in both directions — over-reporting until credibility is lost, or missing genuine PD buried in noise. Phase-resolved classification, multi-sensor correlation, and repeated measurements at different times are therefore essential skills.

Another limitation is that some PD sources, such as deep internal voids in solid insulation, radiate very little detectable signal externally, so a clean survey never guarantees defect-free equipment. Effective programs schedule regular surveys — typically once or twice a year for critical switchgear — combine results with thermography and maintenance history, and escalate to a de-energized internal inspection whenever a sustained rising trend appears, before the defect develops into flashover.

• Inverter and radio interference is the main interpretation trap
• A clean reading does not prove the equipment is defect-free
• Survey on a regular cycle and judge primarily by trend
• Escalate to internal inspection on a sustained rising trend