Partial Discharge

What Is Partial Discharge?

Partial discharge is a localized dielectric breakdown that crosses a portion of the insulation between two conductors. The discharge bridges part of the insulating gap and stops before reaching the opposite electrode, so no fault current flows through the full insulation path and the equipment continues to operate.

That incomplete path separates it from flashover, in which insulation fails outright and a conductive channel connects the electrodes. Discharge instead occurs in three settings: in voids or delaminations inside solid insulation, along insulation surfaces, and at sharp conductor points in gas, where the effect is known as corona. Each event deposits energy into a small volume of material, and the damage compounds with repetition.

Why Partial Discharge Matters

Partial discharge (PD) is both a symptom and an accelerator of insulation degradation. The discharge signals that a defect already exists; at the same time, each event erodes the surrounding material, carbonizes pathways through the insulation, and enlarges the defect. Given enough repetitions, the eroded path eventually bridges the electrodes and the insulation fails.

The equipment affected covers much of a plant's electrical infrastructure: metal-clad switchgear, power transformers, rotating machine windings such as motor and generator stators, and cable terminations and joints. These assets typically sit at the head of the distribution system, so their condition shapes the reliability of the supply downstream.

The escalation pattern follows a consistent sequence: discharge activity appears at a defect, the defect grows as discharges repeat, insulation strength declines, and the asset eventually suffers equipment failure that can trigger unplanned downtime. Because the first stage is measurable long before the last, partial discharge testing has become a standard condition assessment technique for medium- and high-voltage equipment.

The Three PD Mechanisms

Partial discharge takes three main forms, distinguished by where the discharge occurs within the insulation system. Identifying the mechanism matters because each one progresses at a different rate and points to a different root cause.

Internal discharge takes place inside voids, delaminations, or gaps within solid insulation such as epoxy, cast resin, or oil-paper systems. These cavities form during manufacturing or develop through thermal aging and mechanical stress. Surface tracking runs across the boundary between insulation and the surrounding medium, most often on contaminated or moist bushings and terminations. Corona occurs in gas at sharp conductor points, where the local electric field concentrates enough to ionize the air.

How Partial Discharge Is Measured

The conventional measurement reports apparent charge in picocoulombs (pC). Apparent charge is the charge measurable at the equipment terminals that corresponds to the discharge inside the insulation, and it serves as the standard quantity for comparing readings across tests. The IEC 60270 standard defines the conventional offline procedure: the asset is de-energized, connected to a coupling capacitor and measuring impedance, and excited at rated or reduced voltage while discharge pulses are recorded.

Online methods measure discharge while equipment remains energized. Transient earth voltage (TEV) sensors detect the voltage transients that discharge pulses induce on the metal enclosure of switchgear. High-frequency current transformers (HFCT) clamp around cable earth connections to capture discharge current pulses. Ultrasonic and acoustic sensors pick up the sound that discharge events produce, and ultra-high-frequency (UHF) sensors capture the electromagnetic emissions from discharge inside shielded compartments such as gas-insulated switchgear.

Online testing fits naturally into a predictive maintenance program, since it gathers condition data without taking critical assets out of service.

PD Patterns and Interpretation

Raw pulse data becomes usable through phase-resolved analysis. Phase-resolved partial discharge (PRPD) patterns plot discharge magnitude against the phase angle of the AC cycle, and different defect types produce recognizable shapes: internal voids, surface tracking, and corona leave distinct signatures in the pattern.

Trending carries more weight than any individual measurement. A single reading reports the state at one moment; a series of readings shows whether activity is stable, intermittent, or growing. Rising magnitude, a higher pulse repetition rate, or discharge that appears at lower voltage indicates an accelerating defect and calls for shorter retest intervals.

Interpretation also requires separating true discharge from electrical noise. Variable-speed drives, SCR switching, radio transmitters, and loose connections generate signals that can mimic PD on some sensors. Experienced analysts cross-check with multiple sensor types, look for consistency in phase position, and confirm that pulses repeat predictably before classifying a source as discharge.

Practical Program Guidance

A partial discharge program starts with a baseline survey of medium- and high-voltage assets. The survey records each asset's initial signature, giving later tests something to compare against. Assets then move onto a periodic retest cycle, and assets showing activity get shorter intervals and closer attention.

Electrical PD testing works best alongside other inspection techniques. Infrared analysis reveals the heating that discharge and other defects produce, acoustic inspection catches audible and ultrasonic emission, and continuous asset condition monitoring fills the gap between scheduled tests. Combining methods reduces the chance that a single technique misses a developing fault.

Partial discharge testing is specialized work. Measurements on energized high-voltage equipment require qualified personnel, appropriate safety clearances, and instruments suited to the measurement type. Many plants bring in specialist contractors for offline IEC 60270 tests while running simpler online surveys with in-house teams.

Frequently Asked Questions

The Bottom Line

Partial discharge is an early, measurable warning that insulation in switchgear, transformers, machine windings, or cables is deteriorating. IEC 60270 testing quantifies the condition offline in picocoulombs, while TEV, HFCT, ultrasonic, and UHF methods keep energized assets under observation. Treat the results as a trend, because a defect whose activity grows between tests deserves action before a stable one does. Baseline the MV/HV fleet, retest on a cycle, and pair electrical PD testing with thermal and acoustic inspection for the fullest picture of insulation health.