Partial Discharge Testing in Turbine Generators: Interpretation and Follow-Up Actions

Partial discharge (PD) testing is one of the most powerful diagnostic tools available for assessing stator insulation condition in turbine generators. PD activity is the primary mechanism by which stator insulation degrades over time — and PD testing can detect this degradation before it progresses to a failure event.

Despite its diagnostic value, PD test results are frequently misinterpreted or used without adequate context. This article explains what PD testing measures, how to interpret results in context, and what follow-up actions the data supports.

What Partial Discharge Testing Measures

Partial discharge is an electrical discharge that occurs within the insulation system without completely bridging the insulation gap. PD activity occurs when the local electric field exceeds the breakdown strength of a void, delamination, or contamination site within the insulation. Each discharge event erodes the insulation locally, and over time, this erosion progresses to complete insulation failure.

PD testing measures the magnitude and pattern of these discharge events. The primary outputs are the peak PD magnitude (Qmax), the number of discharge events per cycle, and the phase relationship of the discharges to the applied voltage waveform (the phase-resolved PD pattern).

Online vs. Offline Partial Discharge Testing

PD testing can be conducted online (with the machine in service) or offline (with the machine de-energized and the stator winding energized from an external source). Each approach has distinct advantages:

• Online PD testing measures PD activity under actual operating conditions, including the effects of operating temperature, load, and mechanical stress. It can be conducted without an outage and provides data that reflects the machine's actual operating environment.
• Offline PD testing provides more controlled test conditions and allows for more detailed characterization of PD sources. It requires an outage but can provide more definitive information about specific PD sources.

Interpreting PD Test Results

PD test results should be interpreted in context — not against absolute thresholds. The most important contextual factors are: the trend relative to previous tests (is PD activity increasing?); the phase-resolved PD pattern (what type of PD source is indicated?); the machine's age and operating history; and the relationship between PD activity and operating conditions (load, temperature, humidity).

Follow-Up Actions

The appropriate follow-up action depends on the severity and trend of PD activity. Options range from increased monitoring frequency (for stable, low-level PD activity) to offline testing for source characterization (for elevated or increasing PD activity) to accelerated rewind planning (for severe PD activity indicating advanced insulation degradation).