How to detect, interpret and manage partial discharge in 11 kV stator windings so you can plan repairs before catastrophic failure.
Partial discharge (PD) is a localised electrical discharge that only partially bridges the insulation between conductors. In high-voltage motors and generators operating at 6.6 kV, 11 kV and above, PD activity is one of the earliest indicators that stator insulation is ageing or stressed.
PD does not immediately mean imminent failure, but uncontrolled levels or rapidly increasing trends are strong warning signals. The objective of PD monitoring is to detect insulation degradation early enough to plan rewinding or refurbishment without suffering an in-service breakdown.
Common PD sources include voids in groundwall insulation, surface tracking in end-windings, contamination and moisture on insulation surfaces, and sharp edges or protrusions creating high local electric stress.
Manufacturing imperfections, poor impregnation (in older windings), thermal cycling and mechanical vibration can all contribute to the development or aggravation of PD sites over time.
Partial discharge can be measured using offline tests (during shutdown) or online monitoring while the motor is energised. Typical techniques include pulse current sensors, capacitive couplers on phase terminals and high-frequency current transformers (HFCT).
The key outputs are apparent charge (in pC or mV), phase-resolved patterns and trends over time. No single absolute value defines pass or fail; interpretation must consider machine design, operating voltage, service history and reference data from similar machines.
Rapidly rising PD magnitude over successive measurements, a sudden appearance of new PD patterns, or high activity concentrated in a particular phase or direction may indicate developing defects that cannot be ignored.
When PD is combined with other negative indicators—such as decreasing insulation resistance/PI values, visible tracking in end-windings or abnormal thermal images—planning a rewind or major refurbishment becomes a high priority.
At RKELC, PD results are never interpreted in isolation. Our engineers combine PD signatures with IEEE/IEC-compliant electrical tests (IR, PI, tan-delta, surge), visual inspection and operating history to build a complete picture of insulation health.
For motors and generators with concerning PD levels, we recommend a clear action plan: continued monitoring with shorter intervals, targeted repairs (e.g. end-winding re-bracing and cleaning) or full stator rewinding at the next major outage, depending on risk and criticality.
Whether you are planning a maintenance strategy, investigating a failure, or building a predictive maintenance programme, RKELC can support you with NABL-accredited testing, workshop repairs, engineering studies and on-site services across power generation, oil & gas, metals & mining, marine and process industries.
Key Takeaways
Essential insights and actionable points extracted from the article for quick reference.
Partial discharge (PD) is a localised electrical discharge that only partially bridges the insulation between conductors. In high-voltage motors and g...
Common PD sources include voids in groundwall insulation, surface tracking in end-windings, contamination and moisture on insulation surfaces, and sha...
Partial discharge can be measured using offline tests (during shutdown) or online monitoring while the motor is energised. Typical techniques include ...
Rapidly rising PD magnitude over successive measurements, a sudden appearance of new PD patterns, or high activity concentrated in a particular phase ...