Generator and motor stator windings operate under electrical, thermal, mechanical and environmental stress, and their insulation deteriorates over time. Partial discharge (PD) testing is the established online method for assessing stator insulation health – it detects the discharge activity of incipient defects while the machine stays in service. This article explains the common measurement approaches and how to interpret the results.
Why PD testing for stator windings
Stator winding insulation degrades through several mechanisms – thermal ageing, load cycling, contamination, and vibration damage at the end-windings. These mechanisms produce measurable PD well before a failure. Detecting the rise in PD activity allows the maintenance planner to act while the machine is still healthy enough to schedule work.
Measurement approaches
Coupling capacitors (the classic online method)
High-voltage coupling capacitors are connected to the phase terminals, and PD pulses are measured across them. This is the conventional method per industry practice, with the capacitor providing the high-frequency path to the PD detector. It is effective on machines with accessible terminals and is the approach behind many installed online PD monitoring systems.
HFCT sensors at the neutral and ground connections
High-frequency current transformers clamp onto the neutral-to-ground connections or the winding earth leads. They pick up PD pulses flowing to ground, providing a practical alternative that avoids connecting a capacitor to the phase bus.
Offline and bar-to-bar testing
For scheduled outages, offline PD tests – applying voltage from an external source – and bar-to-bar measurements on individual bars allow direct comparison between bars and detection of weak insulation on specific coils. These methods are used during major overhauls.
Interpreting stator PD results
- Trends matter most – a single PD reading is less informative than the change over time. Rising amplitude or discharge rate on the same phase indicates progression.
- Compare phases – PD on one phase significantly above the others points to a specific winding region.
- Patterns identify mechanisms – phase-resolved PD patterns help distinguish internal voids, slot discharge and end-winding surface activity, guiding the inspection focus.
From measurement to maintenance action
- Establish baseline PD levels at commissioning or first test.
- Test periodically (online where possible) and record trends per phase.
- Escalate rising activity to targeted inspection: end-winding examination, contamination check and cleaning, or re-tightening.
- For critical machines with rising trends, move to continuous monitoring.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) supplies PD detection instruments applicable to rotating machines – handheld PD detectors with coupling and HFCT inputs, and online PD monitoring components – supporting utilities and service companies worldwide with OEM/ODM supply and application guidance. Contact the HUWOR sales team for a stator PD testing configuration.