Recent developments in IEEE and IEC standards for off-line and on-line partial discharge testing of motor and generator stator windings

Partial discharges (PD) are small electrical sparks that occur in voids within electrical insulation, or on the surface of motor and generator stator winding coils. Offline and online PD testing has become a widely used tool for infactory quality acceptance and a baseline performance evaluation of new groundwall insulation in stator windings rated 6 kV and above. Many petrochemical plants and refineries perform online PD tests to periodically assess the condition of the stator winding insulation on machines during operation. IEEE and the International Electrotechnical Commission (IEC) have created standards called recommended practices or technical specifications to address offline and online PD measurement using electrical diagnostic equipment, and optical measurement of surface PD. They explain testing objectives and principles, discuss commonly-used methods for measuring PD, and provide guidance for data interpretation. Users of these standards should identify the PD test method and acceptance criteria, as the standards describe several suitable test procedures, but provide no acceptance criteria. This paper provides an overview and comparison of the standards, and addresses some common questions about offline and online PD tests.