Partial-Discharge Measuring Produces Uncertain Results: Important Quality Assesment Updates for Stator Windings

The petrochemical industry considers the offline partial-discharge (PD) measurement an important quality assessment for new medium-voltage motor and generator stator windings. A recent edition of American Petroleum Institute (API) Standard 541 [1] requires an instrument to be compliant with International Electrotechnical Commission (IEC) Standard 60270 [3] for PD measurements on sacrificial stator coils and proposes 100 pC for guidance on acceptance criteria until more data becomes available. Given the industry's faith in PD as an acceptance test based on a representative sample, it warrants closer scrutiny. The characteristic PD distribution pattern produced by each test instrument provides information on the origin of the discharges and, therefore, can be used to determine what type of defect has produced them. This article shows how external factors such as instrument selection, testing environment, and time interval between tests can influence PD test results.