Partial discharge. XXV. Calibration of PD measurements for motor and generator windings-why it can't be done

In most types of high voltage apparatus, a PD acceptance test is required. In contrast, the PD test is very rarely used as an acceptance test for new motor and generator windings. The closest that machine manufacturers come to a PD acceptance test is the power factor tip-up test (IEEE Standard 286), which is an indirect PD test. The tip-up test is sensitive to PD sources when testing individual coils/bars but is relatively insensitive when testing complete windings, due to the presence of stress relief coatings. The lack of a direct PD acceptance test fundamentally results from the inductive nature of a stator winding. This article discusses the reasons for the lack of an absolute PD measurement criteria for stator windings, after a discussion on measurement methods and the quantities used to measure PD activity.