Improvements to partial discharge screening test for stator bars and coils

In 1991 and 1992, the authors published the description of a circuit for measuring partial discharge (PD) associated with individual stator bars and coils using a commercial single channel pulse height analyzer together with a sensing system consisting of an 80 pF coupling capacitor terminated in 50 ohms at the instrumentation. The sensing system has a theoretical lower -3 dB point of 40 MHz, while the bandwidth of the instrumentation extends to 100 MHz. The advantage of such a system over measurements at lower frequency (<1 MHz) is that the response to external discharges on the end arms is reduced, rendering the system preferentially sensitive to PD in internal voids and delaminations. The purpose of this paper is to describe changes to improve the high frequency response of the circuit and the repeatability of the method.