Partial discharges in HV machines; initial considerations for a PD specification

Partial discharge (PD) specifications exist for all types of electrical equipment except motors and generators. Whereas specifications essentially require that the equipment is discharge-free the stator conductor bar insulation of the motors and generators under consideration is micaceous and has excellent PD resistance. Consequently, the intention in a specification would be to permit a certain level of PD. It is therefore particularly important that the problem of overcoming the attenuation between the PD site and the measurement equipment at the winding terminals be solved. Within the general context of examining the feasibility of a PD specification, the problem of PD signal attenuation is addressed. The approach adopted was to seek a region of low attenuation by examining the whole of the possible range of PD measurement frequencies. The investigations described consist of experiments on a full-size 500 MW endwinding model and on two large high-voltage motor windings. Careful screening to reduce radiative pick-up from the calibration generator simulating a PD source and a direct earthing scheme enabled the essential elements of PD pulse transfer through a winding to be determined. The frequencies investigated ranged from kHz to GHz. Pulse transmission and frequency spectrum measurements were made. It is concluded that the PD pulse transmits essentially as a travelling wave, the high-frequency components. being highly attenuated. No significant crosscoupling was apparent through the endwinding region. The windings acted as a low-pass filter, the cut-off frequency being dependent on the winding length, and being below 1 MHz. Consequently, it is concluded that it should be possible to measure PD, using equipment with a passband below the low-frequency cut-off, with low attenuation.