PARTIAL DISCHARGES IN HV MACHINES - INITIAL CONSIDERATIONS FOR A PD SPECIFICATION

Partial discharge (PD) specifications exist for all types of electrica l equipment except motors and generators. Whereas specifications essen tially require that the equipment is discharge-free the stator conduct or 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 th erefore particularly important that the problem of overcoming the atte nuation between the PD site and the measurement equipment at the windi ng terminals be solved. Within the general context of examining the fe asibility of a PD specification, the problem of PD signal attenuation is addressed. The approach adopted was to seek a region of low attenua tion by examining the whole of the possible range of PD measurement fr equencies. The investigations described consist of experiments on a fu ll-size 500 MW endwinding model and on two large high-voltage motor wi ndings. Careful screening to reduce radiative pick-up from the calibra tion generator simulating a PD source and a direct earthing scheme ena bled the essential elements of PD pulse transfer through a winding to be determined. The frequencies investigated ranged from kHz to GHz. Pu lse 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 significan t crosscoupling was apparent through the end-winding region. The windi ngs acted as a low-pass filter, the cut-off frequency being dependent on the winding length, and being below 1 MHz. Consequently, it is conc luded that it should be possible to measure PD, using equipment with a passband below the low-frequency cut-off, with low attentuation.