Frequency dependent PD-pulse distortion in rotating machines

In this contribution, the authors present results from investigations on the distortion of PD pulses in the stator winding of a 10 kV asynchronous motor. In order to determine the distortion of PD pulses from different, well defined parts in the winding, the following two techniques were used simultaneously: (a) ultra wide band detection in the frequency range of 20-400 MHz by means of stator slot couplers under the stator wedges, in order to localize and quantify the discharges; and (b) high frequency measurements in the frequency range of 1-100 MHz by means of high frequency current transformers at the machine terminals. The results show a variation of the attenuation of the discharge pulses inside the machine of about 20 dB highest for pulses from the far end, i.e. the neutral point. The capability of exact localization of the discharges in the winding gives a correct measure of the range of the current transformer based detection method, when being applied to rotating machines. The results are discussed with regard to the practical application of PD detection systems on rotating machines, particularly considering aspects of range and applicability of systems in the HF ranges.