Partial Discharges Pulse Shape Analysis at AC and DC

Partial discharge (PD) signal analysis is a well-known tool for monitoring the conditions of insulation systems of electrical machines and equipment. Classical analyzes of PD are mostly based on the evaluation of the phase angle of the test voltage at which partial discharges occur. At the present, the partial discharge detection is mostly based on the measurement of charging currents that are needed to resume the voltage on the sample that was there before the partial discharge occurred. Other electrical methods are, for example, the scanning of an electromagnetic field emitted during a partial discharge. Another developed methodology in the field of measuring and analyzing partial discharges is the monitoring of pulse parameters of the detected discharge. Experiments suggest that measuring the rise and fall time of the pulse edges and amplitude measurements can evaluate the partial discharge signal. Especially this method could be very helpful at the moment when partial discharges at DC are studied and evaluated. The disadvantage of partial discharges at DC is the fact there is no phase and phase angle position for PD patterns recognition. Thus, another evaluation method development is necessary for PD recognition DC. There is presented in the study an experiment where the PD pulse shapes, in the manner of rise time and fall time, were investigated at laboratory PD models at both voltage types AC and DC. The PD models simulated corona discharges. Each individual pulse of a partial discharge can be detected by a suitable scanning impedance and viewed with fast digital oscilloscopes. As has been said, there is a presumption that different discharge arrangements or even variously degraded insulating materials have different waveforms of the sensed pulses. The main monitored parameters of individual pulses are their rise time and amplitude. Since the occurrence of partial discharge is essentially a random phenomenon, statistical analysis of a significant amount of data is required. A view of the behaviour of the partial discharge pulse in various cases of the discharge activity brings different results in the observation of the pulse rise time and its amplitude in the various observed setups. Monitoring the rise time and pulse amplitude is one way of evaluating partial discharges, regardless of their superimposed position on the sine wave of the supply voltage. Thus this PD monitoring could be a part of PD evaluation at DC. These pulse parameters together with other PD analysis at DC (as pulse sequence analysis) could be used to estimate the partial discharge type as well as to evaluate the deterioration of the insulation of electrical machines.