Study of Partial Discharges at the Positive and Negative Voltage Flanks of Semi-Square Voltages

Experiences from the use of a measuring system for detection of partial discharges (PDs) at a repetitive, unipolar, square-like voltage with a rise time of 2.0 mus, are here presented. PDs were detected during the major part of the voltage flanks, even when the duty cycle of the applied voltage was not completely stable. The voltage was applied for relatively long time and one PD appeared at each voltage flank. As the measurement progressed however, the time intervals within which the PDs appeared shifted. This indicates that comparisons of results from measurements performed at e.g. different rise times or voltage levels can be difficult, due to the inherent stochastic variations in the PD appearance. One example of a time interval where the PDs at the negative voltage flank all appeared during the falling part of the voltage is also shown. Under such circumstances, it is important to be able to measure the PDs also during the voltage flanks to avoid that the test object is considered to be PD free.