Measurement and Analysis of Partial Discharges at Semi-Square Voltages

Licentiate thesis, 2007

Insulation is today being exposed to voltages having considerably higher frequency components than the conventional power frequency AC 50 Hz or 60 Hz, for example in motors supplied by adjustable speed drives based on pulse width modulated converters. This has in many cases caused unexpected behaviour of the insulation systems. One factor influencing the insulation is partial discharges (PDs), and the occurrence of these also indicates the condition of the insulation. There are therefore several reasons why PD measurements at these kinds of voltages are desirable. Measuring PDs at voltages with high frequency contents is however difficult. Conventional methods that rely on the large difference in frequency contents between the PDs and the applied voltages can no longer be utilized. In this thesis, a measuring system that enables PD detection at rapidly changing voltages is presented and measurements performed at square-like voltages with exponentially rising and declining edges, so called semi-square voltages, are reported. Signals measured at a coupling device adapted for these voltages, connected in series with a coupling capacitor, are used for detection of PDs and reconstruction of the shape of the voltage applied at the test object. Considerable contributions from the applied voltage, remaining in the signal used for PD detection, can be handled due to the utilization of a high resolution data acquisition card. The measured signals are processed on-line. The contributions from the applied voltage are removed by subtraction of a signal consisting of an accumulated sum of earlier measured signals. In this way, the measuring system utilizes both the difference in frequency contents between the PDs and the applied voltages and the fact that the PDs appear with a variation in time, for extraction of the PD pulses. In the on-line processing of the measured signals, certain pertinent parameters characterizing each PD are acquired and used for subsequent statistical analyses. The measuring system has been employed for investigations of the influence of both the rise time and level of the applied voltage on the appearance of the PDs. As test object, a needle creating corona discharges was used. Since measuring is most difficult during the rising part of the voltage and since the PD appearing first during the rising part of the voltage is mostly affected by the rapidly changing voltage, the analyses are focused on the first PD at each voltage rise. The distributions of this both in time and in voltage at the test object, are studied and reveal that the PD behaviour is influenced by both rise time and level of the applied voltage. Further, space charges created around the needle appear to have considerable influence on the appearance of the PDs.