Partial discharges measured at semi-square voltages

The increased use of power electronic devices in power systems cause voltage wave forms containing considerably higher frequency components than 50 Hz or 60 Hz. In such cases, detection of partial discharges (PDs) becomes difficult and the conventional approach for detection no longer works. Studies of square-like voltages have earlier shown that PDs occur during the voltage rise. Therefore, it is important to be able to measure also in this region, to understand the behaviour of the PDs. In this study, PDs were detected at so called "semi-square voltages" of two different voltage levels, in a wide range of rise times. Distributions of the voltages where PDs occurred are discussed. The PD signal was detected using a coupling device consisting of a low order high pass filter, a high resolution digitizer and software processing of the digitized records. This technique enables wideband PD detection very close to the switching instant. Subtraction of an accumulated sum of earlier traces of the PD signal improved the detection possibilities. The high voltage applied to the test object could also be reconstructed from a signal detected at the coupling device.