Novel PD Measuring System and Test Circuit for Operating Frequencies of 20 kHz

Inhalt:
Insulation systems are not ideal and have cavities or inhomogeneities. Inside these cavities partial discharges (PD) may occur, which result in degradation of the insulation. This phenomenon depends on the applied voltage waveform, and thus also on characteristics such as amplitude and frequency. The measurement of partial discharges at 50 Hz or 60 Hz is established and regulated in the IEC 60270. However, the specified conventional measurement technology has its limit at frequencies of several 100 Hz and cannot be applied in the kilohertz range. Certain unconventional PD measurement methods have other difficulties (IEC 62478). One approach to characterize an insulation system of an apparatus with operating frequency of e.g. 20 kHz is to perform PD measurements with 50/60 Hz and apply a scaling factor. These scaling factors are not known and usually base on experts experiences. This work presents a novel approach which features a new setup for the voltage supply and electrical PD measurements at high voltages with frequencies in the kHz-range (e.g. 20 kHz). The first part of this paper describes a power inverter combined with an oscillating circuit in resonance as supply for the high voltage in the kHz-range at adequate voltage quality. The second part shows the measurement setup harmonized with the voltage supply. This setup combines a high voltage and a low voltage circuit for filtering and measurement purposes. This method for electrical PD measurements enables calibration and thus detection of the apparent charge as in IEC 60270. The presented results show the applicability of PD measurements with this novel setup even in the kHz-range.