PD measurement at Large Capacitors Using a Modified HFCT

In many measurement technique applications (high voltage engineering, pulse power electronics, electromagnetic compatibility a.s.o.) it is necessary to measure some current pulses with ns durations, superimposed on periodical currents with different shapes or frequencies. When detecting partial discharges (PD), short duration impulses are superimposed on a low frequency (usually power line frequency) carrier signal. If High Frequency Current Transformers (HFCTs) are used to measure PD in large capacitors or capacitor banks, the low frequency component of the complex signal has such amplitude that it leads to magnetic circuit saturation associated with measurement errors. This paper shows how to modify a HFCT so that its sensitivity automatically changes with the measured signal frequency to avoid the saturation at low frequencies. For this purpose, a passive current filter connected in parallel to the HFCT output significantly reduces its sensitivity for a preset low frequency band, so that the nominal sensitivity at high frequencies within the transformer measuring range remains unchanged. The filter design, its practical implementation and laboratory experiments validating the proposed solution are described in the paper.