Band width limits in wide band partial discharge calibration and corresponding countermeasures

Partial discharge (PD) measurement method defined in IEC60270 only captures the low frequency energy of measured signals, while more than 95% of the energy is distributed in the range of 30kHz~30MHz. Thus a high frequency wide band measurement method becomes fashionable to cover the entire bandwidth. In such frequency range, the influence of stray inductance and capacitance become dominant, especially in the conventional calibration process, where the calibration pulses are directly wired to the bushing through long single wires. In IEC bandwidth, this is not a problem since frequency is low. In high frequency, it makes the recorded calibration charge much smaller than the named quantity. Thus the calibration process becomes meaningless. This paper has studied such influence in a real 110kV transformer bushing through circuit analyses and real measurement. An innovative co-axial sensor is presented and shows good frequency response and measurement accuracy.