Limitations of Attempting Calibration of Partial Discharge Measurements in VHF and UHF Ranges

There is an obvious desire to assign units of charge (pC) to on-site/on-line partial discharge (PD) measurements made using radio-frequency (RF) techniques by employing some means of calibration in a way similar to that defined in IEC 60270. While moving just a few MHz past the IEC 60270 mandated maximum cut-off frequency (1 MHz) may still allow the principle of quasi-integration to hold, and thus allow a valid charge-based calibration, this is strongly dependent on the test object and test circuit. However, at higher (RF/VHF/UHF) frequency regions in which PD diagnostics are often being made today on GIS and HV transformers, physical effects profoundly affect the RF signal generated at the PD source and its path to the receiving sensor, thus directly impacting the received RF signal strength. We explain the problems inherent in attempting charge calibration at these higher frequencies by looking at the fundamental difference in how the measurement methods acquire the PD signal, and we point out the relative unimportance of charge level for certain critical defects.