X-ray modulation of PD activity in GIS insulators: application to acceptance testing

A system has been assembled which can perform high-sensitivity partial discharge (PD) measurements under combined electric stress and X-ray irradiation. Sensitivity down to 0.02 pC is achievable at voltages up to 500 kV. The system has been used to test over 250 GIS (gas-insulated switchgear) epoxy insulators (138 kV class, 550 kV BIL) with embedded electrodes and made with a range of controlled defects. Under X-ray irradiation in PD pulse repetition rate is increased and the discharge inception voltage (DIV) is sharply reduced. As a result of the PD modulation, location of the discharge site can be determined. Pulse height and pulse phase patterns are tightened with X-ray irradiation but display a broad range of features depending on the type of defect, voltage, and time on voltage. The reduction in DIV has practical industrial application as acceptance testing of insulators could be performed at much reduced voltages, while providing a more searching test