MODULATION OF PARTIAL DISCHARGE ACTIVITY IN GIS INSULATORS BY X-RAY-IRRADIATION

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 0.01 pC is achievable at voltages up to 400 kV. The system has been used to test over 340 GIS epoxy insulators (138 kV class, 550 kV BIL) with embedded electrodes and manufactured with a range of controlled defects. Under X-ray irradiation, the PD pulse repetition rate is increased and the discharge inception voltage (DIV) is sharply reduced. For example, the DIV of these insulators under X-ray irradiation was recorded at values as low as 25 kV although some samples could not be brought into discharge at 400 kV without X-rays. The modulation effect of X-rays on PD activity permits the location of the discharge site. In general, pulse-height and pulse-phase patterns are more tightly clustered with X-ray irradiation, but the patterns display a broad range of features depending on the type of defect, voltage and time on voltage. The reduction in the DIV has a practical industrial application, because acceptance testing of insulators could be performed at much reduced voltages, while providing a more searching test.