Charge deposition on an insulating spacer under impulse-voltage conditions

The authors describe a study of surface-charge deposition on insulating spacers under impulse-voltage conditions, using an electrode arrangement which allows scanning of the entire surface of the spacer with a capacitive probe. For a plain cylindrical spacer in SF₆ at 1 bar, significant charge deposition was recorded for impulse-voltage levels greater than approximately 70% of the gas breakdown voltage, with the charge density reaching maximum values at the electrode-gas-dielectric triple junction of approximately 20 mu C/m². This has been shown to be consistent with the onset of ionisation in gas gaps of a few tens of mu m at triple-junction defects. With a large ( approximately 1 mm) gas gap formed by a circumferential groove at the triple junction, the maximum charge density was increased to approximately 35 mu C/m² and the charging was almost uniform because of multiple discharge initiation. All triple-junction-initiated discharges deposited charge of the same polarity at the adjacent electrode (homocharge). It is important to control charging processes not connected with triple-junction phenomena when inserts are used.