CHARGE DEPOSITION ON AN INSULATING SPACER UNDER IMPULSE-VOLTAGE CONDITIONS

The paper describes 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). When metal inserts were used to shield the triple-junction region, there was negligible prebreakdown activity but, when ionisation did occur (as at breakdown) the normal component of field associated with the inserts resulted in the deposition of large quantities of charge of polarity opposite to that of the inserts (heterocharge). As this condition reduces the strength of the spacer against subsequent impulse-voltage applications, it is important to control charging processes not connected with triple-junction phenomena when inserts are used.