Pre-breakdown field calculations for charged spacers in compressed gases under impulse voltages

The charges accumulating on the surface of a cylindrical insulating spacer placed in a rod-plane electrode arrangement in compressed SF₆ have been measured in situ. Lightning impulse voltages of positive polarity were used, and tests with gas pressures of 0.1, 0.2, and 0.3 MPa were conducted. Air and nitrogen were also used for purposes of comparison. The voltage distribution and electric field along the spacer surface were calculated using a simplified surface-charge simulation method. The possible role of these charges in the occurrence of flashover and in determining its path is discussed. The deleterious effect of areas on the spacer holding charges for long periods of time after a series of impulse voltages is considered