Significance of switching impulse breakdown voltage characteristics in voltage uprating

The voltage rating of overhead lines is determined by a combined consideration of the stress applied to the line and its electrical strength. In this investigation, the voltage uprating of an existing `L3´ 275 kV transmission line to 400 kV is explored. Minimum air clearances for 275 kV and 400 kV lines are first reviewed and the IEC recommendations for insulation coordination were considered. The importance of switching impulse breakdown voltage characteristics in determining the required electrical clearance has been highlighted through a detailed review of published literature of breakdown characteristics of long air gaps. Based on the data obtained, a new simplified empirical equation is derived which allows determination of air clearances to suit switching and lightning breakdown voltage characteristics of lines.