Negative Impulse Ground Wire Corona Parameters for Backflash Evaluation of High Voltage Transmission Lines

Ground wires (GW) struck by lightning are often exposed to transient impulse voltages in the MV range. This results in intense negative streamer-type corona, causing a considerable increase in the effective wire capacitance and a corresponding reduction in its surge impedance. Such variables have a significant influence on the tower top potential and on the electromagnetic coupling between the ground wire and phase conductors. Presently there is considerable uncertainty regarding the impulse corona parameters to be used in back flashover calculations. In the present paper a new method is formulated for determination of GW impulse corona parameters. The new approach is compared with two existing methods and with available experimental results. It is found that the method usually used within the IEEE has little physical justification and is excessively conservative. It is also shown that such corona parameters significantly depend on the GW height above ground, which has been totally ignored in the CIGRE method. This and other unjustified simplifying assumptions explain the surprisingly large discrepancy between values of the negative impulse corona constant recommended by CIGRE and IEC.