Modeling of lightning impulse behavior of long air gaps and insulators including predischarge current: Implications on insulation coordination of overhead transmission lines and substations

• Accurate estimation of flashover characteristics of external insulation requires the consideration of predischarge current.
• Including predischarge current in simulations yields higher estimates of minimum backflashover current of overhead lines.
• Including predischarge current in simulations results in less severe backflashover overvoltages arising at substations.
• The minimum shielding failure flashover current of overhead lines is not affected by simulating predischarge current.
• Use of CIGRE leader development model is suggested for the prediction of lightning impulse behavior of insulator strings.

Evaluation of the dielectric strength of transmission line insulation subjected to fast-front overvoltages is of major importance for the insulation coordination of overhead lines and connected substations. Among models proposed in literature for the prediction of the dielectric behavior of long air gaps and insulators under lightning overvoltages, leader development models (also called leader progression models) have a greater physical significance. These models consider the predischarge current flowing in the gap during the leader propagation phase preceding breakdown. However, this current is often disregarded for simplicity in fast-front overvoltage simulations. In this study the effects of simulating predischarge current on voltage–time characteristics of long air gaps and insulators, critical currents of overhead transmission lines as well as fast-front overvoltages arising at substations are investigated with the aid of ATP–EMTP. Including predischarge current in simulations, affecting the flashover characteristics of air gaps and insulators, results in slightly higher estimates of the minimum backflashover current of overhead transmission lines. However, it does not affect the estimated minimum shielding failure flashover current of overhead lines. In addition, simulating predischarge current may affect the overvoltages arising at substations due to shielding failure of the connected overhead lines depending on withstand or flashover of line insulation. In the case of backflashover the wavefront steepness and amplitude of the overvoltages are lower. Thus, predischarge current effects should be considered in insulation coordination of overhead transmission lines and substations.