Estimation of flashovers in the EHV/UHV lines on the east coast due to lightning produced by the Bay of Bengal cyclones

Abstract

Electric Power is imperative for the modern world. The quality and reliability of the power supply can be impaired by several factors. Lightning is one such natural threat that disrupts the continuity of supply and can even affect the stability of the power grid. Cyclones can produce many ground flashes in a matter of a few days. The Bay of Bengal produces a considerable number of cyclones that invade the East Coast of India. They can cause structural damage to the lines & substations. On the Extra High Voltage (EHV) and Ultra High Voltage (UHV) grids, the line flashovers dominate over the structural damages. However, no serious effort has been made to estimate the possible number of such flashovers. The present work aims to fill this serious gap. Estimating lightning-induced flashovers requires several aspects, both electrical and cyclone-related. The lightning strike could be intercepted by the tower/ground wire, or it can strike the phase conductor. The electro-geometric model (EGM), suggested in IEEE standards, is employed to assess the normalized number of shielding failure strokes and that intercepted by the tower/ground wire. The associated probabilities are also estimated for typical EHV and UHV lines. The simulations are carried out by modeling the lines in EMTP with a multi-story model for the tower, and the voltage rise in the system is evaluated. Using this information and the Basic Insulation Level (BIL) of the line, the possibility of flashovers is assessed. The cyclone's trajectory, the speed, and the number of lightning flashes produced by them are assimilated from different sources. Modeling the cyclone as a disc-like structure, the line length shadowed as a function of time is calculated. In addition, the equivalent ground flash density per square km per hour is also calculated. Combining all this information, the possible number of lighting-induced flashovers in the EHV/UHV grid along the East Coast is estimated. For a few selected cyclones during the period of five years in the last decade, it amounted to tens of flashovers in 400 kV lines and hundreds of flashovers in 220 kV lines. For engineering purposes, the maximum number of possible flashovers is required. Based on the maximum number of ground flashes per hour across the cyclones for five years, it was estimated. It amounts to a few tens of flashovers in the 765 kV grid, a few tens to a few hundred in the 400 kV grid, and for the 220 kV grid, it amounted to a few hundred to a few thousand. These are quite large for any power system to withstand. This work, which is the first of its kind, would be useful to both system operators and planners.