Design and Simulation of Back-to-Back Transient of Shunt Capacitor Switching

Abstract

Back-to-back switching of capacitor banks, a condition occurring when an energized bank is connected to a bus with other banks already in service, generates some of the most severe inrush current transients in power systems. This study investigates this phenomenon through a design and simulation approach utilizing the Alternative Transients Program (ATP/EMTP). A comprehensive model of a 34.5 kV substation, incorporating two distinct capacitor banks (18 MVAR and 10 MVAR), was developed to meticulously analyse the transient behaviour during switching events. Simulation results demonstrated a peak inrush current exceeding 100 kA, characterized by a natural oscillation frequency of roughly 4.6 kHz. Validation against analytical calculations confirmed a strong correlation in frequency, although the simplified analytical model significantly underestimated the peak current by around 50%. The practical implications of these transients, involving severe electromechanical stresses on circuit breakers, potential damage to capacitor units, and increased risk of flashovers, are discussed. The findings underscore the critical necessity of implementing mitigation strategies, such as inrush current-limiting reactors, particularly in multi-bank installations, to ensure system reliability and equipment longevity.