High-impedance bus differential misoperation due to circuit breaker restrikes

This paper discusses the potential misoperation of a high-impedance bus differential relay when surge arresters are located within the relay zone of protection. Transient overvoltage caused by a circuit breaker restrike during shunt capacitor bank de-energization can cause the surge arresters to conduct. The relay interprets the current flowing through the surge arrester as fault current within its zone of protection and subsequently trips the bus. This paper reviews high-impedance bus differential protection principles and discusses circuit breaker design, voltage rating, and restrikes. Trapped charge on shunt capacitor banks is analyzed, and surge arrester design and operation are reviewed. This paper also analyzes real-world events that show relay misoperation due to circuit breaker restrikes and are validated by computer restrike simulations. This paper shows that a better understanding of transient overvoltages is essential to improving protection settings in order to minimize false trips while maintaining fast, secure, and sensitive bus protection.