A Protection Circuit for the Shunt Capacitor equipped with a Series Inverter under Voltage Sags

This paper proposes a protection circuit for the shunt capacitor equipped with a series inverter (active SC) under a voltage sag in the distribution grid. The voltage sag causes the discharge of the shunt capacitors, resulting in a large inrush current that may damage the series inverter. The proposed protection circuit is composed of varistors connected in parallel with the inverters, and a series AC circuit breaker. When the large inrush current flows into the inverter, all of its power devices are turned off. Then, almost all of the inrush current flows into the varistors. As a result, the proposed method mitigates the inrush current of the inverter. Computer simulations are carried out to investigate the behavior of the active SC during balanced and unbalanced voltage sags. Simulation results demonstrate that the proposed protection circuit is able to mitigate the overcurrent in the inverter by a factor close to 2.5 under a balanced voltage sag. It was also verified that the circuit protects the inverter from unbalanced voltage sags caused by a single and double-line-to-ground faults.