21C capacitor bank protection

For capacitor bank protection design, protection engineers frequently rely on the established voltage differential protection (87V) in feasible applications. This protection scheme aims to detect faults in the Shunt Capacitor Banks (SCB) by measuring a ratio of voltages between two measurement points in the capacitor bank.

Failed capacitor elements, as well as rack faults, cause a change in measured voltages, resulting in a change of ratio. Impedance-based protection for capacitor banks (21C) is proposed to overcome some drawbacks of voltage differential protection (87V) within different capacitor bank configurations or even high tolerance of the measurement of input voltage in protection relays. More specifically, to be more fault tolerant in fuseless capacitor banks. The impedance protection on capacitor banks measures the operating complex impedance of the capacitor bank. Under healthy conditions, this impedance remains stable. A change in the elements of the capacitor bank, for example, a blown fuse, will change the impedance of the capacitor bank. This change is detected by the 21C impedance protection function.