21C Cap bank Protection

When designing the protection of capacitor banks, protection engineers resort to the well-known voltage differential protection (87V), wherever is feasible. This protection scheme aims to detect faults in the Shunt Capacitor Banks 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, from a blown fuse as an example, will change the impedance of the capacitor bank. This change is detected by the 21C impedance protection function.