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In a previous post, we demonstrated how impedance-based protection (21C) is a perfect complement to the more traditional voltage-differential protection (87V). Impedance-based protection is providing a more natural feedback to its users and can be easily used in a condition-based monitoring program for maintenance.

However, one of the drawbacks sometimes raised about this protection scheme is that bank capacitance varies throughout the day...

When designing the protection of capacitor banks, engineers mostly resort to the well-known voltage differential protection (87V). This protection scheme aims to detect faults in the bank by measuring a ratio between two measurement points in the bank. Failed capacitor elements, as well as rack faults, cause a change in measured voltages, resulting in a change of ratio. Depending on the amplitude of this change, the protection relay can issue an alarm to SCADA or a trip signal to the bank breaker...