Fundamentals of adaptive protection of large capacitor banks

Abstract

This paper derives correct balance equations for short circuit protection of shunt capacitor banks taking into account inherent unbalances in the protected bank. Four methods are derived: voltage differential, compensated neutral voltage unbalance, phase current balance, and neutral current balance. As can be seen from key equations the proper way of balancing the bank (or banks) involves instantaneous values of currents or voltages. Subtracting the residual unbalance as a time-delayed signal (a historical, or a constant value), and responding to the delta changes does not constitute a proper, sensitive and secure operating equation for protective relaying purposes. The methods presented in this paper compensate for both bank and system unbalances. Therefore they are insensitive to major system events such as close-in faults. Presently used relaying techniques might misoperate on such system conditions, as they typically disregard system unbalances and compensate for the bank unbalance assuming no, or minor system unbalances.