THE TREND TOWARDS LARGE, fuseless shunt capacitor banks being employed in power systems is due to their lower cost, compact design, higher reliability with less exposed live parts, and lower losses. This article describes a novel application of the impedance method to provide complete bank protection by measuring the impedance of each series string of the capacitor bank. The scheme can identify the string(s) with an alarm or trip condition, keep the bank in service with, pack failures in multiple strings, and trip only when the bank is actually in an unacceptable operating condition. Because this scheme can account for the distributed failure of packs in strings, bank availability can be increased. The North American blackout of 14 August 2003 disrupted power from Toronto to Cleveland and from New York to Detroit. This particular event exhibited a large voltage dip to about 60% rated voltage. Preliminary results suggest a "voltage stability" or a "voltage instability" issue, depending on your particular viewpoint. Large capacitor banks, often similar to the ones described in this article, provide reactive vars to help support the voltage. These capacitor banks are of particular importance if large generators are tripped in the power system.
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