CHAPTER 8: APPLICATION OF SETTINGS PROTECTION METHODS FOR CAPACITOR BANKSC70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 8-588.3 Protection methods for capacitor banks8.3.1 OverviewThe protection of shunt capacitor banks involves both bank and system protection schemes.Bank protection schemes are provided for faults within the capacitor bank itself. Bank protection include items such as ameans to disconnect a faulted capacitor unit or capacitor elements, a means to initiate a shutdown of the bank in case offaults that can lead to a catastrophic failure, and alarms to indicate unbalance within the bank.System protection schemes are provided to protect the capacitor bank from stresses caused by the system and to protectthe substation and system from stresses that can be caused by the operation of the capacitor bank. System protectionincludes items such as a means to limit overvoltage and excessive transient overcurrents, and to disconnect the bank inthe event of a major fault within the capacitor installation. System protection can also include alarms and methods todisconnect the entire shunt capacitor bank to prevent further damage to the capacitors due to abnormal systemconditions.In externally fused capacitor banks, several capacitor element breakdowns can occur before the fuse removes the entireunit. The external fuse operates when a capacitor unit becomes (essentially) short circuited, isolating the faulted unit.Unbalance protection removes the bank from service when the resulting overvoltage becomes excessive on the remaininghealthy capacitor units.Internally fused capacitors have individual capacitor elements within a capacitor unit that are disconnected when anelement breakdown occurs. The risk of successive faults is minimized because the fuse isolates the faulty element within afew cycles. Unbalance protection removes the bank from service when the resulting unbalanced voltage becomesexcessive on the remaining healthy capacitor elements or units.For fuseless or unfused capacitor banks, a failed element is short-circuited by the weld that naturally occurs at the point offailure. Unbalance protection removes the bank from service when the resulting voltage becomes excessive on theremaining healthy capacitor elements or units.8.3.2 Capacitor unbalance protection8.3.2.1 ConnectionsUnbalance protection utilizes the unbalance that occurs in a normally balanced capacitor bank to detect an abnormalityand initiate appropriate action. The most important function is to promptly remove the bank from service for any fault thatcan result in further damage. Capacitor unbalance protection is provided in many ways, depending on the capacitor bankarrangement.A variety of unbalance protection schemes can be applied with the C70 for internally fused, externally fused, fuseless, orunfused shunt capacitor banks, as shown in the rest of this section.The following figure illustrates a single capacitor bank, ungrounded, with no tap available. In this case, a single CT/VTmodule is required.
