Protection Methods For Capacitor Banks; Overview; Capacitor Unbalance Protection - GE C70 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS

PROTECTION METHODS FOR CAPACITOR BANKS

8.3 Protection methods for capacitor banks

8.3.1 Overview

The 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 a
means to disconnect a faulted capacitor unit or capacitor elements, a means to initiate a shutdown of the bank in case of
faults 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 protect
the substation and system from stresses that can be caused by the operation of the capacitor bank. System protection
includes items such as a means to limit overvoltage and excessive transient overcurrents, and to disconnect the bank in
the event of a major fault within the capacitor installation. System protection can also include alarms and methods to
disconnect the entire shunt capacitor bank to prevent further damage to the capacitors due to abnormal system
conditions.
In externally fused capacitor banks, several capacitor element breakdowns can occur before the fuse removes the entire
unit. 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 remaining
healthy capacitor units.
Internally fused capacitors have individual capacitor elements within a capacitor unit that are disconnected when an
element breakdown occurs. The risk of successive faults is minimized because the fuse isolates the faulty element within a
few cycles. Unbalance protection removes the bank from service when the resulting unbalanced voltage becomes
excessive 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 of
failure. Unbalance protection removes the bank from service when the resulting voltage becomes excessive on the
remaining healthy capacitor elements or units.

8.3.2 Capacitor unbalance protection

8.3.2.1 Connections
Unbalance protection utilizes the unbalance that occurs in a normally balanced capacitor bank to detect an abnormality
and initiate appropriate action. The most important function is to promptly remove the bank from service for any fault that
can result in further damage. Capacitor unbalance protection is provided in many ways, depending on the capacitor bank
arrangement.
A variety of unbalance protection schemes can be applied with the C70 for internally fused, externally fused, fuseless, or
unfused 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/VT
module is required.
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C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 8-5