Bank Overcurrent Protection - GE C70 Instruction Manual

9 APPLICATION OF SETTINGS 9.4 SETTING EXAMPLE

9.4.9 BANK OVERCURRENT PROTECTION

Bank overcurrent protection is required to protect capacitor banks against major faults, such as line-to-line or line-to-ground
faults. It is desirable to set overcurrent protection to be as low and as fast as possible, with only enough delay to avoid trip-
ping on external system disturbances.
During bank energization, the capacitor charge current inrush may cause significant neutral currents approaching the bank
phase current levels. Also, during a system SLG fault, unbalanced bank neutral currents can be as high as bank phase cur-
rents. Because of these considerations, the neutral time overcurrent pickup is programmed equal to the phase time over-
current pickup and is set to 1.25 pu. The delay for these elements should be specified longer than the maximum system
fault clearing time (for example 0.5 second).
The phase instantaneous overcurrent element can be set to a value three to four times the capacitor bank rated current to
override back-to-back bank switching.
Negative-sequence instantaneous overcurrent protection is required to provide backup protection for phase current unbal-
ance during rack faults in the banks where phase capacitors are not well separated. Programming the negative-sequence
instantaneous overcurrent pickup at 10% of the nominal bank phase current with a delay of 250 to 400 ms typically pro-
vides adequate coordination for faults external to the bank and protection for internal rack faults.
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GE Multilin C70 Capacitor Bank Protection and Control System 9-21