Bank Overcurrent Protection - GE C70 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS SETTINGS EXAMPLE

8.4.9 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. Set overcurrent protection to be as low and as fast as possible, with only enough delay to avoid tripping on
external system disturbances.
During bank energization, the capacitor charge current inrush can 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 currents. Because of these considerations, the neutral time overcurrent pickup is programmed equal to the phase
time overcurrent pickup and is set to 1.25 pu. Specify the delay for these elements to be 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
unbalance 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 provides adequate coordination for faults external to the bank and protection for internal rack faults.
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C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 8-21