GE C70, Instruction Manual

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GE Multilin C70 Capacitor Bank Protection and Control System 9-11
9 APPLICATION OF SETTINGS 9.3 PROTECTION METHODS FOR CAPACITOR BANKS
9
9.3.5 OVERCURRENT
Phase and neutral time overcurrent elements can be applied with normal settings without encountering false operations
due to inrush currents. The desirable minimum pickup is 135% of nominal phase current for grounded wye banks or 125%
for ungrounded banks. Instantaneous elements (if used) should be set high to override inrush or outrush transients.
For effectively grounded systems with grounded wye capacitor banks, the high-frequency outrush current into an external
ground fault will not normally operate the 51N ground element. The unbalanced capacitor bank load current caused by an
external ground fault may be sufficient to cause the element to pick up and trip the capacitor bank if the 51N is set too low.
To prevent this inadvertent tripping, the trip of the 51N element is normally set above the capacitor phase current.
9.3.6 LOSS OF BUS VOLTAGE
In some cases, it may be necessary to trip a shunt capacitor bank if the supply bus voltage is lost. Two conditions that may
need to be considered are:
• Re-energizing a bank with a trapped charge.
• Energizing a capacitor bank without parallel load through a previously unenergized transformer.
The undervoltage element (ANSI device 27B) will detect loss of system voltage and trip the capacitor bank after a time
delay. This delay prevents tripping of the bank for system faults external to the bank. The undervoltage element should be
set so the element will not operate for voltages that require the capacitor bank to remain in service. Because this tripping is
also not due to a fault within the capacitor bank (like system overvoltage tripping), the bank is not locked out.