GE C70

CHAPTER 8: APPLICATION OF SETTINGS SETTINGS EXAMPLE
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 8-11
8
8.3.5 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. Set the instantaneous elements (if used) 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 does not normally operate the 51N ground element. The unbalanced capacitor bank load current caused by
an external ground fault can 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.
8.3.6 Loss of bus voltage
In some cases, it is necessary to trip a shunt capacitor bank if the supply bus voltage is lost. Two conditions that 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) detects 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. Set the undervoltage element so that the
element does 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.
8.4 Settings example
8.4.1 Overview
Consider protection of a capacitor bank with the following characteristics:
•V
LL
= 138 kV fuseless
•Q
B
= 21 Mvar
• Two parallel strings (S = 2) in each phase with N = 7 capacitor units rated Q
C
= 500 kvar, V
C
= 11.62 kV
• Each capacitor unit consisting of n = 6 series capacitors
The figure shows details. Phase capacitors are connected to ground through a neutral capacitor rated Q
CN
= 167 kvar, V
CN
= 0.825 kV.