GE C70 Instruction Manual page 455

CHAPTER 8: APPLICATION OF SETTINGS
Two voltage differential (87V) stages provide trip and alarm levels during capacitor unit failures in each differential
element. The capacitor units are subject to overvoltage if one or more capacitors units are shorted out. The voltage
differential trip pickup setting is programmed for value of 110% of capacitor rated voltage. As seen from the previous
Unbalance Values for Maximum System Conditions table, a failure of four capacitor elements causes a 10.5% overvoltage.
The pickup setting for the stage 1 differential alarm element is chosen for one element failure.
87V alarm
The alarm delay is set to 10 seconds to avoid alarming during transient conditions.
The pickup setting for the stage 2 differential trip element is programmed as the differential voltage during maximum
system conditions midway between the third and fourth capacitor element failure differential voltages.
87V trip
Therefore, the trip delay is set to 50 ms.
The following setting values for the voltage differential 1 element between two tap voltages are programmed in the
EnerVista UR Setup software.
Note that match factor shown above assumes perfect bank balance. This must be adjusted when the bank is energized
during the differential function auto-setting procedure tune up.
The voltage differential element must be blocked during a VT fuse failure at any tap. The voltage differential element is
dependent on both source VTs. These VTs must be blocked if one or both of them has a VT fuse failure condition. Therefore,
as shown in the following figure, a blocking input for each element is created with FlexLogic.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
= 0.9V = 0.9
diff f1_max ( )
V + V
diff f4_max ( ) diff f3_max ( )
= ---------------------------------------------------------- - =
2
× 0.0249 pu = 0.0224 pu
0.1074 pu 0.0785 pu +
--------------------------------------------------- - = 0.093 pu
2
SETTINGS EXAMPLE
Eq. 8-23
Eq. 8-24
8
8-17