GE C70 Instruction Manual page 469

Capacitor bank protection and control system ur series
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9 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 Unbalance values for maxi-
mum system conditions table above, 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 EnerV-
ista 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. There-
fore, as shown in the following figure, a blocking input for each element is created with FlexLogic™.
GE Multilin
0.9V
=
=
(
)
diff f1_max
V
V
+
(
)
(
)
diff f4_max
diff f3_max
----------------------------------------------------------------- -
=
=
2
C70 Capacitor Bank Protection and Control System
×
0.9
0.0249 pu
0.0224 pu
=
0.1074 pu
0.0785 pu
+
----------------------------------------------------------- -
0.093 pu
=
2
9.4 SETTING EXAMPLE
(EQ 9.23)
(EQ 9.24)
9-17
9

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