Date Code 20020426 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-65
SEL-351-5, -6, -7 Instruction Manual
Power Elements Application—VAR Control for a Capacitor Bank
SEL-
351
(B)
52-B
SEL-
351
(C)
52-C
Transformer
Bank
Trip
Trip
Close
Trip
Block
Close
VAR Control
DWG: M351198
9600 KVAR
Capacitor
Bank
1.0 per unit
VARs (leading)
21.6 KV Bus
2000:5
ct ratio
12,500:125
pt ratio
To other feeder postions
Block Close
Figure 3.39: SEL-351(B) Relay Provides VAR Control for 9600 kVAR Capacitor Bank
The 9600 kVAR capacitor bank in Figure 3.39 is put on-line and taken off-line according to the
VAR loading on the transformer bank feeding the 21.6 kV bus. The VAR loading is measured
with the SEL-351(B) Relay located at bus circuit breaker 52-B.
Two SEL-351 Relays control the capacitor bank—both relays are connected to capacitor bank
circuit breaker 52-C. The SEL-351(C) Relay provides capacitor overcurrent protection and trips
circuit breaker 52-C for a fault in the capacitor bank. The SEL-351(B) Relay provides VAR
control and automatically puts the capacitor bank on-line (closes circuit breaker 52-C) or takes it
off-line (trips circuit breaker 52-C) according to the measured VAR level. The SEL-351(B)
Relay also provides bus overcurrent protection and trips circuit breaker 52-B for a fault on the
21.6 kV bus.
In Figure 3.39, if the SEL-351(C) Relay trips circuit breaker 52-C for a fault in the capacitor
bank, then a block close signal is sent from the SEL-351(C) Relay to the SEL-351(B) Relay.
This prevents the SEL-351(B) Relay from issuing an automatic close to circuit breaker 52-C.
For additional security, the close circuit from the SEL-351(B) Relay to circuit breaker 52-C is
supervised by a block close output contact from the SEL-351(C) Relay. This block close output
contact opens if the SEL-351(C) Relay trips circuit breaker 52-C for a fault in the capacitor
bank—no automatic closing can then take place.
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