Schweitzer Engineering Laboratories SEL-351 - Page 141

Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-63
SEL-351 Instruction Manual
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.
These block close signals seal in when the SEL-351(C) Relay trips circuit breaker 52-C for a fault
in the capacitor bank. Automatic closing of circuit breaker 52-C with the SEL-351(B) Relay can
then take place only after the block close signals are reset. The exact implementation of this block
close logic requires an application note beyond the scope of this discussion.
The rest of this discussion focuses on the determination of VAR levels (and corresponding power
element settings) for automatic tripping and closing of circuit breaker 52-C with the SEL-351(B)
Relay.
Convert three-phase 9600 kVAR (kVA) to single-phase VA (voltamperes) secondary:
9600 kVA/(21.6 kV•√3) = 256.6 A primary
256.6 A primary • (5/2000) = 0.64 A secondary
0.64 A secondary • 125 V secondary = 80.0 VA secondary (single-phase)
The three-phase 9600 kVAR capacitor is corresponded to 1.0 per unit VARs (leading) for dem-
onstration convenience in Figure 3.36. Figure 3.37 shows the per unit VAR levels for putting on-
line (closing circuit breaker 52-C) or taking off-line (tripping circuit breaker 52-C) the capacitor
bank.
DWG: M351199
0.3
0.0
-0.2
-1.2
Per unit VARs
(lagging)
Corresponding
settings
PWRIP, PWRIT,
and PWRID
Corresponding
settings
PWR2P, PWR2T,
and PWR2D
Per unit VARs
(leading)
0.5 per
unit margin
Take
capacitor
bank
off-line
0.3
0.0
-0.7
-1.2
Per unit VARs
(lagging)
Corresponding
settings
PWRIP, PWRIT,
and PWRID
Corresponding
settings
PWR2P, PWR2T,
and PWR2D
Per unit VARs
(leading)
0.5 per
unit margin
Put
capacitor
bank
on-line
Figure 3.37: Per Unit Setting Limits for Switching 9600 kVAR Capacitor Bank On- and Off-
Line
Courtesy of NationalSwitchgear.com