5-4
Applications
Date Code 20010515
SEL-2BFR, -2/BFR Instruction Manual
between the secondaries of potential transformers connected on both sides of the protected
breaker.
If the potential transformers on each side of the breaker do not have identical turns ratios, you
must install a voltage balancing circuit between the secondary on one potential transformer and
the voltage input to the relay. Figure 5.3 shows one circuit you can use to balance the input
voltage.
If you do not wish to use voltage-based breaker protection features such as resistor thermal
protection, flashover protection, and breaker voltage warning, you need not connect the voltage
inputs.
Example Breaker
Figure 5.3 shows the example breaker arrangement. Table 5.1 lists pertinent breaker, line, and
system information.
Table 5.1: Example Line/Breaker Information
System per-unit bases:
525 kV; 100 MVA; 2756
Ω
; 110.0 A
Line
length:
100
miles
Line impedances:
Z1 = Z2 = 0.0218 pu
Z0 = 0.0758 pu
Source impedances:
ZS1 = 0.0034 pu
ZS0 = 0.0055 pu
Three-pole
tripping
Zone 1, three-phase minimum
fault duty:
I
3ph
= 49.6 pu = 5456 A
Line charging current:
I
c
= 350 A
CTR:
3000/5
(600/1)
PTR:
4300/1
Breaker:
Cogenel Type PK-8E (Air Blast)
500 kV, 3000 A, 3800 MVA
Maximum trip time:
2 cycles
Nominal close time:
10 cycles
Close
resistors:
35
Ω
/column, 2 columns
Trip
resistors:
110
Ω
/column, 2 columns
Transfer trip channel time:
0.5 cycle
Scheme Overview
The protection scheme in this example protects the breaker for all failures to trip. When fault
current exceeds the minimum three-phase fault duty for a Zone 1 fault, the fault must be cleared
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