Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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6.163
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
Circuit Breaker Failure Application Examples
Assume that the total load current (I
S
) supplied from Substation S flows through
BK1 only; I
S(BK1)
= I
S
(see Figure 6.46 ). Calculate setting 50FP1 with all the
load current I
S
through Circuit Breaker BK1.
Equation 6.91
A fault study shows that the minimum ground fault current, I
fault minimum
, is
4.2 A secondary when the parallel line is in service at minimum generation. Cal-
culate the 50FP1 setting for dependability at 1/2 of the minimum fault current.
Equation 6.92
Although the result of this setting calculation is below maximum load (see
Equation 6.91 ), use this calculation to set the 50FP1 element for dependability.
50FP1 := 2.10 .
Phase Fault Current Pickup—BK1 (0.50–50 A secondary)
Circuit Breaker Failure Time Delay
NOTE:
This is one method for
calculating setting 50FP1. Use your
company practices and policies for
determining the pickup setting for
your particular application.
BFPU1 (Breaker Failure Time Delay—BK1) is the time delay on pickup for a
circuit breaker trip following a multiphase fault. You can also add an additional
delay, SPBFPU1 (SPT Breaker Failure Time Delay—BK1).
The recommended setting for BFPU1 is the sum of the following (see
Figure 6.47 ):
➤
Maximum circuit breaker operating time
➤
50FA1 maximum dropout time
➤
Safety margin
Figure 6.46 Fault Current Distribution Through Faulted Line at Station S
I
S
I
S(BK1)
BK1
BK2
IW
SEL-421
I
S(BK2)
50FP1 120
%
Percent Current I
S
•
 
• =
120
%
100
%
3.25 A
•
 
•
 
=
3.91 A secondary
=
50FP1 0.5 Percent Current I
fault-minimum
•
 
• =
0.5 100
%
4.20 A
•
 
•
 
=
2.10 A secondary
=