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6.155
Date Code 20171021
Instruction Manual
SEL-421 Relay
Protection Applications Examples
Circuit Breaker Failure Application Examples
The absolute maximum fault clearing time depends on power system transient
stability and the thermal withstand capability of the equipment. If a circuit
breaker fails, the total time required to trip all electrically adjacent circuit break-
ers must be less than this absolute maximum clearing time. Set the time delay on
pickup timer to allow time for the protected circuit breaker to operate and the
instantaneous overcurrent element (50F
1) to reset. Always include a safety mar-
gin, remembering that the operating time of the line relays and the electrically
adjacent circuit breakers limit this margin.
Circuit Breaker Failure Protection—Example 1
Use the SEL-421 to provide circuit breaker failure protection for one circuit
breaker. This is a circuit breaker failure protection Scheme 1 application example
for three-pole tripping circuit breakers (you can also use this scheme for single-
pole tripping applications). For a single-pole tripping circuit breaker application
example (Scheme 2), see
Circuit Breaker Failure Protection—Example 2 on
. This example uses a 230 kV power system similar to the system in
230 kV Overhead Distribution Line Example.
shows the SEL-421 at
the S terminal of the two-terminal line between Harvard and Princeton.
provides the related power system parameters.
Figure 6.41
Scheme 2 Single-Phase Fault Timing Diagram
50F
φ
n Element
Asserts
Transfer
Trip
Asserts
BF Trip
Outputs
Assert
BF Timer Delay
(BFPU)
Communications Channel Time
Absolute Maximum Fault Clearing Time
t
Local Backup
Breakers Operate
Time
Remote Breakers
Trip Time
Protective Relay
Operate Time
Maximum Breaker
Operating Time
50F
φ
n
Dropout Time
Safety Margin
Time
SPBF Timer Delay
(SPBFPU)
Fault Occurs
Trip Asserts
Normal
Operation
Breaker
Failure
Operation
Auxiliary Breaker Failure
Relay Operate Time
Figure 6.42
230 kV Power System for Circuit Breaker Failure Scheme 1
SEL-421
BK1
IW
S (Harvard)
R (Princeton)
BK2
Summary of Contents for SEL-421-4
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