Schweitzer Engineering Laboratories SEL-421-4 - Page 489

6.153
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
Circuit Breaker Failure Application Examples
Scheme Components
The following are components of the circuit breaker failure schemes in the SEL-421:
➤ Circuit Breaker Failure Initiation (BFI3P1 or BFI1)
➤ Phase Fault Current Pickup (50FP1)
➤ Breaker Failure Pickup Time Delay (BFPU1)
For a detailed description see Circuit Breaker Failure Trip Logic on page 5.154.
Circuit Breaker Failure Initiation (BFI3P1 or BFI1)
All circuit breaker trips typically initiate the circuit breaker failure scheme. The
SEL-421 detects power system faults; the relay does not need an external BFI
contact for local circuit breaker failure protection applications.
Scheme 1
Scheme 1 uses initiation SELOGIC control equation BFI3P1 for three-pole trip-
ping applications and BFI1 for single-pole tripping applications.
Scheme 2
Scheme 2 uses initiation SELOGIC control equations BFI1 for both three-pole
(multiphase) and single-phase faults.
Phase Fault Current Pickup (50FP1)
Circuit breaker failure protection must pick up for all faults on the protected line.
Two settings philosophies are prevalent. One philosophy is to set the instanta-
neous overcurrent element (50F1) to pick up above load current and below the
minimum fault current (under minimum generation), if possible (I
load max
<
50FP1 < I
minimum fault
). Another settings philosophy is to set the threshold to
match the line protection sensitivity; this increases circuit breaker failure protec-
tion dependability.
In the following application examples, we use the first settings philosophy
because this approach gives greater security. In either case, when input phase cur-
rents exceed the overcurrent element threshold, the relay asserts Relay Word bit
50F1.
Subsidence current results from the energy trapped in the CT magnetizing branch
after the circuit breaker opens to clear a fault or interrupt load. Subsidence cur-
rent exponentially decays and delays resetting of instantaneous overcurrent ele-
ments. However, the open-phase detection logic causes the SEL-421 50F1
element to reset in less than one cycle during subsidence current conditions. The
open-phase detection logic determines that a pol e is open during the presence of
subsidence current and immediately resets the corresponding current level detectors.
Breaker Failure Pickup Time Delay (BFPU1)
Scheme 1
Relay Word bit FBF1 (Breaker 1 Breaker Failure) asserts when the time delay on
pickup timer BFPU1 expires and the corresponding 50F1 element is asserted.