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6.46
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Assign PT1 to the corresponding Received M
IRRORED
B
ITS
signals.
PT1 :=
RMB1A
.
General Permissive Trip Received (SEL
OGIC
Equation)
PT3 :=
RMB2A
.
Three-Pole Permissive Trip Received (SEL
OGIC
Equation)
Three-Channel POTT Scheme, POTT3
In a cross-country fault situation of a mutually coupled parallel-line system, a
relay using the one-channel POTT scheme will trip all three poles at the remote-
to-fault terminal. This is because the relay at the remote terminal sees a multi-
phase fault and receives the only permissive trip signal. Both transmission lines
will be out of service if even a single-phase ground fault occurs on each circuit.
The two-channel POTT scheme retains the much-desired single-pole tripping in
the event of cross-country faults. However, the relay at the remote terminal has to
delay a single-pole trip until the external fault is cleared at the close-in terminal.
This application example uses direct transfer trips described below to compli-
ment the two-channel POTT scheme and reduce the single-pole trip delay to a
minimum.
As an alternative to the two-channel POTT scheme with direct transfer trips, you
may use the phase-segregate three-channel POTT scheme (POTT3) to correctly
single-pole trip without a delay in the event of cross-country faults. In the previ-
ous cross-country fault example, the SEL-421 on line 1 at Station R will transmit
KEYA to the relay at Station S, which converts it to PTA, a permissive A-Phase
trip signal. The relay then combines a locally detected Zone 2 phase-distance ele-
ment with the received PTA and trips A-Phase only without a time delay. Because
the direct transfer trips are not necessary in the three-channel POTT scheme, the
total communications channels used would be three.
For three-channel POTT applications, the following equations apply:
ECOMM :=
POTT3
.
Communications-Assisted Tripping (N, DCB, POTT,
POTT2, POTT3, DCUB1, DCUB2)
TMB1A :=
KEYA
.
Transmit M
IRRORED
B
ITS
1A (SEL
OGIC
Equation)
TMB2A :=
KEYB
.
Transmit M
IRRORED
B
ITS
2A (SEL
OGIC
Equation)
TMB3A :=
KEYC
.
Transmit M
IRRORED
B
ITS
3A (SEL
OGIC
Equation)
PTA :=
RMB1A
.
A-Phase Permissive Trip Received (SEL
OGIC
Equation)
PTB :=
RMB2A
.
B-Phase Permissive Trip Received (SEL
OGIC
Equation)
PTC :=
RMB3A
.
C-Phase Permissive Trip Received (SEL
OGIC
Equation)
Relay Word bit KEY is general permission to trip.
Single-Line Applications
For single-line applications, the following equations apply:
ECOMM :=
POTT
.
Communications-Assisted Tripping (N, DCB, POTT,
POTT2, POTT3, DCUB1, DCUB2)
TMB1A :=
KEY
.
Transmit M
IRRORED
B
ITS
1A (SEL
OGIC
Equation)
PT1 :=
RMB1A
.
General Permissive Trip Received (SEL
OGIC
Equation)
Relay Word bit KEY is general permission to trip.
Summary of Contents for SEL-421-4
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