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 MIRRORED BITS signals.
PT1 := RMB1A. General Permissive Trip Received (SELOGIC Equation)
PT3 := RMB2A. Three-Pole Permissive Trip Received (SELOGIC 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 MIRRORED BITS 1A (SELOGIC Equation)
TMB2A := KEYB. Transmit MIRRORED BITS 2A (SELOGIC Equation)
TMB3A := KEYC. Transmit MIRRORED BITS 3A (SELOGIC Equation)
PTA := RMB1A. A-Phase Permissive Trip Received (SELOGIC Equation)
PTB := RMB2A. B-Phase Permissive Trip Received (SELOGIC Equation)
PTC := RMB3A. C-Phase Permissive Trip Received (SELOGIC 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 MIRRORED BITS 1A (SELOGIC Equation)
PT1 := RMB1A. General Permissive Trip Received (SELOGIC Equation)
Relay Word bit KEY is general permission to trip.
To Next Page
Loading...
