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5.124
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
Permissive Overreaching Transfer Tripping Scheme
Use M
IRRORED
B
ITS
communications to implement a Permissive Overreaching
Transfer Tripping (POTT) scheme efficiently and economically. M
IRRORED
B
ITS
communications technology improves security and improves the overall operat-
ing speed. If the communications channel is reliable and noise-free (as with fiber-
optic channels), then POTT provides both security and reliability. You can also
implement a POTT scheme with other conventional communications channels
such as leased telephone lines and microwave. The DCUB trip scheme is a better
choice if the communications channel is less than perfect, but communications
channel failures are unlikely to occur during external faults.
POTT Scheme Selection
The SEL-421 offers three POTT schemes: POTT, POTT2, and POTT3. The type
of communications channel(s) in your application best determines which scheme
to implement.
Figure 5.92
DCB Logic Diagram
2
CYC
0
21SD
0
0
1/4
CYC
67SD
0
0
BTXD
Z3XPU
Z3XD
—
+
—
+
—
+
Relay
Word
Bits
Relay
Word
Bits
BT
DSTRT
67G2
67Q2
Z2G
Z2P
50G3
50Q3
67G3
67Q3
Z3G
Z3P
Z3P
Relay
Word
Bits
Z3XT
DSTRT
NSTRT
Z2PGS
67QG2S
STOP
BTX
VPOLV
ICAL
IBCL
IABL
0.1 • I
NOM
SEL
OGIC
Setting
Summary of Contents for SEL-421-4
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