6.173
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
230 kV Tapped Transmission Line Application Example
Permanent 230 kV Line Fault Or Circuit Breaker Failure
If a permanent 230 kV line fault or circuit breaker failure operation occurs at
Substation S or Substation R, the SEL-421 at Substation T executes the following
actions:
➤ Locks open the high-side MOD if there is no voltage on the 230 kV
side for 13 seconds. (You can implement this logically via a timer
and latch bit combination.) The latch resets if the 230 kV system is
hot for 60 seconds and both low-side circuit breakers (BK1T and
BK2T) are open.
➤ Recloses Circuit Breaker BK1T after five seconds if there is nominal
voltage on the 115 kV side and the high-side MOD is locked open.
➤ Recloses Circuit Breaker BK2T five seconds later if there is nominal
voltage on the 115 kV side, the high-side MOD is locked open, and
Circuit Breaker BK1T reclosed successfully.
These actions restore station service and low-side continuity at Substation T.
Substation T operates in this configuration until operations or field personnel
take further action.
SEL-421 Configuration
In this example, the SEL-421 at Substation T measures a single set of three-phase
potentials (input VY) and a single set of three-phase currents (input IW) on the
230 kV side of the autotransformer. The relay uses single-phase potential inputs
VAZ and VBZ to provide synchronism check across the low-side circuit break-
ers. Figure 6.52 shows the connection diagram.
Figure 6.52 SEL-421 Inputs
230 kV
2000/1
3
MOD
V
Y
I
W
V
AZ
V
BZ
SEL-421 Relay
1200/5
1
1000/1
1
1000/1
115 kV
BK2T
BK1T
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