6.172
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
230 kV Tapped Transmission Line Application Example
Automatic Restoration Philosophy
Refer to Figure 6.51 for a timing diagram of the complete automatic restoration
cycle. The SEL-421 at Substation T automatically restores service in response to
the following system conditions:
➤ Low-side circuit breaker or autotransformer failure at Substation T
➤ Successful 230 kV line reclose
➤ Permanent 230 kV line fault or circuit breaker failure at Substation S
or Substation R
Transmission Line Faults or Circuit Breaker/Autotransformer Failure
The SEL-421 at Substation T responds to a 230 kV transmission line fault, low-
side circuit breaker failure or autotransformer failure at the local substation, and
any circuit breaker failure at Substations S and R. For these situations, the relay
does the following:
➤ Sends a pulsed open command to the high-side MOD. The 230 kV
transmission line must be dead for two seconds, and the relay must
have successfully opened the 115 kV circuit breakers.
➤ Disables DTT (direct transfer trip) via MIRRORED BITS
communications (issued at Substation T) when the high-side MOD is
open
➤ Inhibits reception of any DTT via MIRRORED BITS communications
from any of the two remote 230 kV terminals when the high-side
MOD is open
Low-Side Circuit Breaker or Autotransformer Failure
The following actions occur if a low-side circuit breaker or autotransformer fail-
ure occurs at Substation T:
➤ Substation S and R reclose five seconds later.
➤ Lockout relays at Substation T locks out the high-side MOD and
low-side circuit breakers. The relay uses an external lockout relay (86
a and 86 b contacts in the breaker trip and close circuits, and an 86 b
contact in the MOD close circuit); Substation T remains locked out
pending further action from operations or field personnel.
Successful 230 kV Line Reclose
The SEL-421 at Substation T issues the following actions if the 230 kV transmis-
sion line autoreclose is successful at Substation S and Substation R:
➤ Sends a pulsed close command to the high-side MOD at Substation T
to energize the autotransformer if the relay measures balanced
nominal voltage from the high-side PTs for five seconds and the low-
side circuit breakers are open; the tertiary windings of the
autotransformer restore station service.
➤ Recloses Circuit Breaker BK1T five seconds after the high-side
MOD closes, if the voltage is nominal and synchronized.
➤ Recloses Circuit Breaker BK2T five seconds later if the voltage is
still nominal and synchronized.
Thus, the local SEL-421 restores the 115 kV system at Substation T.
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