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5.92
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Functions
Zone Time Delay
Independent Zone Timing
Use Relay Word bits Z
n
PT (Time-Delayed Zone Phase-Distance Protection) and
Z
n
GT (Time-Delayed Zone Ground-Distance Protection) to select independent
zone timing in SEL
OGIC
control equation TR (Trip) (
n
= 1–5).
The example below uses independent timing for Zone 2 phase and ground-dis-
tance protection:
TR := Z1P OR Z1G OR Z2PT OR Z2GT
Common Zone Timing
Use Relay Word bits Z
n
T (Zone
n
Distance Protection) to select common zone
timing in SEL
OGIC
control equation TR (Trip) (
n
= 1–5).
The next example uses common timing for Zone 2 distance protection:
TR := Z1P OR Z1G OR Z2T
If the timer input drops out while timing, the relay suspends the common zone
timer for two cycles. This feature prevents resetting the timer when a fault
evolves (e.g., the fault changes from a single phase-to-ground to phase-to-phase-
to-ground). If the timer expires, the relay blocks the suspend-timing logic. When
the zone timer is set to OFF, the output from the timer is blocked.
Summary of Contents for SEL-421-4
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