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6.110
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Pole-Open Detection
The setting EPO offers two options for deciding what conditions signify an open
pole, as listed in
Select the second option because a 52A contact is available. The relay uses both
open-phase detection and status information from the circuit breaker to make the
most secure decision.
EPO :=
52
.
Pole-Open Detection (52, V)
Pole-Open Time Delay on Dropout
The setting 3POD is the time delay on dropout after the Relay Word bit 3PO
deasserts. The setting 3POD stabilizes the ground-distance elements during pole
scatter when the circuit breaker closes.
3POD :=
0.500
.
Three-Pole Open Dropout Delay (0.000–60 cycles)
POTT Trip Scheme
This application example presents the permissive overreaching transfer trip
(POTT) scheme to high-speed trip for faults along the protected cable.
The POTT scheme logic consists of the following sections:
➤
Current reversal guard logic
➤
Echo
➤
Weak infeed logic
➤
Permission to trip received
Current Reversal Guard Logic
This is a parallel cable application, so you must use current reversal guard. When
the reverse-looking elements detect an external fault, the relay does not key the
transmitter and ignores reception of a permissive signal from the remote terminal.
The Zone 3 Reverse Block Delay (Z3RBD) timer extends these two operations
after a current reversal occurs and the reverse-looking elements drop out.
Table 6.29
Options for Enabling Pole-Open Logic
Option
Description
EPO := V
The logic declares a single-pole open if the corresponding phase undervoltage
element asserts and the open-phase detection logic declares the pole is open.
Select this option only if you use line-side potential transformers for relaying
purposes. A typical setting for the 27PO, pole-open undervoltage threshold, is
60 percent of the nominal line-to-neutral voltage.
Do not select this option when shunt reactors are applied because the voltage
slowly decays after the circuit breaker opens. With this option selected, the
relay can incorrectly declare LOP during a pole-open condition if there is
charging current that exceeds the pole-open current threshold.
EPO := 52
The logic declares a single-pole open if the corresponding 52A contact
(52AA1) from the circuit breaker deasserts and the open-phase detection logic
declares that the pole is open.
Summary of Contents for SEL-421-4
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