6.93
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Occasional loss-of-potential to the distance relay, while unavoidable, is detect-
able. When the relay detects a loss-of-potential condition, the relay can block dis-
tance element operation, block or enable forward-looking directional overcurrent
elements, and issue an alarm for any true loss-of-potential condition.
If line-side PTs are used, the circuit breaker(s) must be closed for the LOP logic
to detect a loss-of-potential condition. Therefore, if three-phase potential to the
relay is lost while the circuit breaker(s) is open (e.g., the PT fuses are removed
while the line is de-energized), the relay cannot detect a loss-of-potential condi-
tion when the circuit breaker(s) closes again. At circuit breaker closing, the relay
can detect one or two missing potentials that occurred while the circuit breaker
was open. See Loss-of-Potential Logic on page 5.28 for more information.
Table 6.24 lists the three choices for enabling LOP protection.
Set ELOP to Y1 for this application example. This choice reduces the chances of
false tripping because of a loss-of-potential condition.
ELOP := Y1. Loss-of-Potential (Y, Y1, N)
Enable the Advanced Settings so you can properly set the zero-sequence com-
pensation factors.
EADVS := Y. Advanced Settings (Y, N)
Phase-Distance Elements (21P)
Mho Phase-Distance Element Reach
Employ each zone of distance protection as follows:
➤ Zone 1—Instantaneous underreaching tripping
➤ Zone 2—Forward-looking fault detector for the POTT scheme and
backup time delayed tripping
➤ Zone 3—Current reversal guard for the POTT scheme, echo tripping,
and weak infeed logic
Zone 1 Phase-Distance Element Reach
Zone 1 phase-distance protection provides instantaneous protection for phase-to-
phase, phase-to-phase-to-ground, and three-phase faults in the first 80 percent of
the cable. Errors in the current transformers, potential transformers, modeled
cable data, and fault study data do not permit a Zone 1 setting for 100 percent of
the cable; unwanted tripping could occur for faults just beyond the remote end of
the cable.
Table 6.24 LOP Enable Options
Option Description
N The LOP logic operates but does not disable voltage-polarized directional ele-
ments, distance elements, and forward-looking directional overcurrent ele-
ments. Use LOP in this case for alarm only.
Y The relay disables all voltage-polarized directional elements and distance ele-
ments, but enables forward-looking directional overcurrent elements. These
forward-looking directional overcurrent elements effectively become nondirec-
tional and provide overcurrent protection during a loss-of-potential condition.
Y1 The relay disables all voltage-polarized directional elements and distance ele-
ments. The relay also disables the overcurrent elements controlled by the volt-
age-polarized directional elements.
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