6.25
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Use the two-channel POTT trip scheme (POTT2) to quickly clear faults internal
to the protected line.
ECOMM := POTT2. Communications-Assisted Tripping (N, DCB, POTT,
POTT2, POTT3, DCUB1, DCUB2)
Fuses or molded case circuit breakers often protect potential transformers. Oper-
ation of one or more fuses, or molded case circuit breakers, results in a loss of
polarizing potential inputs to the relay. Loss of one or more phase voltages pre-
vents the relay from properly determining fault distance or direction.
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 an LOP 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 condition 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 6.9 lists the three choices for enabling LOP.
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 for the zero-sequence mutual coupling between the parallel
transmission lines.
EADVS := Y. Advanced Settings (Y, N)
Ta b l e 6 .9 LO P E n a b l e O p t i o n s
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 an LOP 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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