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5.15
Date Code 20060320
Instruction Manual
SEL-311C Relay
Trip and Target Logic
Weak-Infeed Logic and Settings
Weak-Infeed Logic and Settings
In some applications, with all sources in service, one terminal may not
contribute enough fault current to operate the protective elements. If the fault
lies within the Zone 1 reach of the strong terminal, the fault currents may
redistribute after the strong terminal line breaker opens to permit sequential
tripping of the weak-infeed terminal line breaker. If currents do not
redistribute sufficiently to operate the protective elements at the weak-infeed
terminal, it is still desirable to open the local breaker. This prevents the low-
level currents from maintaining the fault arc and allows successful
autoreclosure from the strong terminal. When the fault location is near the
weak terminal, the Zone 1 elements of the strong terminal do not pick up, and
the fault is not cleared rapidly. This is because the weak terminal protective
elements do not operate. Note that while the weak-infeed terminal contributes
little fault current, the phase voltage(s) are depressed.
SEL-311C Weak-Infeed
Logic
Enable the weak-infeed logic by setting EWFC = Y.
The SEL-311C provides additional logic (see
) for weak-infeed
terminals to permit rapid tripping of both line terminals for internal faults near
the weak terminal. The strong terminal is permitted to trip via the permissive
signal echoed back from the weak terminal. The weak-infeed logic generates a
trip at the weak terminal if all of the following are true:
➤
A permissive trip (PT) signal is received for ETDPU time.
➤
A phase undervoltage or residual overvoltage element is picked
up.
➤
No reverse-looking elements are picked up.
➤
The circuit breaker is closed.
After these four conditions are met, the weak-infeed logic sets the Echo-
Conversion-To-Trip (ECTT) bit in the Relay Word. The ECTT bit is included
in the trip logic (see
) and a trip signal is issued to the local breaker
when the conditions described above are true.
Typical phase undervoltage setting (27PPW) is 70–80 percent of the lowest
expected system operating voltage. The residual overvoltage setting should be
set to approximately twice the expected standing 3V
0
voltage. With the 59NW
element set at twice the nominal standing 3V
0
voltage, the instrument
measures only fault-induced zero-sequence voltage.
KEY—Key Permissive Trip
Signals communications equipment to transmit permissive trip. For example,
SEL
OGIC
control equation setting OUT105 is set:
OUT105 =
KEY
Output contact
OUT105
drives a communications equipment transmitter input
in a two-terminal line application (see
).
In a three-terminal line scheme, output contact
OUT107
is set the same as
OUT105
(see
):
OUT107 =
KEY
Summary of Contents for SEL-311C
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