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5-2
Trip and Target Logic
Date Code 20010625
SEL-311L Instruction Manual
enabled if setting E87L = 3R (three-terminal protection with two communications channels) or
when an 87L communications channel fails and setting E87L = 3,
even if setting EHSDTT = N
.
See
Section 3: Line Current Differential, Distance, Out-of-Step, Overcurrent, Voltage,
Synchronism Check, and Frequency Elements
for more information about setting E87L.
The SEL-311L Relay direct transfer trip signals have outstanding security, resulting in less than
one unwanted trip per 100 million channel noise bursts. The relay maximizes dependability by
transmitting the direct trip signal for at least 8 cycles when an internal fault is detected.
87L Tripping Qualified by the Local Disturbance Detector
Set EDD = Y to qualify 87L tripping for up to two cycles with the local disturbance detector
Relay Word bit DD. There are no other settings associated with the local disturbance detector.
Make setting EDD = Y at all terminals where a weak-infeed or zero-infeed condition cannot
occur. If a weak-infeed or zero-infeed condition can occur, set EDD = N. The disturbance
detector output DD asserts for 10 cycles after the local I1 changes by more than 5 degrees or
more than 2% of nominal current, or the local I0 changes by more than 5 degrees or more than
2% of nominal current.
Setting EDD = Y increases communications channel security. The 87L algorithms have
excellent communications security that results in less than one unwanted trip per five hundred
thousand channel noise bursts when setting EDD = N. When setting EDD = Y, the relay retains
that security even if every noise burst results from an external detectable fault (an unlikely case).
The security increases if some noise bursts do not result from an external fault (a more likely
case).
The relay maintains excellent dependability by limiting disturbance detector qualification to 2
cycles maximum. The local disturbance detector does not qualify 87L direct transfer trips.
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