6.126
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Out-of-Step Logic Application Examples
Equation 6.78
A typical stable swing frequency is slip = 5 Hz. Use this value in Equation 6.79
to find setting OSBD.
Equation 6.79
The OSBD timer settings are in increments of 0.125 cycle; round up to the near-
est valid relay setting.
OSBD := 1.875. Out-of-Step Block Time Delay (0.500–8000 cycles)
Latch Out-of-Step Blocking
The SEL-421 automatically resets the OSB logic if this logic asserts for more
than two seconds while the positive-sequence impedance locus is inside Zone 7.
During an unstable power swing, the relay also resets the OSB logic each time
the swing impedance exits Zone 7. You can latch on the OSB function during an
unstable power swing to continue blocking the distance elements if the power
swing impedance locus moves outside of Zone 7 and before it comes back inside
Zone 7 on its next swing cycle. If latched, the OSB logic resets one second after
the power system stops the out-of-step. Latching the OSB gives you an advantage
in that the relay can successfully block uncontrolled distance element operations
if a fault occurs when the unstable swing impedance is outside of Zone 7. Relay
elements detect internal faults that occur during a power swing and take the
appropriate action (unblock).
OSBLTCH := Y. Latch Out-of-Step Blocking (Y, N)
Out-of-Step Unblocking
The relay disables out-of-step blocking automatically when a fault occurs during
a power swing. Therefore, the distance protection successfully detects all fault
types and trips the circuit breaker(s) during internal faults.
where:
NOM
= nominal power system frequency (Hz)
slip
= maximum slip frequency (Hz)
Ang_R7 2
Z
T
2
---------
R1R7
---------------
atan• =
2
8.8 88 8.00 87.6 3.52 88++
2
--------------------------------------------------------------------------------------------------------------
15.53
--------------------------------------------------------------------------------------------------------------
atan•
=
66.4=
OSBD
Ang_R6 Ang_R7–f
NOM
•
360
cycle
-------------- f
slip
•
--------------------------------------------------------------------------- cycles=
120.9 66.4–60 Hz•
360
cycle
-------------- 5 Hz•
------------------------------------------------------------- cycles=
1.82 cycles=
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