Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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5.63
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Out-of-Step Logic (Zero Settings)
In summary, for an external forward fault, the logic issues a DOSB signal if the
signal from a fault detector has lasted several cycles, no power swing has been
detected, the relay has issued no trip, and at least one of the Zone 1 phase-mho
has picked up or when FZAVG asserts. For a reverse fault, the logic issues a
DOSB signal if a power swing has not been detected, the signal from a fault
detector has lasted several cycles and been cleared, the relay has issued no trip
signal, and a Zone 2 mho-phase has picked up within a time delay.
Depending on the EOOS setting, the relay selects either the logic shown in
Figure 5.44 (EOOS = Y1) or the logic shown in Figure 5.45 (EOOS = Y).
Figure 5.44 shows the dependable power-swing block detector logic.
Figure 5.43 Type of Power Swings Detected by the DOSB Function
Real (Z1)
Trajectories of Z1
during a power swing
following a reverse fault
Trajectories of Z1
during a power swing
following a forward fault
Imag (Z1)