5.57
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Out-of-Step Logic (Zero Settings)
Swing Center Voltage (SCV) Processing and Analog Variables
The detection of a network power swing condition is based on monitoring the
rate-of-change of the positive-sequence swing-center voltage. For the purpose of
implementing the function, the following analog variables are used:
➤ SCV1: per-unit positive-sequence swing-center voltage
➤ dSCV1_Unflt: unfiltered derivative of the positive-sequence swing-
center voltage
➤ dSCV1_UF: ultra-fast derivative (filtered) of the positive-sequence
swing-center voltage
➤ dSCV1_F: fast derivative (moderately filtered) of the positive-
sequence swing-center voltage
➤ dSCV1_S: slow derivative (most filtered) of the positive-sequence
swing-center voltage
➤ d2SCV1_UF: ultra-fast second derivative (not filtered) of the
positive-sequence swing-center voltage
Swing Center Voltage Slope Detector
In Figure 5.38, the top four comparators determine whether the swing is fast
(dSCV1_F remains asserted for 1.75 cycles) or slow (dSCV1_S remains asserted
for 5 cycles) for both negative and positive slopes, if the supervision condition
are met. The supervision conditions are:
➤ No power swing is in progress (OSB_I is de-asserted), and no Zone 2
through Zone 5 distance elements are asserted or
➤ The absolute value of dSCV1_UF is greater than 0.55) or
➤ The absolute value of dSCV1_UF is lower than 0.55 but greater than
0.2 and the absolute value of d2SCV1_UF is greater than 0.23.
Therefore, if OR Gate OR 1 does not assert, one of the timers (Timer 1 through
Timer 4) starts timing. If the conditions prevail, the top input of AND Gate
AND 5 asserts after the appropriate timer expires.
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