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Scheme communication logic for residual
overcurrent protection (EFC)
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Connect the necessary signal from the auto-recloser for blocking of the directional com-
parison scheme, during a single-phase auto-reclosing cycle, to the EFC--BLOCK input
of the directional comparison module.
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In the blocking overreach scheme, a signal is sent to the other line end if the directional
element detects a fault in the reverse direction. When the forward directional element
operates, it trips the line after a short time delay if no blocking signal is received from
the other line end. The time delay, normally 30-40 ms, depends on the communication
transmission time and the chosen safety margin.
One advantage of the blocking scheme is that only one channel (carrier frequency) is
needed and the channel can be shared with the impedance-measuring system, if that also
works in the blocking mode. The communication signal is transmitted on a healthy line
and no signal attenuation will occur due to the fault.
Blocking schemes are particularly favorable for three-terminal applications if there is
no zero-sequence current outfeed from the tapping. The blocking scheme is immune to
current reversals because the received carrier signal is maintained long enough to avoid
unwanted operation due to current reversal. There is never any need for weak-end-in-
feed logic, because the strong end trips for an internal fault when no blocking signal is
received from the weak end. But the fault clearing time is generally longer for a block-
ing scheme than for a permissive one.
If the fault is on the line, the forward direction measuring element operates. If no block-
ing signal comes from the other line end via the EFC--CR binary input (carrier receive)
the EFC--TRIP output is activated after the tCoord set time delay.
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