Toshiba GRL100-701B Instruction Manual page 61

When a sequential fault clearance occurs for a fault on a parallel line, the direction of the current
on the healthy line is reversed. The status of the forward overreaching element changes from an
operating to a reset state at the terminal where the current is reversed from an inward to an outward
direction, and from a non-operating status to operating status at the other terminal. In this process,
if the operating periods of the forward overreaching element of both terminals overlap, the healthy
line may be tripped erroneously. To prevent this, current reversal logic (CRL) is provided. (See
Section 2.3.2.6 for current reversal.)
Since the POP transmits a trip permission signal with the operation of the overreaching element, it
requires multiplex signaling channels or one channel for each direction. This ensures that the
transmitting terminal does not trip erroneously due to reception of its own transmit signal during
an external fault in the overreaching zone.
Scheme Logic
Figure 2.3.2.2 shows the scheme logic for the POP. The POP transmits a trip permission signal to
the other terminal for any of the following conditions.
• The forward overreaching zone 2 or zone 3 selected by scheme switch [ZONESEL]
operates and the current reversal logic (CRL) has not picked up.
• The circuit breaker is opened and a trip permission signal CR is received from the other
terminal.
• The forward overreaching zone 2 or zone 3 and reverse looking Z4 have not operated
and a trip permission signal is received from the other terminal.
The last two are implemented when an echo function (ECH) is selected. (Refer to Section 2.3.2.5
for echo function.)
Transmission of the trip permission signal continues for the TSBCT setting even after the local
terminal is tripped by the delayed drop-off timer TSBCT. This is to ensure that command tripping
is executed at the remote terminal.
The POP outputs single-phase tripping signal S-TRIP or three-phase tripping signal M-TRIP to
the local terminal when the trip permission signal R1-CR and R2-CR are received from the remote
terminals, the current reversal logic (CRL) is not picked up and one of the following conditions is
established.
• The forward overreaching element operates.
• The undervoltage element UVL (UVLS or UVLG) operates and the forward overreaching
and the reverse looking elements do not operate.
The latter is implemented when the weak infeed trip function is selected. (Refer to Section 2.3.2.5
for weak infeed trip function.)
To select the faulted phase reliably, phase selection is performed using the phase selection element
UVC. Phase selection logic is described in Section 2.3.2.7.
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