Toshiba GRL100-701B Instruction Manual page 46

6 F 2 S 0 8 5 0
at the remote terminal. This is because it is difficult for zone 3 at terminals A and C to provide
remote backup protection for the fault shown in Figure 2.3.1.4 due to fault infeed from the other
terminal, whereas reverse looking zone R of terminal B is not affected by this. Zone R trip can be
disabled by the scheme switch [ZRTP].
Zone 3
B
A
Zone R
C
Figure 2.3.1.4 Reverse Zone Protection
To maintain stable operation for close-up three-phase faults which cause the voltages of all phases
to drop to 0 or close to 0, zone 1 for phase faults, once operated, changes its element to a reverse
offset element. This continues until the fault is cleared, and thus it is effective for time delayed
protection.
The reactance element characteristics of zone 1 and zone 2 are parallel lines to the R axis and
provide sufficient coverage for high-resistance faults. The reactance element characteristics of
zone 1 can be transformed to a broken line depending on the load flow direction in order to avoid
overreaching by the influence of load current. The characteristic in the resistive direction is limited
by the mho characteristic of zone 3. The reactive reach setting is independent for each zone. It is
also possible to have independent settings for each individual phase fault and earth fault elements.
With a long-distance line or heavily loaded line, it is possible for the load impedance to encroach
on the operation zone of the mho element. Blinders are provided to limit the operation of the mho
element in the load impedance area.
Zero-sequence current compensation is applied to zone 1 and zone 2 for earth fault protection.
This compensates measuring errors caused by the earth return of zero-sequence current. This
allows the faulted phase reactance element to precisely measure the positive-sequence impedance
up to the fault point. Furthermore, in the case of double-circuit lines, zero-sequence current from
the parallel line is introduced to compensate for influences from zero-sequence mutual coupling.
Considering the case where the impedance angle of positive-sequence impedance and
zero-sequence impedance differ which is the most common in cable circuits, GRL100 carries out
vectorial zero-sequence current compensation.
The autoreclose schemes are utilised with instantaneous zone 1 tripping. When single-phase
autoreclose or single- and three-phase autoreclose are selected, zone 1 executes single-phase
tripping for a single-phase earth fault. In order to achieve reliable fault phase selection even for
faults on heavily loaded long-distance lines or irrespective of variations in power source
conditions behind the relaying point, an undervoltage element with current compensation is used
as a phase selector. Other zones only execute three-phase tripping, and do not initiate autoreclose.
2.3.1.2 Scheme Logic
Figure 2.3.1.5 shows the scheme logic for the time-stepped distance protection. For zone 1
tripping, as described later, it is possible to select instantaneous tripping or time delayed tripping
using the scheme switch [Z1CNT] in the trip mode control logic. (Detail of the [Z1CNT] is
described after.) Zone 2, zone 3 and zone R give time delayed tripping. However, these zones can
trip instantaneously by PLC signals Z∗_INST_TP. Timers TZ2, TZ3 and TZR with time delayed
tripping can be set for earth faults and phase faults separately. Zone 1, zone 2, zone 3 and zone R
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