Toshiba GRZ100 B Series, Инструкция по эксплуатации

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Figure 2.3.3.2 Short-Distance Tapped Line (2)
Permissive Overreach Protection (POP)
The Permissive Overreach Protection (POP) method carries out tripping on condition that zone 2
of each terminal (or zone 3 depending on the setting) has operated for an internal fault.
Accordingly it needs to use a different transmission channel when applied to three terminals.
Zone 2 in the POP method basically covers up to and including the busbar of all terminals at the
remote end of the protected zone. If the source behind each terminal is strong enough, in this
scheme all terminals will operate their distance relays for a fault in the protected zone. However, if
the impedance behind the power source changes, there is a tendency to underreach as a
consequence of the "branch effect." Therefore, it is necessary to check that the relay can operate
for faults in the protected zone even under the worst power source conditions.
Blocking Schemes
With the blocking scheme, a terminal sends a blocking signal to the other terminal for an external
fault and a common power line carrier channel can be used. It can also perform high-speed
tripping even if one end of the multi-terminal line is a non-power source and there is no fault
current infeed.
In the blocking scheme, it is necessary to pay attention to diversion of the outfeed current in the
event of an external fault. In the system shown in Figure 2.3.2.3, if a fault current flows out of
terminals B and C for an external fault, the outfeed currents of terminal B and terminal C are
smaller than the infeed current at terminal A due to the diversion. Therefore, it may be difficult to
operate the external detection relay of one terminal depending on the ratio of diversion.
Terminal B Terminal A
Terminal C