Toshiba GRZ100 B Series, Instruction Manual

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2.3.2.2 Current Outfeed in the Event of an Internal Fault
In the event of an internal fault in a multi-terminal system, a fault current may flow out of a
specific terminal. An example is shown using a three-terminal system with two parallel lines
shown in Figure 2.3.2.2. The figure shows the case where only one circuit is used and another
circuit is open at terminal A. If a fault occurs at a close to terminal C, there is a route through which
the current flows from terminal B via the adjacent line into terminal C and part of the fault current
flows out of terminal B and flows into terminal C again. The magnitude of the outfeed current is a
maximum of approximately 1/2 of the infeed current from terminal A. If the fault point is
examined from terminal A, the impedance of the adjacent circuit between terminal B and terminal
C enters in parallel, and consequently the relay at terminal A sees it as smaller than the actual
impedance up to the fault point, which means this relay tends to overreach.
It is difficult to protect the system when a fault current flows out of one end. Since an ordinary
directional comparison method judges an external fault at one end and sends a block signal, it may
fail to remove the fault.
Figure 2.3.2.2 Current Outfeed in Event of Internal Fault
2.3.2.3 Diversion of Outfeed Current in the Event of an External Fault
If an external fault occurs at terminal C in the three-terminal system shown in Figure 2.3.2.3, the
fault current that flows into terminal A may not only flow out of terminal C but may also flow out
of terminal B and flow into the fault point. In this case, outfeed currents IB from terminal B and IC
from terminal C become smaller than infeed current IA from terminal A. That is,
(Outfeed current) < (infeed current)............................... (2-19)
The directional comparison method sometimes cannot detect external faults under such
conditions, increasing the possibility of unwanted operation due to detection of an internal fault
from terminal A.
Terminal A Terminal C
Terminal B
IB
IC
IA
Figure 2.3.2.3 Outfeed Current in Event of External Fault
Terminal A Terminal C
Terminal B
IC
IB
IA