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• An internal fault occurs on the closed line at the opposite terminal, i.e. Line
1, Terminal B.
In this case, fault current will flow as shown in Fig.K-1. Even though the fault is
internal, the fault current Y from Line 1 at Terminal C flows out of the protected zone.
Assuming the fault current from source S2 is Z, the summation of the fault current
for Line1 at Terminal B is equal to Y+Z. Hence, the current at each terminal is as
follows:
Terminal A : X
Terminal B : Y+Z
Terminal C : −Y
Therefore the differential and restraint currents can be calculated as follows:
differential current：X+Z
restraint current：X+2Y+Z
This point can be expressed in the Id-Ir plane as shown in Fig.K-2. It can be seen that
the effect of the outflow current is to increase the apparent restraint quantity Ir and
thereby shift the point to the right of where it would normally fall.
Fig.K-2 Internal fault in Id-Ir plane including out-flow current
In order to ensure that the GR relay relay will operate correctly in this case, the point
shown on the plot must fall within the operating zone.
According to this requirement, DIFI2 can be calculated as follows.
X +Z > X+2Y+Z−2DIFI2
DIFI2 > Y (K-2)
This means that DIFI2 must be larger than the amount of out-flowing current.
CASE 1. The case with the distance protection applied as a main protection in
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