Toshiba GRL100-701B Instruction Manual page 404

Special case
In the case of a 3 terminal-double circuit line configuration, an additional system condition must
be considered. It is possible, under certain circumstances, that when an internal fault occurs,
out-flowing fault current can also be experienced.
In this case, the effect of the out-flowing fault current must be considered when calculating DIFI1
and DIFI2. For example, the following application can be considered.
Terminal A
S1
The following conditions are assumed as shown in Fig.K-1.
• The three terminals, A, B and C, and the two T-connected transmission lines are assumed to
be live.
• Terminal C is connected to load only.
• Line 2 CB at terminal A is open.
• 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 :
Terminal B :
Terminal C :
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.
Y
Line 2
D
Line 1
X
X-Y
Y
Y
Terminal C
Fig K-1 Special case
X
Y+Z
−Y
 403 
Terminal B
S2
Y
Z
6 F 2 S 0 8 5 0