ABB REL650 series, Applications Manual, Page: 127 / 516

Most multi circuit lines have two parallel operating circuits.
Parallel line applications
In this type of networks, the parallel transmission lines terminate at common nodes at both
ends. We consider the three most common operation modes:
1. parallel line in service.
2. parallel line out of service and grounded in both ends.
3. parallel line out of service and not grounded.
Parallel line in service
This type of application is very common and applies to all normal sub-transmission and
transmission networks.
Z0m
A B
21 21
en05000221_ansi.vsd
FAULT
ANSI05000221 V1 EN
Figure 53: Class 1, parallel line in service
The equivalent circuit of the lines can be simplified, see figure 54 .
A
B
C
Z
0m
Z
0m
Z
0
-
Z
0m
Z
0
-
IEC09000253_1_en.vsd
IEC09000253 V1 EN
Figure 54: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-ground fault at the remote
busbar
If the current on the parallel line has negative sign compared to the current on the protected
line, that is, the current on the parallel line has an opposite direction compared to the
current on the protected line, the distance function will overreach. If the currents have the
same direction, the distance protection will underreach.
1MRK 506 334-UUS A
Section 6
Impedance protection
121
Application manual