Most multi circuit lines have two parallel operating circuits.
Parallel line applications
This type of networks is defined as those networks where the parallel transmission lines
terminate at common nodes at both ends.
The three most common operation modes are:
1. Parallel line in service.
2. Parallel line out of service and grounded.
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.
Let us analyze what happens when a fault occurs on the parallel line see figure
114.
From symmetrical components, we can derive the impedance Z at the relay point for
normal lines without mutual coupling according to equation
112.
ph ph
0 1 ph N
ph 0
1
0
V V
Z Z I 3 K
I 3I
3 Z
I
Z = =
- + ×
+ ×
×
EQUATION1275 V3 EN (Equation 297)
Where:
Vph is phase to ground voltage at the relay point
Iph is phase current in the faulty phase
3I0 is ground fault current
Z1 is positive sequence impedance
Z0 is zero sequence impedance
Section 8 1MRK 506 369-UUS -
Impedance protection
364 Line distance protection REL670 2.2 ANSI
Application manual
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