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 0 N
ph 0
1
U U
Z Z I 3I K
I 3I
3 Z
Z = =
- + ×
+ ×
×
EQUATION2311 V1 EN (Equation 239)
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 239)
Where:
V
ph
is phase to ground voltage at the relay point
I
ph
is phase current in the faulty phase
3I
0
is ground fault current
Z
1
is positive sequence impedance
Z
0
is zero sequence impedance
Section 8 1MRK 506 369-UUS -
Impedance protection
318 Line distance protection REL670 2.2 ANSI
Application manual
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