Most multi circuit lines have two parallel operating circuits.
Parallel line applications
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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 earthed.
3. Parallel line out of service and not earthed.
Parallel line in service
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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
173.
From symmetrical components, we can derive the impedance Z at the relay point
for normal lines without mutual coupling according to equation
294.
Z
U
I I
Z Z
Z
U
I I K
ph
ph
ph
ph N
=
+ ⋅
−
⋅
=
+ ⋅
3
3
3
0
0 1
1
0
IECEQUATION1275 V2 EN-US (Equation 294)
Where:
Uph is phase to earth voltage at the relay point
Iph is phase current in the faulty phase
3I0 is earth fault current
Z1 is positive sequence impedance
Z0 is zero sequence impedance
Z
0m
A B
Z< Z<
IEC09000250_1_en.vsd
IEC09000250 V1 EN-US
Figure 173: Class 1, parallel line in service
The equivalent circuit of the lines can be simplified, see figure 174.
Section 8 1MRK 506 369-UEN B
Impedance protection
340 Line distance protection REL670 2.2 IEC
Application manual
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