1. Class 1: Parallel line with common positive and zero-sequence network
2. Class 2: Parallel circuits with common positive but isolated or separated zero-
sequence network
3. Class 3: Parallel circuits with positive and zero-sequence sources isolated or
separated.
One example of class 3 networks could be the mutual coupling between a 400 kV line and
rail road overhead lines. This type of mutual coupling is not so common although it exists
and is not treated any further in this manual.
The most used configuration is class 1 network: Parallel circuit with common positive and
zero-sequence sources. In this type of networks, the parallel transmission lines terminate
at common nodes at both ends.
For each type of network class we can have three different topologies; the parallel line can
be in service, out of service, out of service and grounded in both ends.
The reach of the distance protection zone 1 will be different depending on the operation
condition of the parallel line. It is therefore recommended to use the different setting
groups to handle the cases when the parallel line is in operation and out of service and
grounded at both ends.
Impact on distance protection
Distance relaying of ph-ph and three-phase faults is not influenced by the parallel line. For
protection of phase-to-ground faults, however a measuring error occurs. In principle, this
error appears due to the fact that the parallel line ground-current (I
EP
= 3.I
0P
) induces a
voltage I
EP.
Z
0m
/3 in to the fault loop.
The distance relay phase-to-ground units measure:
ANSIEQUATION14004 V2 EN (Equation 161)
Where:
V
ph
is phase to ground short circuit voltage at the relay location in the
faulted phase
I
ph
is short circuit current in the faulted phase
Table continues on next page
Section 8 1MRK 506 369-UUS -
Impedance protection
262 Line distance protection REL670 2.2 ANSI
Application manual
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