Classes of networks
From an application point-of-view, there are three types of network configurations
(classes) which must be considered when making the settings for the protection
function.
The different network configuration classes are:
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 earthed 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 earthed 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-earth faults, however a measuring error occurs. In
principle, this error appears due to the fact that the parallel line earth-current (I
EP
=
3.I
0P
) induces a voltage I
EP.
Z
0m
/3 in to the fault loop.
The distance relay phase-to-earth units measure:
IECEQUATION14004 V1 EN-US (Equation 158)
Where:
U
ph
is phase to earth 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
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 245
Application manual
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