Parallel Line Application With Mutual Coupling - ABB REL650 Applications Manual

Section 6
Impedance protection
6.1.2.5
120

Parallel line application with mutual coupling

General
Introduction of parallel lines in the network is increasing due to difficulties to get
necessary area for new lines.
Parallel lines introduce an error in the measurement due to the mutual coupling between
the parallel lines. The lines need not be of the same voltage in order to experience mutual
coupling, and some coupling exists even for lines that are separated by 100 meters or
more. The mutual coupling does influence the zero sequence impedance to the fault point
but it does not normally cause voltage inversion.
It can be shown from analytical calculations of line impedances that the mutual
impedances for positive and negative sequence are very small (< 1-2%) of the self
impedance and it is a practice to neglect them.
From an application point of view there exists three types of network configurations
(classes) that must be considered when making the settings for the protection function.
The different network configuration classes are:
1. Parallel line with common positive and zero sequence network
2. Parallel circuits with common positive but isolated zero sequence network
3. Parallel circuits with positive and zero sequence sources isolated.
One example of class 3 networks could be the mutual coupling between a 400kV 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.
For each type of network class, there are 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. This can be handled by the use of different setting groups for
handling the cases when the parallel line is in operation and out of service and grounded at
both ends.
The distance protection within the IED can compensate for the influence of a zero
sequence mutual coupling on the measurement at single phase-to-ground faults in the
following ways, by using:
• The possibility of different setting values that influence the ground-return
compensation for different distance zones within the same group of setting
parameters.
• Different groups of setting parameters for different operating conditions of a
protected multi circuit line.
1MRK 506 334-UUS A
Application manual