or more. The reason to the introduced error in measuring due to mutual coupling is the zero
sequence voltage inversion that occurs.
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 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.
Those 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 class3 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.
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 zone1 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.
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.
Most multi circuit lines have two parallel operating circuits. The application guide
mentioned below recommends in more detail the setting practice for this particular type of
line. The basic principles also apply to other multi circuit lines.
Parallel line applications
This type of networks are defined as those networks where the parallel transmission lines
terminate at common nodes at both ends. We consider the three most common operation
modes:
Section 8 1MRK 506 369-UUS -
Impedance protection
286 Line distance protection REL670 2.2 ANSI
Application manual
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