ABB RET670 Applications Manual page 201

1MRK504116-UUS C
Application manual
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 to experience
mutual coupling, and some coupling exists even for lines that are separated by 100
meters 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:
• Parallel line with common positive and zero sequence network
• Parallel circuits with common positive but isolated zero-sequence network
• 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.
Section 3
IED application
195