ABB RET670 Applications Manual page 274

Section 3
IED application
268
blinder might cut off a larger part of the operating area of the circle (see to the right of
figure 115).
It is recommended to use at least one of the load discrimination functions for long
heavy loaded transmission lines.
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 lines. The lines need not to 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 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 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.
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 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.
Five zone distance protection, mho characteristic function (ZMHPDIS) 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:
1MRK504116-UUS C
Application manual