Two transmission lines that are within close proximity of each other are “mutually coupled” to each other in the zero sequence.  By “mutual coupled” we mean that the fault current flowing on one of the lines causes a smaller current to flow -- in the opposite direction -- on the neighboring line.  The modelling and simulation of mutual coupling between lines is important to the design of ground protection schemes.

Mutually coupling data are specified between a pair of lines at a time.  You can specify up to 5 segment of mutual coupling between any two lines without the use of artificial tap buses. The mutual coupling parameters of each segment include:

•      The per-unit mutual coupling impedance of segment.

•      For each line, the beginning and ending percentage that is mutually coupled to the other line.

The mechanics of how to enter these data are explained in the documentation of the Network | New | Mutual Coupling Pair command in Section 3 of this manual.

There is no industry standard on how close two line have to be, or how long the two lines have to run alongside together, before you model their mutual coupling.  As a result, the number of mutual coupling pairs in network models varies widely from utility to utility.  A utility with a large number of mutual in its network model may want to know whether the mutual couplings are slowing down the short circuit solutions.

To explore the effect of mutual coupling on solution speed, we suggest you execute the Fault | Options command.  The dialog box shown below will appear.  Near the bottom of the dialog box is the parameter “Ignore Mutuals < This Threshold”.  If you set this threshold to 9999, then all the mutuals in your network will be ignored.  Near the other extreme, if you set the threshold to 0.05, for example, OneLiner will ignore only mutuals with mutual-coupling impedance lower than 0.05 pu.  Some utilities may find a small threshold will help speeding up the fault solutions without scarifying accuracy.