The correct operation of ground distance relays is highly dependent on the correct application of the residual or zero-sequence compensation factor. But what are these factors. Various relays vendors have different forms of naming, defining and applying these factors and this confuses relay application engineers. Names like KN, K0, KE, KG, KZN, RE/RL, XE/XL, Z0/Z1 and others are used; some factors are simple scalar numbers and others are in Vectorial form. A factor K0 in one relay can have a different definition from that of another relay that uses exactly the same name, and this may be true whether the relays come from different vendors or even from the same vendor.  An incorrect compensation setting can result in either misoperation of the relay or its failure to operate for ground faults. This paper provides some derivations of residual compensation and a survey of the various forms of compensation, their definitions and how to convert from one form to another.

Another confusion in the application ground distance relays that have polygon-shaped characteristics is the way the resistive reach is defined. In some relays the relay resistive reach setting is defined as a per phase resistance, which can be affected by the residual compensation factor; while in another relay it could be the ground loop resistance, which is not affected by the residual compensation factor. In addition the angle of the resistive blinder may or may not be influenced by the residual compensation angle. Hence, there is a need to accurately model and visualize the relay characteristic consistently, especially when comparing relays.

In order to ensure that the relay is set correctly and operates as expected it is necessary to test the relay characteristic for ground faults and verify that it matches the relay settings and expected characteristics. Modeling the ground relay characteristic and using the correct residual compensation setting can be a complex subject for the test technician. Depending on how the relay resistive settings are entered, questions arise on how to model the relay polygon characteristics - should it be modeled in the per phase plane, or in the loop plane, or should the reactance line be modeled per phase and the resistance line modeled per loop since many distance relays enter the settings as such? Even mho ground characteristics can be a challenge when the residual compensation setting is Vectorial with angles far away from zero.

When testing the ground distance relay the capability of the protective relay test system becomes another important factor since the test system may or may not have the capability to exactly model the relay characteristic and residual compensation factor. One may have to convert the compensation to another form and model the characteristics in another way when testing and viewing the results of the test. This paper describes some available methods to do this and offers suggestions for testing ground distance relays.

A good understanding of these concepts is essential for relaying personnel in order to properly set the relays and test them. Post mortem analysis of relay operations also requires a good understanding of these concepts.