Comparison of five methods of compensation for the ground distance function and assessment of their effect on the resistive reach in quadrilateral characteristics

This article compares five compensation methods for the ground distance function, in order to show the maximum fault resistances that the relays can see when they have quadrilateral characteristics. These five compensation methods are based on the description of real protective functions, found in manuals of commercial relays. For a power system taken as an example, the impedances seen by the relays are computed for each form of compensation and for different system conditions. The results show that: (a) the locus of the apparent impedance is very different for each compensation method; (b) the maximum fault resistance seen with each compensation method can be very different, although the same quadrilateral characteristic is adjusted in the relays. Three of these compensation methods (A-C) are based on the positive-sequence impedance of the line, and two of these compensation methods (D, E) are based on the impedance of the ground-fault loop. The results also show that: (a) methods D and E tend to over-reach or under-reach, for solid faults; (b) the coverage for resistive faults tend to be greater for methods A and B than for methods C, D, and E; (c) the loci of the apparent impedance tend to be flat only for method C. In general, the knowledge of the behavior of the compensation method for the ground distance function is important because it should be considered when the relay settings are computed and/or when the faults are analyzed.