Some factors affecting the accuracy of distance-type protective equipment under earth-fault conditions

Abstract
Distance-type protective schemes are widely used in conjunction with e.h.v. overhead lines. Because of imperfections in the measuring elements, transformer errors, etc., it is necessary to allow a factor of safety when applying such schemes, and therefore instantaneous operation is only provided for faults in part of the line length. Improvements in relay performance have tended to reduce the normally adopted safety margin. It was felt that the magnitudes of certain errors which can arise were not sufficiently appreciated and that they should be investigated, to indicate the possible risks incurred when reducing the safety margin. The first part of the paper is concerned with the errors introduced when schemes based on transposed-line performance are applied to an untransposed single-circuit line, this practice being invariably adopted. It is shown that, when earth faults occur, schemes may overreach appreciably (10% or more) under these conditions and that false tripping could occur if first-stage settings much in excess of 80% of the line length were used. While this source of error could be eliminated by using correctly set sound-phase compensation, it is unavoidable if residual compensation is employed. The second part deals with errors caused by the mutual effects between adjacent circuits of a double-circuit set of lines. It is shown that schemes used to compensate for the mutual effects can lead to incorrect operation of an equipment when earth faults occur on the adjacent line. Such schemes are therefore unsatisfactory. If the practice of neglecting the mutual effects is adopted, there are measuring errors, but these do not tend to lead to maloperation whatever first-stage settings are used, and thus this method appears to be preferable.

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