The paper deals with the errors inherent in the application of sound-phase and residual compensation to distance-relay protection of transposed and untransposed double-circuit lines, taking into account the combined effect of load and fault currents. The treatment thus extends consideration beyond the cases where the two main compensation systems are assumed to be ideal, under transposed conditions, and are then applied to practical untransposed systems, usually neglecting the effect of load currents. Earlier work suggested that the sound-phase-compensation scheme was basically acceptable; it is shown, however, that there are substantial measuring errors by the relays in the faulted and unfaulted circuits of the double-circuit system, whether transposed or not. The errors for both types of compensation are determined for a range of source impedances, and are presented as loci of apparent impedances `seen???????? in the complex RX plane. The errors inherent in the compensation systems, as a consequence of assuming that their complex-number ratios are scalar, are also derived. The errors associated with sound-phase-compensation and residual-compensation systems for cases of simultaneous earth faults are considered.
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