Performance analysis of microcomputer based differential protection of UHV lines under selective phase switching

The effects of primary and residual faults, as well as of couplings of the neighboring phases or lines, on the sensitivity of operation and threshold settings of microcomputer-based differential protection of UHV lines were investigated. The microcomputer-based differential protection appears to have poor performance when applied to uncompensated lines employing selective pole switching. A computer-based method which, for improved accuracy, utilizes the ABCD generalized line constants has been developed and used to deduce a set of analytical expressions for computing the residual fault currents, recovery voltages, and coupling effects of neighboring lines under varying isolated and connected fault conditions. The performance analysis and digital simulation of UHV lines ranging from 345 kV through 1500 kV have revealed that the residual faults, recovery voltages, and coupling of neighboring lines are rather large and have an adverse effect on the performance, sensitivity of operation, and threshold settings of differential protection and other protection algorithms. A microcomputer-based single-equation algorithm, independent of fault types, for per-phase differential protection without restraint is described. Shunt inductive and capacitive compensation has been proposed for improved performance