This paper examines the effects of primary and secondary fault quantities as well as of mutual couplings of neighboring circuits on the sensitivity of operation and threshold settigns of a microcomputer based differential protection of UHV lines under selective phase switching. Microcomputer based selective phase switching allows the disconnection of minimum number of phases involved in a fault and requires the autoreclosing of these phases immediately after the extinction of secondary arc. During a primary fault a heavy current contribution to the healthy phases tends to cause an unwanted tripping. Faulty phases physically disconnected constitute an isolated fault which being coupled to the system affects the current and voltage levels of the healthy phases still retained in the system and may cause an unwanted tripping. 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 have 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 a per phase differential protection without restraint has been described. Shunt inductive and capacitive compensation has been proposed for improved performance. © 1990 IEEE
