Comparison of time coordination feasibility criteria for a subtransmission system protection scheme

This paper presents the evaluation of a line protection scheme for interconnected power subtransmission systems by comparison of the feasibility of relay time coordination, under different criteria. The protection scheme consists of distance relays as primary protection and directional overcurrent relays as back-up relays at 69 kV. A coordination computer program that uses linear programming and goal programming techniques is employed to determine the relay settings. Three cases are considered: (1) independent coordination of inverse-time relays and distance relays; (2) combined coordination of inverse-time directional overcurrent relays considering distance relays; and (3) combined coordination of definite-time-high-current setting directional overcurrent relays. The test case consists of a system located in the Eastern zone of Venezuela with 108 buses, 86 lines, 61 transformers and 97 directional overcurrent relays. It is shown that all three cases have advantages and disadvantages regarding the feasibility and viability of relay coordination, however the case with definite-time-high-current setting directional overcurrent relays is simpler for time coordination achievement.