Coordination of over-current relays for cascaded parallel feeder

Abstract

This paper presents an idea of the relay coordination for cascaded parallel feeders fed from both ends as if it were a part of the interconnected power system. The criterion for setting of the relays in radial feeder is not exactly applicable to the parallel feeders fed from both the ends. The settings of the relays, placed on the diagonal corners of the parallel feeder should be such that they must provide back-up protection to each other. This requirement is satisfied easily, if the parallel feeder is fed from one end. But problem becomes more complicated when the parallel feeder is fed from both the ends as may be the case in an interconnected power system. For parallel feeder, it is hard to say that which relays are on source side and which are on load-side. The changing generating pattern, which may be due to maintenance work or due to the forced outage of the generators, introduces complexity in deciding the relay settings of directional over-current relays placed on parallel feeders. Also, different possible network configurations may give rise to the situation where the fault current is less than the rated current which necessitates the need of voltage monitored over-current relays. Reversal of power flow during healthy condition (not due to fault) in the lines of the power network, which is a very common phenomenon in an interconnected power system, affects the back-up protection criterion. It becomes very difficult for a power system engineer to make the relays to distinguish the difference between the current flowing in one direction without fault with maximum generation and the current flowing in the same direction with fault with minimum generation, particularly in the case of parallel feeder fed from both the ends. The primary-back-up relay pairs may exchange their main roles due to the reversal of the direction of the power flow. Still their settings should be such that they operate in proper sequence, not violating the principles of the protection. (7 pages)