Algorithms employing sparse techniques for coordinating directional overcurrent and distance relays on transmission network are described with an example system. Analytical expressions are derived for determining the limiting values of settings for zones 2 and 3 of distance relays on systems with both two and three-terminal systems. These values are then used to completely coordinate all the three zones of distance relays and to ensure maximum coverage of the lines by these zones. Some of the desirable enhancements added to the existing overcurrent relay coordination algorithm have also been described. 1.INTRODUCTIONThe basic role of a transmission protection system is to sense faults on lines or at substations and to rapidly isolate these faults by opening all incoming current paths. The usual practice of utilities to achieve protection is to provide high-speed primary protection followed by slower backup protection which is initiated only if the primary protective device fails. The coordination issue is that there should be sufficient time delay between the primary and backup relay operations thereby giving the primary relay a chance to operate first for any fault in its zone of protection. The number of primary/backup (P/B) relay pairs and the faults for which these are to be coordinated grow dramatically with the system size. Hence computer aids are necessary to carry out the coordination process of even a medium sized system. This paper presents a set of algorithms developed at the University of Washington (UW) to coordinate directional overcurrent and distance relays of a typical transmission/sub-transmission system. The process of coordinating a system of directional relays (either overcurrent or distance) involves setting relays one by one so that at each stage the relay being set coordinates with all its primary relays. The inherent multi-loop structure of a typical transmission system entails a large number of relay setting calculations to be done iteratively around all the loops of the system until system-wide coordination is obtained. In order to ensure the fast convergence of this iterative process, it has been found that an "optimal" sequence of relays is to be obtained before starting the relay coordinating process. Knable [1] has suggested a heuristic scheme to obtain this relay sequence. Dwarakanath and Nowitz [2] used linear graph theory to obtain a Relative Sequence Matrix (RSM) which determines the optimum relay sequence.This concept was further applied in detail to develop a CAD program for setting directional overcurrent and distance relays by Damborg et.al., [3,4]. Most of the other work related to computer aids [5-7] addressed only the coordination process and no mention of any particular relay sequence was made. The relays were considered in a random order. These coordination programs also did not perform complete coordination of zones 2 and 3 of all the distance relays.The algorithms presented in this paper are the enhanced versions of the earlier ones reported by us [3,4]. These enhance...
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