An application of rule based system to the coordination of directional overcurrent relays

In this paper, we apply a rule based system, in which production rules are utilized to solve a domain problem, to the relay settings and its time delay coordination for directional overcurrent relays. In order to proceed them efficiently, we first propose a simple algorithm for finding the order of relay settings by using heuristic rules which are mainly based on four different categories of knowledge. These rules check all primary-backup relay pairs to detect a specific loop from system configurations, identifying a minimal set of relays to be set initially (break point set). Then, the order of relay settings is identified from the obtained break point set. The effectiveness of the proposed algorithm is demonstrated by some examples. The relay settings and its coordination are also determined by applying experimental rules obtained from the process in constructing the proposed rule based system. In case of the setting of overcurrent relays, we must select two parameters for each relay as follows: time dial settings and pickup current settings. The proposed system allows for discrete pickup setting values in order to avoid rounding off the obtained solutions to the nearest discrete values. The setting rules in the expert system are applied in the order of the relay settings determined by the proposed algorithm, trying to keep the minimum coordination time intervals. The proposed system has been developed on a personal computer by making use of OPS83 and applied to a simple system to verify its effectiveness