Improving the Protection of Ehv Teed Feeders Using Local Agents

The protection of multi-terminal lines is not as simple as that of two-terminal lines. They usually experience additional problems caused by the intermediate infeed from the third terminal, an outfeed, different line lengths to the tee point, etc. The limitations of non-pilot distance relays for their protection have been acknowledged for decades. These limitations lead to the traditional distance relaying approach to experience severe underreach and overreach problems. The relay first zone reach may not even extend beyond the tee point in some underreach cases (1),(2). An approach based on adaptive relaying to face such a situation and some fundamental concepts concerning it was presented by Rockefeller (3). Horowitz et al (4) define adaptive relaying as a protection philosophy which permits and seeks to make adjustments to various protection functions in order to make them more attuned to prevailing power system conditions. An adaptive setting concept for two and three terminal lines, which can respond to changes in the network conditions, was proposed by Xia et al (5), (6). Stedall et al (7) also proposed an investigation into the use of adaptive setting techniques for improved distance back-up protection. This paper discusses the adaptation of the settings of distance relays for multi-terminal lines employing agents. Agents are software processes capable of searching for information in networks, interacting with pieces of equipment and performing tasks on behalf of their owners (relays). Moreover, they are autonomous and cooperative. Very few publications concerning the application of agents in the protection field have been reported in the literature. Tomita et al (8) proposed a cooperative protection system utilizing agents. Relay agents were constructed, and the cooperation of the main agents was simulated for primary, backup and adaptive protection. Results illustrating the improved performance of the adaptive method proposed compared to conventional fixed settings are presented in the following sections.