Topological processing of mutually coupled circuits for directional overcurrent protection

Well planned and adjusted protection devices increase the reliability of the system against fault events. The directional overcurrent relays, for example, play an important role in the protection of meshed transmission systems. They are usually employed as backup protection of distance relays. However, even as backup protection, the correct coordination can be set to speed up the fault mitigation and to minimize the affected areas. The requirement for setting the directional overcurrent relays is the knowledge of the short circuit currents under specific conditions. The presence of mutually coupled lines may cause significant impact on the short circuit calculation, thus affecting the currents for which the protection devices have been adjusted to. In many cases, the coupled lines are formed by sections and only some parts of the transmission lines are under direct influence of those effects. Even so, it is important to perform a topological processing to identify the real extension of the lines for the correct calculation of the currents related to mutual impedance. This paper shows the effect of the mutual coupling on the calculation of currents for different operating conditions and the the topological processing to provide the correct results.