Adaptive distance protection of a double-circuit line

Due to changes in the power system, such as generator and line outages and changes in load and generation, the performance of distance relays can vary. In the case of a distance relay protecting a phase of a double-circuit line, the state of the parallel circuit is of major importance. Simulations show that, depending on the power system state, a distance relay can cover from less than 50% up to far more than 100% of the total line length. This is demonstrated with a double-circuit line under the single-line-to-ground fault (SLG) fault condition, since this is the most common type of fault. In this paper the distance protection of a double-circuit line under the SLG fault condition is formulated. To achieve correct operation, the relay does not only use the measured quantities of the circuit-to-be-protected, but also the zero sequence current of the parallel circuit. Such a relay requires extra measuring equipment, and, moreover, the zero sequence current of the parallel circuit cannot always be measured. Therefore, another approach is chosen. A correction factor is introduced, set adaptively according to the actual power system state. In this way, the appropriate setting of the relay is provided, in relation with the actual power system state. A side-effect of the adaptive setting of the relay is that the safety margin in the relay settings is decreased, due to the uncertainty in the power system state. By adapting the relay to the actual power system state, maximum selectivity is achieved, and the protection system as such will be more reliable