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 relay s can vary. In the case of a distance relay protecting a phase of a do uble-circuit line, especially 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 t han 100% of the total line length. This is demonstrated with a double- ircuit line under SLG fault condition, since this is the most ocuuring type of fault. In this paper the distance protection of a double-circ uit line under SLG fault condition is formulated. To achieve correct o peration, the relay does not use the measured quantities of the circui t-to-be-protected only, but also the zero sequence current of the para llel circuit. Such a relay requires extra measuring equipment, and, mo reover, the zero sequence current of the parallel circuit cannot alway s be measured. Therefore, another approach is chosen. A correction fac tor 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 s ettings is decreased, due to the uncertainty in the power system state . By adapting the relay to the actual power system state, maximum sele ctivity is achieved, and the protection system as such will be more re liable.