Back-up protection of distance relay second zone by directional overcurrent relays with combined curves

In this paper, a novel approach for simultaneous setting of distance and directional overcurrent relays is presented. When the line protection schemes are composed of distance relays and directional overcurrent relays the setting of the relays must be computed considering both relays. Separate relay computation would lead to loss of selectivity. If the second zone time setting is fixed, automatic determination of the settings could lead to a non-feasible situation, indicating the impossibility of finding a selective set of settings. Thus the second zone time must be manually changed until the optimization problem is feasible. In this method, the best setting for the second zone that assures selectivity could be larger than the classical setting, so, it could lead to the unfavorable timing of the second zone. In the proposed method, instead of changing the time of the second zone, the shape of the backup directional overcurrent relay would be changed adaptively with the fault location and current. The idea of reducing the overcurrent protection response times at higher fault currents is achieved using a universal protection device with a software platform that can facilitate designing time-current characteristic curves of different shapes, all in the same hardware. The simulation results indicate the effectiveness of the method