عنوان مقاله [English]
In this paper, a new method is presented to determine the optimal settings of instantaneous unit and delay unit of combined overcurrent relay using probabilistic modeling of uncertainties. The uncertainties such as measurement error, operating conditions of system and fault conditions, are modelled probabilistically and use the Monte-Carlo method to determine the interval fault current passing through the relays. Two probabilistic sensitivity and selectivity indexes are defined for instantaneous unit of relay and the problem of determine the optimal setting of instantaneous unit is formulated as an optimization problem. The objective function of this problem is maximize the probabilistic sensitivity index and the complete selectivity is as constraint. Then, an interval method is used to solve this optimization problem and determine the optimal settings of instantaneous unit of relays. Also, using probabilistic modeling of uncertainties, the problem of determine the optimal settings of delay unit is formulated as an optimization problem and a hybrid Genetic Algorithm-Monte Carlo method has been used to solve it. The numerical results in two 8-bus and 14-bus IEEE test system showed that if using the proposed method, the more area of line is protected instantaneously and the remaining line has been protected by delay unit in minimum possible time and with complete selectivity.