Optimal Coordination of Directional Overcurrent Relays for Microgrids Using ‎Hybrid Interval Linear Programming - Differential Evolution

The relay coordination problem of directional overcurrent has  been an  active research issue in distribution networks and power transmission. In general, the problem of relay coordination is the nonlinearity of the optimization problem, which increases or decreases with different network structures. This paper presents a new method with directional overcurrent relay coordination approach to reduce the operating time of the relays between the primary and backup relays by using hybrid  programming  of  ILP  (interval  linear  programming)  and  DE (differential  evolution).  Due to the difference in short circuit current level from grid connected to the isolated mode, therefore, it is necessary to use a reliable protection solution to reduce this discrimination time and also to prevent the increase of coordination time interval (CTI). The ability of the objective function used in this paper is to reduce the discrimination time of primary and backup relays and simultaneously reduce the operating time of primary and backup relays by introducing a new method. The basic parameters of the directional  overcurrent  relay  (DOCR)  such  as  time  multiplier  setting  (TMS)  and  plug  setting  (PS)  have been adjusted  such  that  the relays  operation  time  should  be  optimized.  Optimization  is  based  on  a  new objective function,  described  as  a  highly  constrained  non-linear  problem  to  simultaneously  minimize  operating  time  in backup  and  primary  relays.  A  function  of  penalty  is  also  used  to  check  the  problem  constraints  in  case  the backup relay time is  fewer  than  that of  the main relay. The method is implemented on modified IEEE 14- and 30-bus distribution networks. The results demonstrate the efficiency of the method, and the values are optimal compared to those of other algorithms. MATLAB program has also been used to simulate optimization.