Smart overcurrent relay for operating in islanded and grid-connected modes of a micro-grid without needing communication systems

As the penetration of distributed generation resources in distribution networks increases, the need for designing micro-grids also increases. Micro-grids can operate in islanded and grid-connected modes. When a microgrid is connected to the main grid, it economically exchanges power with the upstream grid. However, in the islanded mode, the micro-grid can provide its own local demand. In addition to the technical and economic advantages due to the presence of micro-grids, there are some issues about the protection of distribution systems. The most important issue is the short circuit level in both islanded and grid-connected modes of operation. In this paper, a smart overcurrent relay is proposed that can adapt itself to the operation modes of a micro-grid. This relay detects the operation mode of the micro-grid using an adaptive algorithm. Two main features of the suggested adaptive method are the optimized accuracy and higher execution speed of the algorithm in determining the operation mode of the micro-grid. The optimal setting that is predefined in the relay is then activated based on the corresponding mode. The entire processing is performed in the relay itself by sampling the voltage and current of the relay line. Consequently, this relay can be installed and used in any location of a network without needing any communication infrastructure. Finally, the proposed method is implemented on a micro-grid. According to the numerical results, the coordination performance superiority of this smart relay over conventional overcurrent relays is proven.