Microgrid Protection Strategies for the Swedish Power System

Abstract: This thesis mainly focuses on the protection strategies of microgrids for the Swedish power system in the medium voltage distribution system and specifically on the three-phase fault and earth faults protection strategies. For the analysis of both faults, different grid-connected, and islanding operations are simulated using DIgSILENT Power Factory. The simulation shows that using a distance protection strategy for a three-phase fault in the microgrid, the fault can be detected on cable in grid-connected and in islanding operations but it creates blinding of the relay in backup protection when the fault location is near the grid side in grid-connected mode and near the distributed generator side in islanding mode. In Sweden, the medium voltage distribution system uses peterson coil earthing thus fault current is low during an earth fault. Directional earth fault protection with a communication line method is employed for the selective detection of the earth fault of the system. By using this method, the earth fault can be selectively cleared during grid connection and islanding operation with low fault resistance value. For high fault resistance values, the fault current is lower than the rating of the current transformer, so it is not possible to detect earth faults.