Ground fault analysis of low voltage DC micro-grids with active front-end converter

When a converter is used to interface an ac distribution system and a low voltage (LV) dc micro-grid, several combined options for ac and for dc grounding can be used. According to the selected grounding scheme, zero-sequence dc or high frequency currents may flow in specific sections of the circuit. These, in turn, can cause EMC issues, unwanted protection trips and other problems. In the present article a dc micro-grid in which the neutral point of the MV/LV transformer is directly grounded and the dc micro-grid is floating (i.e., there is no grounding connection) is analysed. When a ground fault occurs, potentially harmful fault currents flow through the front-end converter, depending on the fault resistance and on the load conditions. The methodology proposed in this work protects the front-end converter while the task of feeding the dc-side loads is committed to storage systems. The calculation of the minimum fault resistance that the front-end converter can tolerate is provided and the effectiveness of the proposed methodology is tested by means of numerical simulations.