Novel Voltage Support Strategies for DGs in Hybrid AC/DC Distribution Networks Under Faults

According to the grid codes, the higher the voltage at the point of common coupling (PCC), the longer duration that the voltage-source converters (VSC) can stay connected to the grid. In hybrid AC/DC distribution networks (HADNs), owing to the low capacity, VSC-based distributed generator (DG) closest to the fault point may fail to significantly support the voltage at the PCC through current injection. Therefore, other large-capacity converters such as flexible AC/DC converters (FACs) should also inject currents to help it. However, in resistive-inductive HADNs, the voltage support effect is related to the current phases of converters, and the fault conditions. In this paper, circumstances under two fault position categories (upstream and downstream fault positions of DG) for double line-to-ground fault and three-phase short-circuit fault are analyzed. The interconnected sequence networks of HADNs under these two fault positions are proposed. Based on the proposed networks, the optimal current phase angles of DG and FAC for maximumly supporting voltages at the PCC of the DG are proposed. Moreover, the line sequence impedance calculation method is proposed. The simulation results verify the proposed strategy. The proposed strategy is suitable for low-capacity DGs in resistive-inductive HADNs to achieve voltage support and fault ride-through.