Study and application of mega-watts interconnections for future urban distribution networks

Traditional AC medium-voltage distribution network is designed in the open-loop mode and the contact switch between different regions is normally in cold standby state. However, with the rapid increasing of DC household appliances, distributed photovoltaic sources, energy storage equipment, charging facilities, DC data centres and so on, aiming at high safety, efficiency and reliability for power supply, the research and application of hybrid distribution network has abundant application scenarios and tremendous potential values. First of all, flexible interconnection of AC and DC grid is described and analysed. And then a novel scheme to establish AC/DC hybrid distribution network by adding the multi-terminal flexible DC interconnection into the traditional medium-voltage AC networks is proposed, which is capable of controlling bi-directional power flow for multiple feeders and providing not only various interfaces for the forementioned new types of loads and renewable energy, but also mutual power supply during fault-on occurrence. The basic composition of flexible DC interconnection adopts mega-watts active-neutral-point-clamped converters based on IGCT-in-series, with advantages of modularity, scalability and high power density. Besides, the security and stability improvement of the proposed hybrid interconnected distribution network for urban power grid is presented and discussed, and a converter prototype is built-up to verify the feasibility with basic experiment results.