Hybrid DC Power Supply Technology Based on CNPC for Isolated Microgrid Containing High Proportion of Renewable Energy

To improve the stability of isolated microgrid operation with high proportion of renewable energy, a hybrid DC power supply technology scheme based on cascaded neutral point clamped three-level converter (CNPC) is proposed, which integrates the technical and economic advantages of LCC-HVDC and VSC-HVDC technologies. The transient control strategies and characteristics of CNPC are studied. Simulation shows that CNPC starts smoothly, DC voltage is balanced, negative sequence current can be effectively suppressed under unbalanced system conditions, AC fault ride-through can be achieved, and AC voltage/frequency of the isolated microgrid can be controlled according to the set control objectives in islanding mode. The LCC-CNPC hybrid DC power supply system scheme is studied, and we recommend that a symmetrical bipolar wiring mode is adopted on DC side to improve the reliability and operational flexibility; an isolated neutral system is adopted on AC side to improve power supply reliability; and low-resistance grounding method is adopted on DC side to effectively suppress fault current. The prototype test shows that the LCC-CNPC hybrid DC system has fast response capability, and the load start performance in islanding mode is verified.