Decentralized Multi-Time Scale Power Control for a Hybrid AC/DC Microgrid With Multiple Subgrids

To facilitate the connection of various renewable ac and dc sources and loads to power systems, the hybrid ac/dc microgrid with multiple subgrids has been grasping more and more attention, in which the rated ac frequencies and dc voltages in different ac subgrids and dc subgrids can diversify for various applications. As the interaction among these subgrids is highly complicated, the stable and efficient operation of the hybrid microgrid is very challenging. Focusing on this problem, this paper proposes a decentralized multi-time scale power control method to make the interacted subgrids operate in coordination and support each other when the power of the system is fluctuated. The proposed control method stabilizes the power fluctuation in three time scales, of which the first two are conducted at the subgrid level and the last is conducted at the system level. In the first time scale, the ac frequency or dc voltage is changed to regulate the load power through the droop control. Further, in the second time scale, the output power of distributed generations is changed according to the deviations of ac frequency or dc voltage. Through these two coordinated control strategies, the power fluctuation is primarily stabilized. In the third time scale, the ac frequencies and dc voltages of all the subgrids are changed according to a designed coordinated power control strategy with consideration of the capacities and load types of each subgrids, which can make the subgrids support each other. Through this coordinated control strategy, the proper power interaction among different subgrids can be realized and the power fluctuation is further stabilized with the support of other subgrids. The effectiveness of the proposed power control method is verified by the real-time hardware-in-loop tests.