Research On Operation Control Method Of Hybrid AC/DC Microgrid With Single Subgrid And Multiple Subgrids

Hybrid AC/DC microgrid has advantages of high conversion efficiency,system flexibility and reliability,making full use of the form of AC power supply and DC power supply.Therefore,constructing of hybrid AC/DC microgrid has become a trend of distribution system development in the future.The hybrid AC/DC microgrid with single subgrid in stand-alone operation mode without the support of the utility grid,can easily occur the fluctuation of AC bus voltage\frequency and DC bus voltage due to the changing load power.Meanwhile,unreasonable power distribution and low inertia will also affect the stability of hybrid AC/DC microgrid.The hybrid AC/DC microgrid with multiple subgrids in stand-alone operation mode not only has the problem of fluctuating AC bus voltage\frequency and DC bus voltage,but also owes the complex power coordination control system because of the coupling between multiple subgrids.At the same time,since there are multiple coupling relationships and interactions between subgrids,the analysis in case of system failure is more important,especially the influence of faults on control strategy.When hybrid AC/DC microgrid changes the operation mode from stand-alone to grid-connected,it may produce large impulse current,which may damage the grid-connected inverter.When hybrid AC/DC microgrid operates in the grid-connected mode,the coupling of output power will further affect the distribution accuracy of output power in hybrid AC/DC microgrid.Based on the problems mentioned above,the operation control method of hybrid AC/DC microgrid with single subgrid and multiple subgrids are studied in this thesis.The main research results are as follows:(1)Aiming at the problem of power distribution and low inertia of hybrid AC/DC microgrid with single subgrid in stand-alone operation mode,a flexible power control method with the suppression of AC bus voltage fluctuation based on the coordination of bi-directional DC/AC converter(BAC)and energy storage(ES)unit are proposed.Autonomous power allocation in AC and DC subgrids and power mutual assistance between subgrids are realized.Furthermore,the inertia of hybrid microgrid is improved,and the fluctuations of AC voltage/frequency and DC voltage are reduced.The correctness and effectiveness of the proposed control strategy are verified by hardware in the loop experiment.(2)Aiming at the power coordination control problem of hybrid AC/DC microgrid with multiple subgrids in island operation state,the multi-functional control strategy of bi-directional DC/DC converter(BDC)and BAC is proposed,also with the power coordination control strategy based on model prediction.The coordinated power distribution within AC and DC subgrids and among multiple subgrids are both realized,enabling BDC has the support capability of DC subgrid and DC common bus(DCB)voltage.On the other hand,BAC owes the ability to support AC subgrid at the same time.The correctness and effectiveness of the proposed control strategy are verified by hardware in the loop experiment.(3)Aiming at the double frequency disturbance of DC voltage and DC power caused by asymmetric fault of AC subgrid,the suppression control of double frequency disturbance in DCB voltage are proposed,effectively suppressing the double frequency disturbance of DCB voltage,and reducing the fluctuations of AC frequency,DC voltage and power.In addition,aiming at the frequency stability of AC subgrid after faults removal,the frequency support is designed to realize the frequency support of fault AC subgrid.The correctness and effectiveness of the proposed control strategy are verified by hardware in the loop experiment.(4)Aiming at the problem of excessive grid connected impulse current in the process of grid-connected mode,and output power coupling after grid connection,a fast and shock-free grid connection control strategy and a power decoupling control strategy based on linear active disturbance rejection control(LADRC)are proposed.The accurate voltage tracking before grid connection is realized to reduce the grid connection impulse current.The decoupling of active power loop and reactive power loop is designed to improve the accuracy of output power distribution.Finally,the correctness and effectiveness of the proposed control strategy are verified by hardware in the loop experiment.