Stability Analysis And Islanding Operation Control Of Hybrid AC/DC Microgrids Based On MMC

To deal with the severe challenges brought by climate change,environmental pollution,and energy structure transformation since the 21 st century,the third industrial revolution characterized by the deep integration of distributed renewable energy and information technology has begun."Energy Internet" is an indispensable core technology in the industrial revolution,and also represents the future development direction of the energy industry.As a friendly,flexible generation and distribution system and efficient energy management unit,the microgrid plays an irreplaceable critical role in the energy internet,which connects the traditional large power grid with the emerging distributed generation system.With the diversification of distributed power supply and the continuous increase of load types and demands,the application scope of AC-DC hybrid microgrid is gradually expanding,and the research on the optimization of microgrid structure and efficient operation control has received more and more attention.Aiming at the problems of high voltage harmonic content,low voltage level,and difficult capacity expansion in AC/DC hybrid microgrid interconnected by two-level voltage source converters,this dissertation introduces modular multilevel converter into AC/DC hybrid microgrid as the energy router between AC/DC sub-grids and constructs a modular multilevel converter-hybrid microgrids(MMC-HMGs).This dissertation focuses on the island operation mode,power transfer characteristics,stability control of interfaced MMC,operation control strategy,start-up and pre-charging control,modeling,and small signal stability analysis of MMC-HMGs system:1)Firstly,multi-bus AC/DC hybrid microgrids system with MMC as an interfaced converter is constructed,and the "grid to grid" power transmission characteristics of the system is analyzed based on the power circle diagram.Based on the islanded operation mode,the generalized equivalent short circuit ratio is defined to measure the relative strength of the AC sub-grid.According to the relative strength of the AC subgrid,MMC is positioned from the perspective of the control function.Based on the critical generalized equivalent short circuit ratio,the control roles of MMC is respectively positioned as system stability converter and power regulation converter.2)When the generalized equivalent short-circuit ratio of the MMC-HMGs system is small,the AC sub-grid behaves as the weak grid.Aiming at the problems of low inertia and small damping when the weak AC sub-grid operates in an isolated island,combined with the control requirements when MMC acts as the system stable converter,the virtual synchronous machine control strategy of MMC is studied.MMC rectifier operation and inverter operation are equivalent to the virtual synchronous motor and virtual synchronous generator,respectively,to realize bi-directional active power flow scheduling of the MMC-HMGs system.At the same time,the arm current is used as the direct control quantity to control the AC side current and suppress the circulating current,which simplifies the design of the current loop.3)When the generalized equivalent short circuit ratio of the MMC-HMGs system is large,MMC acts as a power regulation converter to improve the voltage and current dynamic performance of the AC sub-grid and improve the power supply quality of the system.A passive control method based on the port Hamiltonian model is proposed to realize phase independent control by selecting arm differential mode electromotive force and unbalanced current as control variables.In order to solve the problem of nonobservability of some state variables,a state estimator is constructed and the global stability of the system is proved.4)Aiming at the possible "all black" state of the MMC-HMGs system in islanded operation,this dissertation studies the black start recovery strategy based on the hierarchical idea and analyzes the Boost series equivalent circuit of DC sub-grid controllable charging process,and establishes the equivalent model of the pre-charging stage.On this basis,the pre-charging controller of MMC is designed.5)Combined with the dynamic characteristics of AC/DC micro source and energy storage in the MMC-HMGs system,the state-space model of the system is established based on the dynamic phasor method,and the simulation results are compared with the electromagnetic transient model to verify the accuracy of the dynamic phasor statespace model.The small-signal model is obtained by linearizing the state-space model of the system.Based on this model,the influence of line parameters and control parameters on the small-signal stability of the system are analyzed.