Research On Coordinated Control Strategy Of Hybrid AC/DC Microgrid Based On Virtual Synchronous Machine Technology

In order to meet the major needs of national energy structure transformation,renewable energy power generation technologies develop rapidly in recent years.With the development of distributed energy and energy storage technology as well as the increase of load types,the AC-DC hybrid microgrids,which combines the advantages of AC microgrids and DC microgrids,become an inevitable trend in the future development of microgrid technology.Due to its complex structure,numerous indexes,and consideration given to the power balance and voltage/frequency stability on both sides,the coordinated control of AC-DC hybrid microgrids is particularly important,especially in the island mode when microgrids lack the support of the whole power grid.As a bridge between AC and DC buses,the interlinking converters(ILCs)are the core device of a hybrid microgrid,which play a significant role in coordinating the AC and DC subsystems.In recent years,with the continuous increase of the penetration of renewable energy,Distributed Generations(DGs)characterized by intermittence,fluctuation and randomness often cause voltage or frequency fluctuations of AC and DC buses.AC-DC hybrid microgrids often suffer from low inertia and stability problems.The Virtual Synchronous Machine(VSM)technology can effectively increase the inertia of power electronics converter by simulating the inertia and damping characteristics of traditional synchronous machine with specific control strategies.However,at present,researches on VSM technology are mainly focused on large power grid or AC microgrid.Therefore,based on the idea of VSM,this paper studies the coordinated control strategy of AC-DC hybrid microgrid to improve its inertia and stability while to coordinate the power distribution as well.The detailed arrangements are as follows:Firstly,the basic frame structure of AC-DC hybrid microgrids and the traditional coordinated control strategies of microgrids are described.Then,the mathematical model of the core device ILCs,and their basic power coordinated control strategies,are derived.Then,the basic principle of VSM technology is analyzed,including the process of imitating synchronous machines and small signal models.These researches lay the foundation for the following research of AC-DC hybrid microgrid coordinated control strategy based on VSM technology.Secondly,based on traditional normalized droop control,the idea of VSM,and the analysis of the autonomous operation of AC-DC hybrid microgrids,a virtual inertia control strategy of ILC is proposed.This strategy aims at the poor DC bus voltage dynamic response under DC loads switching disturbance in AC-DC hybrid microgrids.This strategy includes two steps:direct DC bus voltage control and virtual inertial control.Without additional energy storage equipments,the proposed strategy can realize power sharing function and increase the inertia of DC subgrid simultaneously,then improve the inertia and system stability of the whole AC-DC hybrid microgrid.In addition,according to the small signal model,the dynamic characteristic analysis and stability analysis are carried out.The influence of parameter selection on dynamic and steady characteristics of the system is explored.The parameter setting method is obtained.The effectiveness of the proposed strategy is verified by simulations and hardware-in-the-loop experiments at last.Thirdly,a multi-layer virtual machine architecture coordinated control strategy of hybrid microgrid is proposed,aiming at the problems of AC-DC hybrid microgrids,such as difficult to coordinate,low inertia,low damping and poor stability.The concept of AC-DC hybrid microgrid multi-layer virtual machine architecture coordinated control is introduced.The specific structure and control strategy of virtual machines in each layer of this architecture are discussed,including the improved virtual DC machine control in DC layer and the improved VSM control in AC layer.Then the design of several core control parameters of this architecture is emphatically explored through the small signal stability analysis.The effect of the proposed multi-layer virtual machine architecture coordinated control strategy is verified by simulation.Finally,the RT-LAB hardware-in-the-loop simulation platform and the physical prototype platform are introduced.The advantages of RT-LAB simulation system are summarized,such as the excellent real-time performance and high simulation accuracy.The modeling and simulation processes are introduced.In addition,a prototype of ILC based on SiC MOSFET is developed and a hybrid microgrid experimental platform is built.The complete design processes,including hardware and software design,are discussed in detail.Feasibility and effectiveness of the proposed control strategies are verified by the above two platforms.