Research On Control Strategy Of MMC Interconnected Converter Based On Virtual Synchronous Machine

As a key component of new energy power system,AC / DC hybrid microgrid plays an important role in improving the utilization rate of new energy,maintaining the stability of new energy power system,and promoting the balance between distribution network and large grid.Because a large number of power electronic equipment are used in AC / DC microgrid,its inherent small moment of inertia seriously affects the stability of power system.In order to maintain the stability of AC / DC microgrid,the modular multilevel converter(MMC)is used as the bus interconnection converter of AC / DC hybrid microgrid,and the MMC mathematical model under AC / DC hybrid microgrid is established,and its working mechanism,control strategy and stability are analyzed.The relevant algorithm is verified through model simulation and experiment.The main research contents and innovations of this paper are as follows:(1)A virtual synchronous machine control strategy with adaptive virtual parameters is proposed.Aiming at the problem that the traditional virtual synchronous machine control can not adaptively change the dynamic inertia level of the system according to the frequency change,this paper first analyzes the mathematical model,working mechanism,basic principle of virtual synchronous machine algorithm and the causes of the problem in the AC / DC hybrid microgrid.On this basis,the power transmission rules of MMC interconnector are analyzed and studied.Aiming at active power balance between hybrid microgrid and load,a virtual synchronous machine control strategy with adaptive virtual parameters is proposed.The system can adjust the inertia level of the system with the change of frequency under the premise of ensuring the steady-state performance of the system.Finally,a simulation model is built for simulation experiments,and the simulation results show the effectiveness of the proposed method.(2)The stability of the control strategy of the virtual synchronous machine is analyzed in detail by using the root locus method.Aiming at the problem that the parameters of the virtual synchronous machine have great influence on the system stability,the small signal model of the control strategy of the virtual synchronous machine is derived.Then,the influence of moment of inertia and damping coefficient on the stability of the system is analyzed by root locus and Routh criterion.It is concludedthat when the damping coefficient is small,the output power of the interconnected converter has low frequency oscillation.With the increase of damping coefficient,the damping ratio increases and the oscillation decreases.The stability of the system can be improved by reasonably designing the parameters of the virtual synchronous machine.(3)A control method of modular multilevel converter based on distributed measurement is proposed.Aiming at the disadvantages of traditional MMC,such as lack of flexibility,scalability and heavy communication caused by centralized control strategy,A new distributed control strategy is proposed.The arm current is controlled by the central controller,and the differential current control,capacitor voltage control and pulse width modulation are distributed to the local controller,which can significantly improve the scalability of MMC and the capacitor voltage The control only depends on the local sub module voltage measurement,and does not need to transmit capacitor voltage signal through intensive communication in each control cycle,which can significantly reduce the communication burden of the control system.