Research On Optimal Control And Parallel Operation Power Sharing Strategy Of Virtual Synchronous Generator

In recent years,in the context of energy and environmental issues,renewable energy distributed power generation technology has developed rapidly.Renewable energy sources generation usually has the characteristics of geographically dispersed,small capacity,large quantity,and large output fluctuations.Therefore,it is usually combined with energy storage and inverters to form a microgrid for operation.The microgrid can be operated on an island or as a whole by a large power grid to coordinate and dispatch.It is an effective form to solve a large number of distributed power generation and energy efficient utilization.Compared with traditional generators,power electronic inverters lack rotational kinetic energy,which is not conducive to the stable operation of microgrids.VSG technology introduces the rotor motion equation and stator voltage equation of the SG into the frequency and voltage control of the inverter,which makes the output dynamic characteristics of the inverter can also simulate the SG.The support ability of distributed power sources to the system and the frequency characteristics are improved,however,the dynamic regulation capability is reduced.In actual systems,to meet the demand for electricity supply,virtual synchronous generators are usually required to operation in parallel,but the mismatch of line impedance and virtual stator impedance makes the reactive power unable to be divided equally according to the capacity of each VSG,resulting in circulating current.In response to the above problems,this article mainly conducts the following research:This paper first analyzes the principle of the virtual synchronous generator and the voltage and frequency controller structure,establishes the mathematical model of the inverter and inner double-loop control,and gives the corresponding control structure.The virtual synchronous generator model was built on the Matlab/Simulink,and the simulation results of island and grid-connected operation verify the performance of the analyzed model.According to the mathematical model of VSG,the influence of the rotational inertia and damping coefficient on the transient performance is analyzed.Based on the analysis of the dynamic process of virtual synchronous generator grid-connected and parallel-connected network operation,an adaptive control strategy of rotational inertia and damping coefficient is proposed.Through the output power and frequency information of the inverter,the current virtual rotor operating state can be judged,and then the rotational inertia and damping coefficient can be adjusted.It can be seen from the simulation results that the proposed control strategy can improve the transient stability and the dynamic performance of the VSG grid-connected operation and multi-machine parallel networking operation system.The conditions for the precise allocation of active power and reactive power of virtual synchronous generators according to capacity are analyzed.According to the reason why the reactive power of VSG cannot be allocated accurately,an improved control strategy based on distributed secondary controller is proposed.The consistency algorithm is introduced in the control.Using the distributed secondary to generate the virtual stator reactance compensation value,so that the output reactive power of each VSG can be accurately allocated according to the VSG capacity ratio,and still have a certain compensation ability in the case of communication failure.The proposed control strategy reduces the bandwidth requirements and reliability requirements of the communication system.The proposed control strategy is simulated and verified by Matlab/Simulink.