Research On VSG Frequency Response And Power Allocation Strategy Based On Model Predictive Control

As more and more distributed energy is connected to the power grid,the traditional grid-connected inverter as a power electronic device can not provide inertia damping for the system and other problems,domestic and foreign experts and scholars proposed the control strategy of virtual synchronous motor,to improve the micro grid itself small capacity,weak inertia,small damping problems.Considering the frequency fluctuation and frequency drift caused by the output oscillation of the distributed power supply,the increase of nonlinear load,the fault of the grid-connected end and the link change,many expert scientists have done extensive research on the frequency modulation of VSG.At present,most of the frequency response control strategies of VSG proposed by most scholars are mainly based on primary frequency modulation,whose core is to improve the tracking performance of distributed generation devices for frequency deviation and eliminate frequency static difference.However,they can do nothing for frequency deviation in the transient process,and it is difficult to effectively participate in secondary frequency modulation.In order to improve the existing Virtual synchronous generator(VSG)existing in the control strategy of the secondary frequency modulation ability is insufficient,and the frequency modulation performance is greatly influenced by moment of inertia and damping parameters of faults,in this paper,the VSG is a frequency modulation,secondary frequency modulation and moment of inertia and damping effects on frequency response study,based on MPC is put forward,the Model Predictive Control,the MPC-VSG feedback Control method and the two-step Model Predictive Control,etc.The improved Control strategy,the specific research work is as follows:(1)The frequency modulation strategy and VSG frequency response of microgrid are analyzed theoretically.Expounding the importance of frequency response in microgrid and summarizes several control methods for improving the frequency response of current grid-connected inverters.The main influencing factors of frequency response in virtual synchronous generator grid-connected microgrid are studied,and the advantages and disadvantages of drop control,adaptive moment of inertia,and frequency self-recovery FM control are analyzed.(2)A frequency modulation control method for VSG based on MPC is proposed.The optimal model of VSG was established by the rotor motion equation and droop governing equation.The model was discretized to obtain the trajectory of rotor angular frequency,and the first term of the optimal predictive control input vector was taken as the feedback input of the controller at the next time.The feedback channel between angular frequency increment and torque increment is constructed to realize real-time prediction and compensation of frequency deviation.Through the analysis of the influence of feedback controller parameters on system stability,gives the basic principles of parameter selection,construction of the feedback controller with proposed inverter/grid simulation model,the simulation experiments,contrast analysis and Droop control,Adaptive control of inertia,Frequency self-recovery three ways to no difference in Frequency control,automatic recovery,and the performance of the system damping,etc.(3)A frequency modulation control method for VSG based on two-step MPC is proposed.In traditional model predictive control precision is not enough,and the frequency response adjustment problems for a longer time,a two-step MPC method is put forward: first,to establish prediction model based on VSG frequency adjustment,then using the method of Lagrange interpolation output adjusting control strategy of the two-staged prediction of the VSG VSG controller and applied to micro grid.The simulation results show that the two-step MPC based on VSG can control the frequency response within a reasonable range,and can effectively improve the frequency response of the system.Compared with the traditional model predictive control,the improvement effect is better.(4)The power allocation method of VSG multi-machine parallel system based on MPC is studied.For traditional VSG multimachine parallel problems of active vibration and frequency fluctuations would appear,research under the condition of multiple VSG parallel active and reactive power rationing,line impedance analysis,the influence of inertia,capacity of power allocation,build more VSG parallel system based on model predictive control,through simulation experiment,this paper puts forward the model predictive control method in the VSG multimachine parallel system can improve the frequency response,difference frequency modulation is realized.