| The shortage of non-renewable energy has led to the extensive research and application of new energy technology.Photovoltaic(PV)DC microgrid power generation system is being popularized by more and more countries for its remarkable environmental and economic benefits.In PV DC microgrid system,front-end step-up DC-DC converter serves as an interface between PV array and DC bus,and the converter is the main energy source channel of the microgrid system.Bi-directional DC-DC converter serves as the interface between energy storage device and DC bus,which is a storage and supplemental channel for the system.Both the converters play key roles in the quantity and quality of the electrical energy in the system.Therefore,the research on the DC-DC converter and its control of the photovoltaic microgrid is of great significance in both theoretical research and practical application.Model Predictive Control(MPC)is being paid more and more attention due to its excellent ability of dealing with constraint conditions.Therefore,combined with model predictive control,the structure and control of the DC-DC converter in which the front-end boost DC-DC converter is integrated with the bi-directional DC-DC converter are studied in this thesis.The main works accomplished in this study are as follows:(1)The structure of a three-port DC-DC converter is studied and its matheatical model is constructed.Firstly,the connection modes of the front-end boost converter and bi-directional DC-DC converter in PV microgrid are investigated,and their functions are integrated by using an asymmetric boost integrated phase shifted full bridge three-port DC-DC converter.Then,working principle of the three-port DC-DC converter is analyzed.In the end,mathematical model is established according to its topology structure,which lays the foundation for the rsearch of maximum power point tracking(MPPT)and current sharing control technology.(2)The simulation model of photovoltaic array is established and the MPPT method used in this thesis is studied.Firstly,the mathematical model which consists of four parameters is set up based on the equivalent circuit of photovoltaic cell,and the PV array simulation model is established.Then,by analyzing and comparing the advantages and disadvantages of several typical MPPT methods,the variable step perturbation and observation(VS-P&O)method is adopted to make up for the shortcomings of traditional methods.Finally,the method is verified by simulations.(3)The maximum power tracking method based on model predictive control is studied.Firstly,the basic principle of MPC is introduced,and the prediction model of the three-port converter for MPC-MPPT is established.Then,one-step and two-step prediction of MPC-MPPT algorithm are studied.Finally,the MPC-MPPT method proposed in this chapter is verified.The simulation results show that MPC-MPPT has better dynamic performance than traditional MPPT.(4)The current-sharing method based on model predictive control is studied.Firstly,the cause of the circulating current in the parallel DC-DC converter system is analyzed,and droop method is proposed to solve the problem.After that,the current-sharing control prediction model is established and the model MPC-droop scheme is designed.Finally,the simulation verifies the correctness of the method. |
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