Study On The Coordinated Control Strategy Of The Isolated Direct Current Microgrid

With the rapid development of social economy,a series of problems such as energy crisis,climate warming and environmental deterioration are becoming more and more serious.In this context,countries around the world pay more and more attention to microgrid.Compared with AC microgrid,DC microgrid has no phase synchronization and skin effect,simple structure and better controllability.In addition,the DC microgrid reduces the converter link and improves the energy efficiency.Therefore,the research work of this paper has strong pertinence and certain practical significance.Firstly,this paper describes the composition and structure of DC microgrid,establishes the mathematical model of photovoltaic cells,storage batteries and super capacitors,and introduces the output characteristics and important parameters of photovoltaic cells in detail,and analyzes the output characteristics of photovoltaic cells under different light intensity or temperature through MATLAB / Simulink simulation.The working mechanism of the converter of distributed generation is studied.Secondly,according to the requirements of micro grid system for photovoltaic power generation module,the working mode is divided into two modes: maximum power point tracking(MPPT)and constant voltage control(CVC).Through the analysis and comparison of several common MPPT control algorithms,the MPPT control algorithm based on the variable step disturbance observation method is proposed,the power tracking speed is analyzed,the corresponding control strategy is designed,and the simulation model before and after the improvement is built in Matlab / Simulink.Through the simulation comparison,it shows that the variable step disturbance observation method proposed in this paper has higher tracking efficiency The stability of the output is better.For the hybrid energy storage system composed of super capacitor and battery,super capacitor can effectively suppress the power fluctuation with high frequency and fast response speed due to its outstanding characteristics such as more charging and discharging times and long service life;The battery can make up for the power fluctuation of higher energy density and lower power density.They complement each other and bear the high-frequency and low-frequency fluctuation components of power in the system respectively.In order to solve the problem of intermittence and randomness of photovoltaic power generation,the battery adopts the combination of charge discharge control and variable droop control to detect the state of charge.In view of the randomness of photovoltaic power generation and the uncertainty of load switching,it is easy to cause DC bus voltage fluctuation,In this paper,the improved droop control and voltage feed-forward method are combined to further suppress the power fluctuation.Finally,in view of the coordinated control strategy and the stable operation mechanism of microgrid,this paper analyzes the changes of load and light intensity,divides them into three cases,a total of nine modes,and gives the power distribution of each distributed generation in each mode.The simulation results show that the DC microgrid can maintain stable operation when the light intensity and load change,which proves that the proposed coordinated control strategy is effective and feasible.