Researchs On Dc Microgrid Control Based On Bus Voltage Signal

In order to efficiently use distributed DC micro-sources such as photovoltaics,fuel cells,and energy storage batteries,and to adapt to the increasing demand for DC loads,DC microgrids have gradually become a new direction for microgrid research and development.However,the DC microgrid system has many constituent units and complicated operating conditions,so the corresponding control strategy is also complicated.In this paper,the common distributed power photovoltaic array,energy storage battery,grid-connected converter,corresponding controller and load are used as the basic components of the microgrid system,and the hierarchical coordinated control strategy of DC microgrid based on bus voltage signal is mainly studied.The details are as follows:In the first part,the research background and significance are introduced,and the research status and key technologies of DC microgrid are briefly summarized.The second part,on the basis of introducing the whole structure of the system and the working state of each component unit,deeply analyzes the interface circuit model of each unit and calculates the component parameters of each circuit.The research focused on the engineering mathematical model of photovoltaic cells,and the simulation analysis of the output characteristic curve of photovoltaic with temperature and light intensity conditions.By modeling the topology of each unit interface,it is prepared for the subsequent study of DC bus voltage control strategy.In the third part,According to the power conservation principle of the system,with the goal of maximizing the use of photovoltaic energy,reducing the charge and discharge times of the battery and improving the voltage range of stable operation of the system,a unified coordinated control strategy of the DC microgrid for off-grid and grid-connected operation was proposed.Using the DC bus voltage as the switching signal,a total of five voltage levels,that is,five-layer mode are divided,and the operating status of the photovoltaic unit,energy storage unit,and grid-connected converter unit in each hierarchy mode is determined.The photovoltaic converter includes MPPT and constant voltage droop control state;the grid-connected unit includes constant current inverter,rectification and droop inverter,and rectifier control state;the battery includes droop constant voltage charge,discharge,constant current charge and discharge state.The transfer function of each unit is obtained by building a small signal model,meanwhile,the bode diagram is drawn.The parameters of the voltage and current regulator are designed by using the frequency domain method.Then,a simulation model was built under MATLAB/Simulink environment to verify the feasibility and effectiveness of the control strategy corresponding to the converter of each unit in different working states.Finally,the simulation of the whole DC microgrid system is carried out.In the fourth part,in practice,considering the voltage and power fluctuation and overcurrent caused by the negative sequence current when the DC microgrid system is connected to the grid and the main network is not balanced,the inverter under unbalanced conditions is first modeled,then the power under the unbalanced condition of the power grid is analyzed,and an improved control strategy of inverter that can be used in both normal and unbalanced conditions and adapted to the above-mentioned bus voltage-based hierarchical control system is proposed,finally,simulation results show the effectiveness of the proposed method.