Research On Power Balance Of Islanded DC Microgrid Based On Nonlinear Adaptive Backstepping Control Strategy

With the application of renewable energy and smart grids today,the research on DC micro-grid that use new energy sources(such as photovoltaics)as distributed power supplies has received extensive attention.The use of micro-grid can effectively solve the problem of grid connection of various distributed power sources,and meet the power supply needs of remote and mountainous areas that are inconvenient to connect to the grid.The micro-grid system in this thesis includes the solar photovoltaic power generation system,the battery energy storage system(BESS),diesel generator system in stand-by mode and load.Among them,the solar photovoltaic power generation system is used as a renewable energy distributed power source,and the diesel generator is equipped with a rectifier as a backup power source to maintain the continuity of power supply in an emergency.In this thesis,a comprehensive control strategy for photovoltaic islanded DC micro-grid is used to design nonlinear adaptive backstepping controllers(NABC)for photovoltaic system,energy storage system and diesel generator system of micro-grid,so that each component of micro-grid can work in coordination to maintain the power balance and DC bus voltage stability of the whole micro-grid system.Based on the Lyapunov control theory,the controllers of the various components of the micro-grid are recursively designed.Since many parameters in their dynamic models are unknown,the adaptive law is used to estimate these unknown parameters,and a suitable control Lyapunov is established during the design process.Finally,the semi-negative qualitativeness of the control Lyapunov function is verified as a stability criterion to ensure that the designed controller can make the DC micro-grid system stable.The main research contents of the thesis are as follows:(1)The structure of photovoltaic Island DC micro-grid system is established.The principle of power balance and the relationship between power balance and DC bus voltage stability are analyzed.Three working modes of micro-grid system are analyzed in detail when the photovoltaic output changes due to the change of external environment.(2)The principle of photovoltaic power generation is analyzed.The commonly used maximum power point tracking(MPPT)algorithm and constant voltage control algorithm are studied.At the same time,the working principle of battery and DC/DC converter in energy storage system is analyzed.The dynamic models of photovoltaic system,energy storage system and diesel generator system are established to facilitate the design of controller for different modules.(3)Since the nonlinear controller is independent of the working point,it can overcome the limitation of the linear controller to keep running in a large working area.A nonlinear adaptive backstepping control strategy is proposed.For the unknown parameters of the system,based on the Lyapunov control theory,the corresponding control Lyapunov function(CLF)is established according to the tracking error,and the final CLF is derived layer by layer by using the backstepping method.In order to balance the power of micro-grid,the stability criterion requires that the derivative of the CLF satisfy the semi negative qualitative.The corresponding controller control law and the adaptive law needed to estimate the unknown parameters in different component models are obtained.(4)The designed nonlinear adaptive backstepping control strategy is verified by simulation and the results are analyzed.The power variation and DC bus voltage of DC micro-grid system under different conditions and working modes are measured,and the response time,regulation time and overshoot data are compared with several common controllers to verify the superiority of the designed controller.