Research On Bus Voltage Oscillation Mechanism And Nonlinear Control For DC Microgrid

Distributed generations such as photovoltaics and wind power have the characteristics of intermittent and uncertainty,which connected to the grid directly will affect the reliability of power supply.As a link connecting the grid and distributed generations,microgrid can improve the reliability of power supply and the ability to absorb clean energy.DC microgrid has the advantages of high energy efficiency and simple control.Maintaining the stability of bus voltage is the key to safe and reliable operation of DC microgrid since there is no need to track the voltage phase frequency or synchronous adjustment.The bus voltage oscillation will reduce the reliability of power supply and m ay even affect the normal and safe operation of DC microgrid system in severe cases.Therefore,it is of great significance to analyze the bus voltage oscillation mechanism and suppression technique of oscillation.DC microgrid in the island operation mode is taken as the research object in this article and the basic structure of typical DC microgrid is introduced.An equivalent circuit model of DC microgrid system in island operation mode is established based on the analysis of the operating characteristics and ideal models of photovoltaic unit,energy storage unit and load unit.Several common causes of DC microgrid bus voltage oscillation are analyzed in this article combined with literature research,including the power fluctuation of distributed generation,the load fluctuation of DC bus and the interaction between cascaded converters.This article focuses on bus voltage oscillation problem caused by constant power loads.The constant power load behaves negative impedance characteristic,which will reduce the system damping.The bus voltage may oscillate when the load power fluctuates greatly.In order to identify the region where the system is stable,the mixed potential function model of energy storage unit under charge and discharge mode is established based on mixed potential function theory proposed by Brayton-Moser and the Lyapunov energy function is constructed.The region of asymptotic stability of energy storage under charge and discharge mode is estimated according to Lyapunov’s stability theorem and the influence of the power of constant power load on the region of asymptotic stability is analyzed.The effectiveness of the stability analysis method can be verified by the results of simulation.The DC microgrid system with constant power load is an affine nonlinear system essentially.A nonlinear control method based on differential geometry theory is proposed to suppress bus voltage oscillation in this article.The affine nonlinear system is transformed into linear Brunovsky standard form by coordinate transformation and linear state feedback controller is designed by the pole placement theory.Then the state feedback control law of energy storage converter is obtained according to the relationship of control variable between linear system and nonlinear system.The simulation results show that the stability of bus voltage can be maintained by the nonlinear control law of energy storage converter proposed in this article.