Research On Sliding Mode Variable Structure Control Strategy For Hybrid Energy Storage Units In Microgrid

DC microgrid is the most commonly used form of photovoltaic power generation system.Compared with traditional AC microgrid,it has the advantages of better power quality,stronger power supply reliability and less energy loss.However,due to the random meteorological conditions,the output power of photovoltaic system fluctuates constantly,which will affect the stability of DC bus voltage.Therefore,the efficient maximum power tracking method and reasonable energy storage control technology have become the research focus in the field of microgrid.In this thesis,an photovoltaic DC microgrid is designed by using batteries and supercapacitors as energy storage components and the sliding mode control strategy of hybrid energy storage system is deeply studied.Firstly,aiming at the photovoltaic system module,the equivalent model of photovoltaic cells is established and its output characteristics are analyzed.On this basis,LC parameters in Boost converter are determined according to the ripple voltage and ripple current requirements of the system.Aiming at the problem that the output power of photovoltaic cells is fluctuant,the maximum power tracking point control strategy is adopted to improve the energy conversion efficiency of photovoltaic cells and reduce the circuit loss.Secondly,the lithium battery and supercapacitor in the hybrid energy storage system are modeled and their charging and discharging characteristics are analyzed.Secondly,the small signal model of bidirectional DC-DC converter is established,and its LC parameters are determined according to the constraints of BDC in Boost and Buck modes.Aiming at the the DC microgrid with the photovoltaic and energy storage system(HESS),a fixed frequency PWM sliding mode controller based on reaching law and a terminal sliding mode controller are designed by using sliding mode variable structure control method to improve the problems of slow voltage response and weak robustness of DC bus.In order to prevent overcharge and overdischarge of lithium battery,the State of Charge of lithium battery is monitored,and the upper and lower thresholds are set,so as to slow down the completion of a charge and discharge cycle of lithium battery and prolong its service life.Finally,the DC microgrid with the photovoltaic and HESS is built in Matlab/Simulink,and the differences between the proposed sliding mode control strategy and traditional PI control strategy are compared and analyzed.The simulation results are consistent with the expectation,which fully verifies the effectiveness and superiority of the adopted control strategy.