Research On Improved Sliding Mode Control Strategy Of High Frequency Isolated Quasi Z Source Photovoltaic Grid-connected Inverter

With the use of the traditional non renewable fossil energy,the problems include greenhouse effect,energy depletion and environmental pollution make people to explore the new energy which is green,efficient and environmental.Among them,solar energy has become a hot research and development direction in the world because of its rich resources,green and clean,huge reserves and other advantages.The core of photovoltaic power generation system is the inverter system.In the face of the complex situation such as the variable input voltage of photovoltaic power generation system,the traditional grid connected inverter system has the problems of poor external anti-interference ability,small input voltage range,low efficiency,large failure rate and so on.Therefore,a high frequency isolated quasi-z-source inverter is proposed.The inverter is a new topology,which has the advantages of large input voltage range,high voltage rise ratio,electrical isolation,and is suitable for photovoltaic power generation system.High frequency isolated quasi-z-source inverter has obvious nonlinearity because of its special topology.When the input voltage is disturbed,the DC link voltage will appear over harmonic oscillation,resulting in the poor stability of the final output voltage and current.Sliding mode variable structure control is a mature nonlinear control strategy,which is widely used in power electronic switching device control,but the traditional sliding mode control strategy has the problems of slow response and large chattering.Therefore,the improved sliding mode control strategy can be used for DC link voltage control of high frequency isolated Quasi-Z source inverter to achieve stable and reliable output of the whole inverter.In this paper,the current situation of photovoltaic power generation system at home and abroad is summarized firstly.The current research status of sliding mode control strategy and Z-source inverter are analyzed;Secondly,The input and output characteristics of photovoltaic cells are analyzed.In view of the shortcomings of the traditional incremental conductance method,an incremental conductance method based on variable step size is proposed.its faster tracking speed is verified through simulation experiments;Then,the working principle of the high frequency isolated Quasi-Z source inverter is introduced.Its small signal model is established.Transfer function is derived;The paper focuses on the overall control strategy of the inverter.The appropriate modulation strategy is selected.The paper adopts theimproved sliding mode control strategy based on the piecewise power approach law to realize the control of the DC chain voltage,and designs the sliding mode surface.The proof of its existence,accessibility and stability is completed.The piecewise power approach law is designed.The performance of the approach law is investigated from the aspects of buffeting stability,approach rate,steady-state error bound,upper bound of convergence time and approach law parameters.It is compared with the traditional fast power approach law and double power approach law.Then,the control strategy of grid connected current is designed.The quasi PR regulator is adopted,and its control parameters are analyzed.The main circuit,peripheral circuit and DSP software system of the high frequency isolation quasi-z-source inverter are designed,and a 300 W prototype is built.Finally,the simulation experiment is carried out under the condition of the change of input voltage and reference voltage,and the change waveforms of DC chain voltage,transformer secondary capacitance voltage and grid connection current are analyzed.The experiment is carried out on the physical platform to verify the correctness of theory and simulation.