Research On Grid-Connected Of Photovoltaic Based On Improved Z-Source Cascade Multilevel Inverter

Solar energy has been widely concerned by its abundant resources and other advantages as a new type of renewable energy.Photovoltaic power generation which is based on the combination of semiconductor technology and new energy demand is a renewable energy technology,and the further study on it is the goal of our country.The situation of photovoltaic power generation in China and abroad is analyzed in this paper,and the topology and control strategy of existing PV grid-connected inverter is also described.On this basis,the Z-source cascade multi-level inverter is choosen as the research object which combined the advantages of the Z-source and multi-level inverters.Although it can overcome the shortcomings of the traditional inverter,it still remains many deficiencies.For example,the boost capacity of Z-source network is limited and it has higher capacitor voltage stress during steady-state and so on.Therefore,the Z-source network topology is improved in this inverter,then the working principle of the structure is described in detail.Finally,the improved Z-source cascade multi-level inverter is applied to the photovoltaic power generation system.Firstly,the working principle and mathematical model of the photovoltaic cell are analyzed,and the variation rules of the corresponding parameters are obtained,next the dynamic simulation model of the photovoltaic cell is established in MATLAB,and the corresponding characteristic curve is obtained.Then the maximum power point tracking principles is introduced,and the MPPT tracking algorithm which based on perturbation observation and conductivity method is used to set up the models in simulink environment.The conclusion is proved by simulation results which based on the analysis of effectiveness and characteristics of the two algorithms.Secondly,the topology of Z-source inverter is improved,and three topologies are proposed,then a detailed analysis of these topologies is given in this chapter and the relative performance is compared.The boost capability curves of three topologies with the same through duty ratio are presented,a Z-source network topology with the strongest boost capability is obtained.When the boost capacity is the same,the capacitive voltage stress in these inverters is compared.The conclusions are verified by the simulation.Considering all aspects,a topological structure is chosen as the model of the improved Z-source network.Furthermore,the Z-source H-bridge module in the improved Z-source cascade multilevel inverter is taken as the research object,a small signal model is established by using the state space averaging method and the corresponding small-signal transfer function is deduced in this chapter,which the control strategy is designed on the premise of measuring the capacitor voltage to controlling DC bus voltage indirectly.The control strategy can guarantee the balance of the DC bus voltage in the occasion of power fluctuation.Finally,the multiple closed loops which includes grid connected current double closed-loop,DC bus voltage loop,photovoltaic array MPPT voltage loop is used in the single-phase grid-connected PV system with improved Z-source cascade five-level inverter.Then the appropriate controller is selected and the parameters of the controller is set.The simulation results show that under the effect of control strategy,this system can maintain the balance of DC bus voltage in all cascaded cells.It is also ensure that the grid current and voltage with the same phase,high power factor is achieved,and this grid connected system has good dynamic performance and stability.