Research On Topology And Control Strategy Of Micro Inverter

The impact of traditional energy on the aspect of environment cannot be ignored.As a clean energy,photovoltaic has a tendency to gradually replace traditional energy.In recent years,with the popularization of photovoltaic power generation,the region for photovoltaic power generation has also changed,gradually from the western to the central and eastern of China.In the central and eastern,the building density is large,and the traditional centralized and group string photovoltaic power generation is inconvenient.Micro inverter refers to a small power inverter with single block photovoltaic cell maximum power point tracking function.As a distributed generation form,the micro inverter grid connected system is easier to popularize in the central and eastern.And the micro inverter can track the maximum power by single PV cell.Compared with the traditional centralized grid system,the utilization rate of PV is higher.However,the traditional micro inverter has poor schedulability,the cost for unit capacity is high,therefore,this paper proposes a two-levels micro inverter based on parallel DC module and Y-Source inverter,so as to solve the existing problems of traditional micro inverter.Firstly,the parallel DC module unit is studied.The DC module unit based on Boost circuit is designed.The dual closed loop control strategy of input voltage and inductor current is used to control the Boost circuit,and the robustness and stability of the system are improved.The variable step size perturbation and observation method based on the optimal gradient method is applied to the maximum power point tracking,which solves the problem of the fluctuation of PV cell output near the maximum power point.Modeling and simulation are carried out by Matlab/Simulink,the control strategy and maximum power point tracking method of the parallel Boost DC module are verified.Secondly,the research of Y-Source inverter is carried out.The principle of Y-Source DC/DC converter is researched,and the characteristics of boost and inverter of Y-Source inverter are researched.Based on the characteristics that the DC side of the Y-Source inverter is not directly connected to the capacitor,a modulation method without adding dead time in the driving signal is designed,which solves the dead time problem of the traditional inverter.With the condition with no dead zone,starting from the characteristics of switch,the reason and rule of Y-Source inverter's shoot through state are analyzed,and the influence of dead time modulation on output of inverter is studied.When the bipolar SPWM modulation is adopted,the impact of the inverter on the output of the inverter is counterbalanced in each switch period,so it has no influence on the fundamental and low harmonic waves.When the single pole double frequency SPWM modulation is adopted,the output voltage of the inverter is periodically influenced bythe period of 0.02 s.The method to add the shoot through time to the normal modulation mode of the inverter is studied so as to add boost characteristics to Y-Source inverter.Under the modulation of bipolar SPWM,the direct time is added by prolonging the drop edge of the driving signal.Under the modulation of single pole frequency double frequency SPWM,the direct time is added to the 0 vector.Matlab/Simulink is used to model and simulate,the two characteristics which are the boost and inverter for the Y-Source inverter is verified.Again,based on the above research content,the micro inverter topology and control strategy is designed.According to the actual situation,the parameters of the system components is designed,and modeling and simulation by Matlab/Simulink is done,so as to verify the rationality of the overall design of the system.Finally,the hardware platform is built,the open loop experiment of Y-Source inverter and the grid connected experiment for all the system were carried out to verify the contents of the paper.