Research On Optimal Control Of Photovoltaic Grid-connected Based On Quasi Z-Source Inverter

With the development of social economy,more and more attention has been paid to the development and utilization of new energy.As a new energy generation method,photovoltaic power generation has become a research hotspot in recent years.Meanwhile,photovoltaic power generation system is an important way to realize commercialization.In order to ensure the safe,stable and efficient operation of the photovoltaic power generation system,output high-quality electricity,avoid pollution to the power grid,and achieve the purpose of grid-connection,it is particularly important to study the topology structure of grid-connected inverter and the control method of grid-connected system.The design of a new quasi Z-source grid-connected inverter can improve the conversion efficiency of electric energy,improve the voltage boosting capacity of the inverter,reduce the voltage stress of the device,improve the power quality and reduce the system cost.After the establishment of the photovoltaic grid-connected system model,the entire system is analyzed,including the topology structure of quasi z-source inverter,voltage outer loop control,grid-connected current inner loop control,filtering circuit,maximum power point tracking(MPPT)direct voltage zero vector loop control.In this paper,the above part is studied deeply,and different control methods are designed to optimize the performance of photovoltaic grid-connected system.The main contents of the paper are as follows:Firstly,as the existing Z-source inverter topology has a series of problems: the capacitor voltage stress is higher,the boost range is narrower,the boost capability is insufficient and the gain is small,in order to improve the above problems,this paper proposes a new type quasi Z-source inverter topology.The topology is transformed on the impedance network of the traditional quasi Z-source inverter,and a new switching inductance unit is designed to replace an inductor in the traditional quasi Z-source impedance network to achieve the purpose of improving the boosting capacity.The structure can obtain a larger boost factor at a smaller duty cycle.The simulation results show that the designed new quasi Z-source inverter topology solves the problem of insufficient boost capability of the traditional Z-source inverter,and at the same time greatly improves the boost range and boost gain,and can achieve unity power factor grid connection,with good grid connection effect.Secondly,the output efficiency of solar photovoltaic cells is easily disturbed by external environmental factors.How to use solar energy efficiently requires research on the tracking control of the maximum power point.Through analysis of various control methods,a fuzzy quasi-proportional resonance(Quasi-PR)control method is applied.Simulation experiments verify that the control method can achieve the maximum power point without static error tracking,and in the case of sudden changes in external conditions such as strength,the maximum power point is quickly tracked,which effectively improves the efficiency of photovoltaic system power generation.At the same time,the system has a fast-tracking speed and good stability.Thirdly,for the control of the current inner loop of the grid-connected inverter side,a fuzzy proportional complex integral(PCI)control method is applied.This method can adaptively eliminate steady-state errors and communicate with specific frequencies.The disturbance signal has zero steady-state error adjustment function,which has better control effect than proportional integral control.Finally,the problem of natural resonance has always been a problem with LCL(L-Capacitor-L)filters.In order to solve this problem,suppress the resonance peak at the resonant frequency and reduce the harmonic content,Clark transform is used to convert the three-phase coordinate system.Converted to a two-phase static coordinate system,the control method of the combination of capacitive current feedback and active damping was used to analyze and design the parameters of the LCL filter.On this basis,the capacitor current feedback coefficient is improved to make the control method more effective.Finally,based on the LCL filter and control method,the controller parameters are set.Simulation experimental results show that the LCL filter is proposed,under the control strategy of the harmonic suppression of the LCL filter,the harmonic content of Z source photovoltaic grid system is greatly reduced,and can realize on the net current control,to achieve the purpose of power factor interconnection,weakened the natural resonance.At the same time,LCL filter has reduced the computation complexity,and solved the coupling problems.