Research On Control Strategy Of Isolated Quasi-Z Source Photovoltaic Grid-Connected Microinverter In Weak Grid

The depletion of traditional fuel energy and environmental pollution have accelerated the development of renewable energy technology.Distributed solar photovoltaic power generation is widely used because of its large storage capacity,no geographical restrictions,clean and pollution-free advantages.Grid-connected inverter as the core component of energy transmission in photovoltaic power generation system,its performance directly affects the quality of grid-connected power.The increasing penetration of long transmission lines and distributed generation equipment at all levels makes the grid present the characteristics of weak grid,which puts forward higher requirements for grid-connected inverter control.In this paper,the isolated quasi-Z-source micro-inverter is taken as the research object,and the topology of the inverter,maximum power tracking,DC link voltage regulation and harmonic suppression of grid-connected current are studied as follows :Firstly,the double diode photovoltaic cell model is established,and the maximum power tracking control is carried out by variable step perturbation and observation method.The working principle of isolated quasi-Z-source inverter is analyzed,and the small signal model is established by using the state space averaging method to obtain the system control characteristic equation.Secondly,in the ideal grid,the system adopts the simple boost SPWM modulation.In view of the non-minimum phase phenomenon between the DC link voltage of the inverter and the passing function of the direct duty cycle,and the overshoot oscillation problem that is easy to occur when the system is disturbed,this paper adopts the sliding mode control strategy to control the DC link voltage.The sliding surface parameters are designed from existence,reachability and stability.In order to ensure the dynamic quality of the system,the multi-power reaching law is selected,and its control performance is investigated from three aspects of steady-state buffeting,reaching rate and disturbance stability bound,and the simulation comparison is made with the traditional exponential reaching law.In order to improve the quality of grid-connected current and suppress the resonant peak of LCL filter,the active damping control method combining capacitive current feedback and quasi-PR controller is adopted,and the control variable method is used to design the parameters of quasi-PR controller.Thirdly,the equivalent model of the grid-connected system is established in the weak power grid,and the influence of the grid impedance on the system under the traditional proportional feedforward control is analyzed.Therefore,an improved feedforward control method is proposed.The frequency selection characteristics of the multi-resonant controller are used to maintain the feedback characteristics of the system at the main frequency and attenuate the rest frequency bands.In order to adapt to the impedance change in a wide range,the small signal harmonic injection method is introduced to conduct the online detection of the grid impedance to realize the adaptive parameter optimization of the quasi-PR current controller.Finally,the main circuit,sampling circuit and control circuit of the system are designed.According to the relevant parameters,a 300 W isolated quasi Z-source inverter experimental prototype is built,and the correctness of the theoretical analysis and simulation results are verified on the experimental prototype.