Research On Three-phase Photovoltaic Grid-connected Inverter And Control Technology

PV grid-connected inverter is the core component of grid-connected PV systems. In thisthesis, taking two-stage and three-phase grid-connected inverter for the study, there are twocrucial problems that are maximum power point tracking (MPPT) and PV inverter gridcontrol double-loop PI control problem. Maximum power point tracking speed andSteady-state oscillation of contradictions are resolved, and the network side voltage can beprecisely tracked by the grid current from inverter.For the problem of Maximum power point tracking slow and steady oscillation,conductance increment method and other typical MPPT algorithms are analyzed andcompared. A improved variable step incremental conductance speed maximum power pointtracking algorithm is proposed. The algorithm combines the advantages of constant voltagemethod and variable step incremental conductance method. The simulation results show thatthe improved variable step incremental conductance method can quickly track the currentmaximum power point and it enter into stable state.In order to achieve grid inverter current error-tracking network side voltage problems,two-stage and three-phase grid-connected inverter is used as the object of study and the gridinverter main circuit topology is determined. Advantages and disadvantages from thetechnology that three-phase SPWM and SVPWM are applied in the inverter modulation areanalyzed. Three-phase inverter model is established and simulation. And the results ofsimulation show that the output voltage and current of inverter modulated by SVPWM havebetter harmonic performance and the DC voltage has highly utilization. At the same time, theinfluence of inverter output produced from dead time is simulated and analyzed. And thedesign and filtering performance of L and LCL filters are also analyzed.The entire PV grid-connected inverter system is simulated and researched. Current loopwith current feed-forward function and system simulation model of double-loop PI controlstrategy from voltage loop and network are set up. The simulation results show grid control ofinverter can be realized better by double-loop control strategy with current feed-forwardfunction. Network side voltage frequency can be traced precisely by inverter current,reflecting better sinusoidal small harmonic. In the case of illumination changes, power outputis adjusted rapidly by inverter and there are no oscillations in the transition process.