Research On Single Stage PV Grid-connected High Frequency Isolated Inverter And Its Precise Control Technology

Due to the shortage of traditional fossil fuels is becoming more and more serious,photovoltaic(PV)power generation has the advantages of pollution-free,clean,safe and convenient,rich resources and so on,attracting more and more attention.As an important link of photovoltaic power generation system,photovoltaic(PV)grid-connected inverter has become a research hotspot in the field of photovoltaic power generation.In recent years,power frequency isolated PV grid-connected inverter has been increasingly replaced by high frequency isolated PV grid-connected inverter because of its shortcomings such as large volume,heavy weight and loud noise.In this paper,a single stage grid-connected PV high frequency isolated inverter is studied,voltage spikes and oscillations on the secondary side of the transformer are eliminated through the clamp circuit and its corresponding modulation strategy.The operation principle of the circuit and the mechanism for double-line frequency ripple voltage occurring on the clamp capacitor are analyzed.The maximum power tracking(MPPT)technology and closedloop control strategy are studied in detail.The MPPT is realized in the single-stage structure,and the accurate control of grid-connected current is realized.The harmonic content of grid-connected current is less than 5%,which conforms to the national gridconnected standard.Firstly,this paper introduces the PV power generation system in detail and discusses the basic classification of PV grid-connected inverter and the future development trend of PV grid-connected inverter technology.The classification of PV grid-connected highfrequency isolation inverter is briefly introduced,the circuit topology of the single-stage PV grid-connected high-frequency isolation inverter is introduced,and the operation principle of the circuit under the corresponding modulation method is analyzed.The equivalent circuit model of the single-stage PV grid-connected high-frequency isolation inverter is established,and a mathematical model is derived.The double-line frequency ripple voltage on the clamp capacitor is studied,and the formula of maximum ripple voltage is derived.The relationship between maximum ripple voltage,clamp capacitance and transformer leakage is clarified.Secondly,this paper analyzes and introduces the relevant principles and techniques of MPPT.The equivalent circuit model of PV cells is established,and the output characteristic curves of PV cells are analyzed when the temperature and light intensity change.The basic principle of MPPT and several common MPPT control methods are introduced,and the MPPT control scheme selected in this paper is determined after Matlab/Simulink simulation analysis.The three-loop control strategy including MPPT power loop,voltage loop and current loop is adopted in this paper.Due to it is very difficult to achieve grid-connected requirements by controlling the output voltage waveform in grid-connected system,this paper mainly studies the control strategy of the current loop.The PI control and multiresonance control are analyzed.Considering the deficiency of these two control strategies in system control,a phase compensation multi-resonance control is studied to control the grid-connected current,and the compensated phase angle of the phase compensation multi-resonance control is derived.The grid-connected current under the three control strategies is analyzed by matlab/simulink simulation,and the advantages of phase compensation multi-resonance control are verified.Finally,this paper describes the design of hardware and software of the single-stage PV grid-connected high-frequency isolation inverter,and sets up a 1k W experimental platform to verify the theoretical analysis of the previous sections.The experimental results show that the single-stage PV grid-connected high-frequency isolation inverter and its control strategy can not only complete the maximum power tracking in a single stage,but also realize the accurate control of the grid-connected current.