Research On Grid-connected Control Strategy Of Quasi-Z-Source Cascaded Multilevel Photovoltaic Inverter

In the environment of increasing energy shortage and rising prices,the energy utilization methods of countries around the world are developing in the direction of high quality,low cost,clean and low carbon.With the continuous progress of power electronics technology,photovoltaic power generation has developed rapidly and has become one of the main goals of new energy development.The quasi-Z-source cascaded multilevel photovoltaic grid-connected inverter can simultaneously complete DC-DC conversion and DC-AC inverter through single-stage power conversion,and can realize independent MPPT control of each stage unit.The application in photovoltaic system can improve the reliability of the system and the quality of grid-connected current,and reduce the cost of photovoltaic power generation.Therefore,this paper takes the quasi-Z-source cascaded multilevel photovoltaic grid-connected inverter as the research object.The specific research contents are as follows:Firstly,the working principle of quasi-Z-source inverter is analyzed,the boost mechanism is deduced,and the parameters of quasi-Z-source impedance network are designed.The corresponding transfer function is obtained by the state space equation of a single quasi-Z-source H-bridge inverter unit,and the equivalent model of quasi-Z-source cascaded multilevel photovoltaic grid-connected system is established.The over-modulation mechanism under system power imbalance and the influence of over-modulation on gridconnected current and system stability are analyzed.Then,based on the analysis of the switching state of a single quasi-Z-source H-bridge inverter unit,the single-arm chopper modulation strategy with the same arm complementary in unipolar modulation is combined with the modulation strategy of the traditional quasi-Zsource cascaded multilevel inverter to obtain an improved carrier horizontal phase shift modulation strategy that fuses the shoot-through duty cycle.In the control strategy of quasiZ-source cascaded multilevel photovoltaic grid-connected system,the former stage realizes the independent maximum power point tracking control of each stage of quasi-Z-source Hbridge inverter unit by perturbation and observation method,and then completes the DC-DC transformation through voltage and current double loop control,and designs the controller parameters.The latter stage realizes the constant voltage of the DC bus through the power outer loop control,and the DC bus voltage balance between the units at all levels.The photovoltaic grid-connected control is completed through the current inner loop.Aiming at the over-modulation phenomenon in the quasi-Z-source cascaded multilevel photovoltaic grid-connected system under power imbalance,this paper proposes an optimal third harmonic compensation strategy to expand the operating range of the system.According to the modulation ratio margin of the quasi-Z-source inverter,the optimal third harmonic compensation coefficient is designed in combination with the polynomial fitting method,so as to obtain the optimal third harmonic,which is injected into the modulation wave of the over-modulation unit to suppress the over-modulation phenomenon in the system under severe power imbalance.At the same time,according to the third harmonic distribution strategy proposed in this paper,the equal reverse third harmonic is compensated to the nonovermodulation unit to eliminate the third harmonic component in the grid-connected current.Finally,the real-time simulation model of three-unit quasi-Z-source cascaded multilevel photovoltaic grid-connected system is built.The maximum power point tracking control,DC-DC double-loop control,independent DC bus voltage balance control,gridconnected current control and optimal third harmonic compensation control under power imbalance are verified by RT-LAB real-time simulation platform.The simulation waveforms of the system under different working conditions are analyzed.Experiments are carried out on a 600 W quasi-Z-source cascaded multilevel inverter prototype.The simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed modulation strategy and grid-connected control strategy.