| With the energy crisis and the environmental pollution caused by traditional energy,renewable energy is being widely studied around the world because of its low cost,wide source,clean and environmental protection.Among them,photovoltaic power generation will be further developed worldwide because of its static,safe and green characteristics.However,natural conditions such as temperature and light will affect photovoltaic power generation.Its power generation process is accompanied by randomness,instability and seasonality,and the voltage of a single photovoltaic panel is low,which requires power conversion to meet user needs.Firstly,aiming at the uncertainty of photovoltaic power generation and the voltage demand of users,this paper adopts a quasi-Z-Source cascaded multilevel photovoltaic inverter.The topology combines the advantages of both.The cascaded multilevel inverter can reduce the rated voltage of the power switching devices used,improve the output characteristics of the circuit and reduce the harmonic content in the output voltage.Through the addition of Z-source network,the function of single-stage control of two-stage power conversion can be realized in each stage.At the same time,each inverter unit can achieve maximum power tracking,maintain DC side voltage stability,and allow the upper and lower arm switches of H-bridge to be turned on at the same time without damage.These will further promote the development of photovoltaic power generation application scenarios and efficiency improvement.Secondly,the control strategy of Z-source cascaded multilevel photovoltaic inverter is designed.Firstly,the quasi-Z-Source inverter is modeled and analyzed.Through the corresponding state space equation,the transfer function of the single-stage quasi-Z-source inverter is derived.The DC-side double-loop control scheme is designed by using the transfer function to achieve DC-side voltage stability and maximum power tracking.The post-stage adopts a control strategy of active and reactive power decoupling.Through the coordination of power decoupling distribution and two-stage transformation,the system operation range is broadened and the system voltage dynamic stability is realized.In addition,aiming at the problem of power imbalance between photovoltaic panels caused by manufacturing tolerance,partial shading,fouling and thermal gradient of each photovoltaic module of cascaded photovoltaic inverter,this paper analyzes the relationship between output voltage component of each module and power generation through vector diagram,and expounds the power distribution principle of this scheme,realizes independent active and reactive power distribution,and uses reactive power compensation strategy to alleviate the overmodulation problem caused by power imbalance.Finally,based on the Matlab/Simulink platform,a quasi-Z-Source cascaded multilevel photovoltaic inverter grid-connected system is built,and the effectiveness of the control strategy is verified.The control strategy of this paper is further verified by the semi-physical simulation experiment and the 600 W cascaded photovoltaic inverter prototype. |
