| The fast development of social economy, resulting in the rapid consumption of fossil energy and the deterioration of social environment, it is imminent to exploit renewable energy. Because the solar energy is clean, pollution-free and inexhaustible, it gets wide attention. Traditionally, Photovoltaic generation system is current-controlled, it has high power factor and controlled simply, but can only operate in grid-connected mode, when switching, it will also cause surge current. Because of the advantage of flexible configuration and seamless modes switching, the voltage-controlled photovoltaic inverter becomes the focus of the present study. This paper researches on voltage-controlled grid-connected strategy to make the grid-connected system operates reliably.Firstly, the topology structure of PV inverter and its inductance and capacitance parameters are designed, and its mathematical model in islanded and grid-connected mode is established. The influence of the grid harmonic on grid-connected current is analyzed, and harmonic detection strategy is proposed to restrain current distortion.The voltage-controlled grid-connected strategy based on droop control is proposed, and droop equations adjust to resistive impendence are used. Because of the influence of PV power, grid voltage and frequency fluctuations on output power of inverter, droop curves are translated to keep output power and DC bus voltage stable. At the same time, grid harmonic is obtained by harmonic detection algorithm, and through harmonic compensation, the grid current distortion rate is decreased. The improved droop control based on line impedance ratio is proposed to decouple output power when inverter operates with resistor-inductance line impedance. Simulation model is built to prove the strategies are right.Finally hardware experimental platform based on STM32F103 is set up to verify that the voltage-controlled grid-connected strategy based on droop control and decoupled droop control scheme are effective. |
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