Research On The Control Strategy Of Grid-connected PV Inverter Based On Droop Control

With the consumption of traditional energy,the energy crisis has become a worldwide problem.Therefore,people pay more attention to the renewable energy,such as photovoltaic,wind power,tidal power and so on.Photovoltaic power generation is an important part of renewable energy generation because of its low cost of generating electricity.But the access of distributed generation equipment may lead to the instability of the main grid.Microgrid can not only work in grid connected state to generate power transmission to main power grid,but also operate in island mode for local load.So it has become the important part of the development of smart grid.According to the requirement of Microgrid,a universal control strategy about two-stage PV inverter and the matching island detection method is proposed in this paper,and the application circuit is designed.The universal control strategy which is based on the droop control equation can implement the island operation,grid-connected operation and seamless transfer between them.According to the disadvantages of basic droop control,the problem of maximum power generation,grid voltage fluctuation and PV power fluctuation is considered in the improved control strategy.Compared with the traditional PV inverters which work as controlled current sources in the grid-tied mode while operating as controlled voltage sources in the islanded mode,the universal control avoid the control reconfiguration and guarantee the stability of inverters.Then the simulation about the PV inverters with the universal control is completed,and the results of simulation show that the proposed control strategy in this paper can be applied to the single-phase two-stage PV inverters.The experimental platform of the single-phase two-stage PV inverter is built.The IGBT is used as the switching device of the PV inverter,and DSP is used as the core of the system.Experiments have realized that the island operation,the seamless transfer and the grid-connected operation can be achieved.So the practicability of the proposed control strategy is proved.Besides,an island detection method based on droop control strategy is proposed in this paper.The amplitude disturbance is injected to grid by changing the virtual resistance.The amplitude of output voltage will change rapidly when the inverters operate in the state of unplanned island mode.The inverter will switch to island operation when the output voltage beyond the normal range.The simulation about inverters with the island detection method is completed,and the results of simulation show that the proposed island detection method in this paper can be used to single inverter and parallel inverters system.