Research On The Grid Connected Control Of Multiple Inverter Based On LCL Filter

The rapid development of new energy power system makes the inverter and LCL filter is widely applied in recent years. Meanwhile, in order to expand the capacity and improve the redundancy of the system, the system is often used in multiple inverters parallel mode. The grid-connected inverter needs to address the problems of improving the grid-connected power quality and controlling the DC voltage and power under unbalanced grid voltage, The main contents are as followed:1) LCL filter is easy to cause resonance, especially considering the grid equivalent inductance effect, the resonance phenomenon is more severe and complex, thereby measures must be taken to curb its resonance and improve the grid-connected power quality of the inverter system.2) The parallel operation of inverters exist circulation problems which affect the efficiency and reliability of the inverter parallel operation, it is necessary to take measures to curb its circulation.3) Under unbalanced grid voltage, the DC voltage and grid power fluctuations of the grid inverter need to be addressed.In order to solve the problems, this paper has established a mathematical model of the L and LCL filter in different coordinate and compared the advantages and disadvantages of two kinds of filter; Then a novel active damping method considering the grid equivalent inductance effect is proposed by analyzing the resonant mechanism and factors. The grid-connected power quality of the inverter is improved effectively and the designd methods of the current controller parameter are given. On this basis, an improved method based on compensatory current is applied to reduce the circulation between parallel inverters for improving the efficiency and reliability. The advantages of LCL filter to suppress the high-frequency harmonic circulation are also analyzed. Finally, the secondary ripple problems of the inverter DC voltage and grid power will be caused by the unbalanced grid voltage, so this paper presents a control method based on the superposition of positive and negative sequence and effectively solve the DC voltage and grid power fluctuations. A simulation mode is build in PSCAD/EMTDC for proving the feasibility and effectiveness of the proposed theories> strategies and methods, it provide a reliable reference for practical engineering applications.