Research On Seamless Transfer Strategy Between Parallel Mode And Grid-Connected Mode Of Multi-Inverter System Based On Droop Control

The technology of microgrid and distributed generation not only greatly make up for the defects of the centralize-controled main grid, but also ease the issue of energy and environmental. In this paper, the operation mode and the mode switching method of multi-inverter system are studied.Firstly, this paper introduces the research status of parallel mode, grid-connected mode and the transfer strategy of microgrid, and analyzes the advantages and disadvantages of various strategies. The wireless parallel technology based on droop control has been widely used because of its good distribution and high reliability. This paper introduced the improved droop control method based on communication lines and droop control. This method make full use of the central controller, which must needed in mode switching mode. This strategy can greatly improve performance of current sharing. The primary advantage of improved droop control is that the system contains the communication line but does not depend on it. The system can operate in parallel normally with the traditional droop method as communication line fault. This method further enhance the redundancy of this system.Due to fact that improved droop control method and voltage-source grid-connected control strategy has the almost same control equation, in order to realize the seamless switch mode, this paper chooses voltage source grid-connected method based on droop control. However, because the voltage-source grid-connected method indirectely control the grid-connected current and grid voltage contains rich harmonics, the quality of the grid current is bound to be very poor, which can be verified from the experiment results. In this paper, a method based on harmonic compensation is proposed to solve this problem, and the THD of the grid connected current is reduced from 9.8% to 2.3%. Due to the fluctuation of the power grid, the steady-state error of the output power can be very large. In this paper, a method based on voltage frequency compensation is proposed, which greatly reduces the steady-state error of active power. The steady-state error is controlled within reasonable range. After control strategy of stand-alone and grid-connected mode has been determined, stretegys of mode transfer is design, which includ voltage synchronization design, transfer strategy of stand-alone mode to grid connected mode, transfer strategy of grid connected mode to stand-alone mode.In addition, this paper introduces the design of the LC filter of the single inverter, the design of the PI parameters in the voltage control loop and the design of the system monitoring and control system Lab VIEW etc. Finally, the effectiveness of the mode switching strategy is verified by the PLECS simulation platform and the hardware platform including three inverters. Based on the completion of this topic, this paper made a summary of the subject. What’s more, the future work of this project is also introduced.