Study On The Seamless Transfer Between Islanded And Grid-connected Operation Mode Of Three-level-inverter Based On Droop Control Strategy

With the increasingly seriousness of environmental and energy issues, more andmore attention has been paid on the development and utilization of renewable energy. Thedistributed generation technology can make the fullest use of widespread renewableenergy, which has become the primary technical mean in the field of renewable energyapplication. The combination of grid and distributed generation system is the mainresearch direction in the future, which can reduce the investment and energy consumption,as well as improve the reliability and flexibility of the system. The seamless transitionbetween grid-connected and islanded modes of distributed generation system is the keyresearch point of the combination of grid and Distributed Generation System. And thisissue is deeply researched in this paper.Firstly, this paper analyzed the operational principle and modulation method of thediode-clamp three-level inverter. The traditional Space Vector Pulse Width Modulation(SVPWM) method has many problems, such as the complex sector judging and too manytrigonometric function calculations. To solve these problems, an improved SVPWMmethod is presented in this paper, which can determine the sector of the target vectordirectly and done away with trigonometric function calculations. It could save a lot ofresources and time when runs on the FPGA. The active synchronization algorithm ofdroop control is then studied. By analyzing the results of previous studies, this paperimproved the controller structure of active synchronization control, which makes thecontrol more simple and effective. The problems of the active power impact at switchingmoment and reactive power exchange at grid-connected time will appear when the inverteris controlled by traditional droop control method. An improved droop control method ispresented to solve these problems in this paper. This method eliminates the activeovershoot and reactive power exchange, which can realize the seamless switching betweenthe two modes.At last, the simulation results verified the improved control strategies presented bythis paper. An experimental platform of diode-clamp three-level inverter has been built, which is controlled by DSP and FPGA. The control programs have been completed. Andthe control strategy has been verified through experiments.