Research On Control Strategy For Microgrid Double-Mode Inverter

ABSTRACT:Double-mode inverter is an important part of microgrid, it is able to function in both grid-connected mode and stand-alone mode. The transfer between grid-connected mode and stand-alone mode should be seamless to ensure the uninterrupted power supply. Meanwhile, double-mode inverter should equipped with reliable islanding detection to avoid mode transfer malfunction. This thesis mainly researches stand-alone function, seamless transfer and islanding detection.The control strategy of double-mode inverter running in grid-connected mode is single current loop control. Modeling and key parameters are figured out. Simulation aiming at load saltation confirms that double-mode inverter has stability.Double-mode inverter should keep the V and f of microgrid constantly when running in stand-alone mode. In this thesis, V/f control and droop control are compared. The droop control strategy based on P-f and Q-V is selected. This thesis analyzes the impact of grid impedance and inertial element on droop control and the stability of microgrid consists of three inverters. The feasibility of the control strategy is confirmed by simulation and experiment.Smooth transfer between grid-connected mode and stand-alone mode is gordian technique of double-mode inverter. A method with an additional regulator of the output closed-loop tracking is proposed for the transfer between two function modes. With this method, double-mode inverter could smoothly and steadily transfer between grid-connection mode and stand-alone mode. The feasibility and smooth voltage and current response of the proposed method is proved by both of simulation and experiment results.Effective islanding detection is necessary to double-mode inverter, since the detection result decides the time of transfer. Non-detection zone and negative influence to power quality are common for the existing islanding detection. In this thesis, negative-sequence voltage positive feedback and grid impedance estimation based on DFT are considered to be islanding detection methods for double-mode inverter. But, double-mode inverters in parallel must use grid impedance estimation alternatively to avoid interaction, which needs more time to finish islanding detection. So negative-sequence voltage positive feedback is selected.This thesis covers four fields which are gordian technique of double-mode inverter: grid-connected mode of double-mode inverter, stand-alone mode of double-mode inverter, seamless transfer between two modes, islanding detection technique.