Study On The Mode Transfer Technique Of Three-phase Grid-Connected/Islanded Dual-Mode Inverter

Grid-connected/islanded dual-mode inverter is mainly applied in new energy distributed generation(DG) system. As for the positive influences on environment, society and economy, DG system has been paid more and more attention, the key techniques of which have been the hot pot of research.In this thesis, the operation of three-phase dual-mode inverter has been analyzed. The dual-mode inverter has been modeled and control methods in grid-connected/islanded operation mode have been studied. Simulation results have verified the effectiveness and correctness.In addition, the mode transfer process of dual-mode inverter has been analyzed. The basic algorithm of the mode-transfer method for three-phase dual-mode inverter has been introduced. Different mode-transfer processes based on different operation sequence of the grid switch and the mode switch have been analyzed. As for the saturation of PI regulator output during the mode transfer process from grid-connected to islanded mode, a novel PI regulator in voltage loop has been proposed. In order to guarantee the seamless transfer of dual-mode inverter from islanded mode to grid-connected mode, a droop control based synchronization strategy has been proposed. Simulation results have verified the effectiveness of the proposed mode transfer method.In order to realize the rapid and accurate detection of islanding operation, the basic principle of impedance estimation based islanding detection method has been introduced. Switching harmonic method and small-signal interruption method have been analyzed respectively and comparatively. The non-characteristic harmonic small-signal interruption based grid impedance estimation method has been utilized to detect the islanding. Simulation results have verified the validity of the proposed islanding detection method.In order to verify the correctness of related principles, a 5k VA three-phase dual-mode inverter experimental platform has been built. The main circuit parameters have designed and the components have been selected. The control algorithm of system has been designed as well. Experiments of the operation of dual-mode inverter in grid-connected/islanded mode and the mode transfer between the two operation modes have been implemented. The results have verified the effectiveness of the proposed mode transfer technique for dual-mode inverter.