Study On The Control Strategy And Mode Switching Technology Of Inverter In Microgrid

As ties of distribution network and distributed generation, Microgrid makes different species and large numbers of dispersed and intermittent distributed energy connected to the grid with minimum impact. Inverter in the Microgrid can work on grid-connected mode, off-grid mode and can achieve smooth transition between those two modes. Taking two voltage source inverters as an example, the principle of parallel operation of the inverter under grid-connected mode and off-grid mode is analyzed in this paper based on droop control theory. Combining with the dynamic model of the power transmission, the system stability is analyzed with different droop control parameters. In view of the inverter power allocation problem, the conditions of average power allocation based on traditional droop control strategy is analyzed in this paper, and the influence of transmission line impedance on reactive power distribution is suppressed using the improved droop control method.For the realization of the inverter output voltage tracking the droop control voltage instruction, this paper studies the common used voltage and current double closed-loop voltage type inverter control strategy. The inductance current inner loop and the capacitive current inner loop control are analyzed respectively in the view of output impedance of the inverter and dynamic performance under different load conditions and the controller parameters is designed. Finally, in the case of LCL filter and LC filter, the influence of transmission line impedance on inverter filtering capacity and harmonic disturbance resistance is analyzed.For the realization of the inverter smooth transition between grid/off-grid mode, this paper studies the grid-connected/off-grid dual mode switching technology. Adopting the synchronization technology, the smooth switching of the inverter from off-grid mode to grid-connected mode is achieved, and the detail transition process is revealed in this paper. The influence of synchronization process on the output power of the inverter is studied, and solutions for off-grid mode and grid-connected mode are studied respectively.Finally, in order to verify the correctness of related principles in this paper, two 5kVA three-phase dual-mode inverter experimental platforms have been built.The single inverter off-grid operation experiment and grid-connected operation experiment, double inverters parallelled operation experiment, synchronization experiment, off-grid mode to grid-connected mode switch experiment, grid-connected mode to off-grid switch experiment are implemented. The results of experiments verified the correctness and effectiveness of theory in this paper.