| With the electrical loads growing, the energy crisis and environmental problem getting worse, the distributed power generation is undergoing a significant development all around the world. Although the widespread application of distributed generation systems will bring about new opportunities and challenges to power grid operation, control and electricity market, there are still a lot of problems. The most important point is that as an uncontrollable power interface, the distributed power generation (DG) unit will produce some negative effects on the stable operation of the grid. Therefore, there are lots of difficulties in large-scale distributed generation’s grid-connected operation. Micro-grid is proposed in this case, aiming to solve the contradiction between the grid and distributed power and take advantage of distributed generation systems in large-scale. The micro-grid, as an effective carrying way of distributed generation, can fully take advantages of distributed generation and support each other with the grid, which makes users obtain uninterrupted reliable power supply. The micro-grid seamless transfer between grid-connected mode and islanding mode is the premise of ensuring the normal operation of sensitive load in micro-grid, which becomes one of the important contents of micro-grid research.In the thesis, the traditional droop control strategy were analyzed, and a droop control strategy based on voltage and current double-loop for multi-inverters parallel operation in micro-grid was proposed by introduction of virtual negative resistance; Then the operating characteristic in both island and grid-connected modes of micro-grid inverter are depicted, and a novel pre-synchronizing control strategy is proposed for the micro-grid inverter to transfer smoothly between two different modes. Finally, based on Matlab/Simulink software, the simulation model of micro-grid is built to verify the effectiveness of droop control strategy for micro-grid inverter. |
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