Abstract
In past ten years, energy saving and carbon reduction become more and more important issues around the world. Because the shortage in fossil fuel, the prosperity of non-nuclear activities and the trend of eco-conscious. Hence, the research topics that relate to alternative energy resources are the most popular issues. There are more and more distributed energy resources (DERs) which are connected to the grid through interface converters. The electric utility power company requires the DER generation systems to remain grid-connected during grid voltage sags to ensure the operation stability and reliability. The grid-connected converters usually link to the grid through a transformer. The low voltage ride-through (LVRT) requires grid-connected converter should still be on-line when voltage sags occur, so the transformer between grid and grid-connected converter needs to experience the voltage sags. These unbalanced voltage will make the transformer saturated and will cause inrush currents when voltage recovers. Inrush currents may harm the transformer physically and reduce the life of the transformer. The thesis proposed a flux compensating method to fix the dc deviation of magnetic flux linkage caused by voltage sags under these unbalanced voltages. The saturation of transformer can be eased and the inrush current caused by transformer saturation can be also reduced. Its operation principle and control method are explained and analyzed. Simulation and laboratory test results are presented to validate the effectiveness of the proposed method. Comparison of the results with and without flux compensating method is also presented.