Research On Voltage Dip Measurement And It's Application In LVRT Of Grid-connection Of Wind Generation

Voltage dips are short-duration reductions in voltage magnitude. As one of the most concerned issues in power quality, voltage dip has done tremendous harm to both users and power grid. To users, it may cause halts or damages for those voltage-sensative equipments that are widly-used in industury, which will bring about great finacial losses; To the power grid itself, it has made a huge challenge the grid-connection of clean energy generation, which has gained increasing concerns nowadays.When wind power generation, as the representative of clean energy generation, is put into the mode of grid-connected generation, the mutual influence between the grid and the wind turbine is to be more and more prominent, especially when the voltage dip appears because of the hitch of the grid, which will do enomorous harm to the turbines; meanwhile, the wind turbine, during the period of voltage dip, is required to keep in grid-connection, i.e. the function of LVRT, in case of the surge to the grid caused by disconnection of the wind turbine, which will increase the difficulty of recovering the grid system.Starting from the analysis of the voltage dip, the present thesis firstly analyzes the causes and features of voltage dip at different positions of the grid caused by different hithes, and secondly introduces the types of voltage dip and its transmission in the grid done by the transformers; Last but not the least, the thesis puts forward the approches of testing and compensation the voltage dip. On the basis of the analysis of the voltage dip, the present thesis puts forward the notion-LVRT of grid-connetion of the wind power generation:the ability of the LVRT of grid-connetion of the wind turbine refers to the situation when the wind turbine merges into the state grid and the grid experiences the voltage dip, the generator still keeps grid-connetion of wind generation till normal. And claims that the protection of it should be from two aspects-first, the testing and the compensation of the voltage dip in the gid have to be strengthened in order to reduce the harm towards the grid-connection; Second, the fuction of wind turbine's LVRT should be strengthened to make the turbines keep working for the grid-connection in face of the voltage dip.The present thesis analyzes the harm to the wind turbines brought by the voltage dip, and uses "the dubly-fed-induction wind turbine" as the example to introduce the transient characteristic of the wind turbine's being in the period of voltage dip. Finally, by using the method of putting together the voltage and the LVRT, the author puts forward the application of test of the voltage dip to LVRT, and by comparing different testing methods according the national requirements on LVRT, the author draws the conclusion that how to apply the most approperiate method of testing according to different curcumstances.