Research On Low Voltage Ride-through Of Wind Turbines With Doubly-fed Induction Generator

In the background of the global energy crisis and the vigorous supports to the new energy industry, wind power becomes a research focus in today’s world. With the increase of the capacity of wind power generation system connected to the grid, the proportion of the wind power in power system is also growing, and the characteristics of the wind turbine while faults occur become more and more significant to the grid. When the external power grid transient voltage fluctuations, the wind turbines’ability to maintain connected to grid will be a vital role in the stability of the system. When the voltage sags caused by the grid’s fault, if the wind turbines are off-grid by a large scale, it will bring secondary impacts to the weakly grid which is in fault or even voltage collapse and greater challenges to the safe and stable operation of power system. Therefore, it is very important to improve the uninterrupted operation of the wind turbine capacity when the grid’s voltage sags.In this thesis we’ll study the wind turbine’s fault ride-through capability i with doubly-fed wind power generation system. At first, we gave a detailed analysis of the basic working principle of the doubly-fed wind power generation system, established a mathematical model of static ABC coordinate system and synchronous rotating d-q coordinate system, designed the back-to-back PWM converter control strategy and achieved and simulated in the PSCAD/EMTDC platform. Then we can establish Transient Mathematical Model of Doubly-fed wind turbine by Laplace transform based on steady-state mathematical model. Both in theory and simulation we analyze the dynamic response characteristics of a doubly-fed wind power generation system in the different amplitude of the voltage sags. The result indicates that voltage sags magnitude has a significant influence on the transient characteristics of the wind turbine, and appropriate low voltage ride through schemes can be used to achieve uninterrupted operation the appropriate in conditions of different ranges of voltage sags. At last, we derive low voltage ride through schemes in different situations according to the simulation analysis based on the fault characteristics of the wind turbine low voltage ride through schemes in different situations. When minor voltage sags occurs, we can achieve fault ride-through by improved flux compensation control strategy and statcom control strategy of grid-side PWM converter. When large voltage sags occurs, we can achieve fault ride-through of doubly-fed wind turbine by cooperative control among crowbar circuit of the rotor side, DC chopper and reactive power compensation. We mainly research the influence to low voltage ride through of crowbar switching time and bypass resistance and finally raise optimum low voltage ride through control scheme. And through simulation analysis, the scheme can increase low voltage ride through capacity of wind turbine and optimize transient voltage characteristics of the system.