Study On The Control Strategy Of Lvrt For DFIG-based Plants With CROWBAR

In the wake of 21st century, the wind as an emerging green energy has aroused wide public concern. There are plenty of countries and regions have realized the importance of wind energy , also, in our country the wind power is in vigorous development. With the booming of the wind turbine generator system WTGs, the wind power generation capacity is rapidly increasing, its influence on grid is becoming more and more remarkable.The grid connection DFIG as one of the dominant types, its steady characteristic, dynamic response characteristic and transient characteristic exert significant effects on the stability of the power grid. When grid failure or disturbance causes the wind farm's network connection point voltage dips, the wind turbine installations have to stay connected to the grid for voltage dips above a certain threshold, which means that the wind turbine has to provide the capability of riding through these voltage dips (LVRT).People began to attach great importance to LVRT.According to recent grid codes, this paper proposes a novel control strategy of LVRT for DFIG-based plants with crowbar. The main content is following:(1) The paper analyzes the difference between constant speed constant frequency (CSCF) and variable speed constant frequency (VSCF), and expounds the operation principle of VSCF wind power generation of DFIG.(2) The aerodynamic model, shaft system model, DFIG model and converter model of the wind turbine generator system are presented. Among them, the paper derives the dynamic model and steady-state models of the DFIG expressed in the synchronous dq reference frame and abcreference frame.(3) This paper introduces the reason of DFIG's stator side over-current and rotor side over-current when sever grid voltage dips occur. There is a detailed analysis on rotor side over-current, and it's verified by MATLAB/Simulink. Then proposes a novel control strategy of LVRT for DFIG-based plants with crowbar when sever grid voltage dips occur.(4) Introduces two mainstream crowbar circuit structure, by comparison, the paper evaluates the different effect of the active crowbar and the passive crowbar. A novel crowbar type of rectifier-bridge with detection and control module is proposed and verified by oscillogram. It has been shown that the key of a successful ride through is the novel active crowbar. At last, the paper analyzes the influence of crowbar protection effect due to two factors: cast cutting time and bypass resistance.