Research On Transient Characteristics Of Doubly-fed Induction Generator And Crowbar Protection Strategy Under Fault Conditions

With the decreasing of traditional fossil energy and the deterioration of the globalclimate, clean and renewable energy such as solar and wind become more and morepopular in many countries. Since China is a country with a vast territory and has abundantwind energy resources, the fast development of wind power industry has made chinabecome the country with largest capacity of installed wind power in the world. However,most of these wind farms are built in rural areas, such as northeast, northwest, north ofchina, and southeast coast, the electric power grid in these areas are normally weak, andthe grid’s fluctuation may cause unstable operation of wind generators. Besides, voltagedrop fault may cause large scale disconnection of wind turbines, which will further causegrid’s frequency drop. To avoid this situation, the disconnection of wind turbine duringvoltage sag is not permitted, and wind turbines should provide active and reactive powerto help the grid voltage recover. Doubly-fed induction generator(DFIG) that has theadvantages of wide speed regulation range and small converter capacity becomes moreand more popular. Meanwhile, the drawback of DFIG that it can be easily affected by gridfluctuation makes low voltage ride through(LVRT) technology a research hotspot in powerconversion field. It has important practical significance and industrial value to improve theability of LVRT.In this paper, the research status of DFIG’s transient characteristics and crowbardesign has been summarized. Then accurate expressions of stator flux, stator current androtor current were deduced by analyzing of the process of LVRT based on detailed modelof DFIG. Thus the characteristic of these variables could be calculated and then analyzed.The analytical expressions were verified by making comparison betweenMATLAB/Simulink simulation and analytical results. From these formulas, relationsbetween crowbar resistor value and the pulsation amplitude of electromagnetic torque,output power were revealed. Taking into account the dc-link clamp effect and influence to the performance of DFIG during LVRT, a new crowbar resistance design method wasproposed, which may be helpful in industrial application.By calculating rotor side Induction Electromotive Force, the control limits of currentLVRT control method have been analyzed. Conclusion can be made that crowbar isindispensable when cope with deep voltage drop. Then in this paper, a new crowbarcontrol strategy has been proposed: when grid voltage suddenly change, rotor side currentwill increase rapidly, crowbar soon engages and the outputs of converter are turned off toavoid destruction of converter, during LVRT period, Induction Electromotive Force iscalculated in real time. Once Induction Electromotive Force attenuates under DC-linkvoltage, crowbar can disengages. Compared with current crowbar control strategy, the newmethod in this paper is more flexible and adapted to any type of voltage fault, the worktime of crowbar is depend on the depth of voltage sag.