Research On The Control Strategy For Doubly Fed Induction Wind Power Generators During Grid Faults

As an effective way to change wind energy into electrical energy, wind power hasgot more and more attention and been widely used in the world. With the increasingof capacity of the wind power system, its impact on the grid security and stability ismore and more obvious. The power system requires that the grid-connected windfarm must have a good power quality and fault ride through capability.With the support of National Natural Science Foundation of China (No.51077034)and Ordinary University Natural Sciences Research Project of Anhui Province(No.2011AJZR0063), the variable speed constant frequency (VSCF) doubly fedinduction generator (DFIG), which is widely used in the wind power market, actingas the object to research the grid fault ride through in this paper. And in-depthanalysis and verification are made by theory, simulations and experiments.It can besummed up as follows:1) After rigorously theoretical derivation, the DFIG mathematical equations inthree-phase stationary frame and two-phase synchronous rotating frame areobtained, and the T-equivalent model and Ï„-equivalent model are established, thevector control model for the converter machine side of DFIG based on statorvoltage orientation is established. It provides a theoretical basis for studying thecontrol of DFIG under grid fault.2) In order to protect the circuit without triggering the crowbar under gridvoltage dip, the control technology based on the active damping is studied toimprove the ability of the DFIG fault ride through. The validity of active dampingcontrol technology to the LVRT is confirmed by rigorous theoretical analysis andnumerous simulation results. For the improved active damping technology, thetechnology based on active inductor and active impedance is studied to restrict therotor over current and shorten the transient time.3) The variable active damping technology for LVRT is proposed for differentfault operating conditions, and the damping size of different depth of the drop,different duration of the grid fault and different speed of the DFIG are analyzed tofix the LVRT requirements best.4) An in-depth theoretical analysis for the transient process of DFIG underthree-phase unbalanced fault is made, and the impact of unbalanced on the windturbine is pointed out. A detailed analysis of the characteristics of unbalancedcontrol technology using dual-dq synchronous rotating frame, and simulationanalysis of the unbalanced control effect in dual-dq frame is made, and the activedamping control analysis to deal with the unbalanced grid is conducted, based onthe analysis, the active damping control analysis under unbalanced grid dip is alsomade.5) A11KW DFIG platform is built in the laboratory, the low voltage ride throughexperiment and unbalanced voltage ride through experiment for DFIG based on the active damping control are conducted. It verifies the correctness and validity of thetheoretical analysis and provides a basis for engineering application.