Research On Adaptive Grid-Connection Control Of Converters For Doubly-Fed Wind Power Generation System

Converter capacity of double-fed wind power generation system is double-fed motor slip capacity with features of small size, light weight, low loss characteristics etc, which has become the mainstream model on the market at home and abroad. With the expansion of the scale of wind power, DFIG system must be equipped with the ability of low voltage riding through, resisting the influence of unbalanced grid voltage and reactive power compensation, namely, to enhance the grid-connection adaptability. Combined with the research project "Modeling and Simulation on Adaption of Grid-Connection Technology for Wind Turbine" authorized by Electric Power Research Institute of Yunnan Grid, converter control strategy of DFIG system was studied. The main contents are as follows:The mathematical model of DFIG system was built. Combined with the mathematical models of wind speed, wind turbine, doubly-fed generator and converters, stator flux oriented vector control strategy of DFIG, grid voltage oriented vector control strategy of grid-side converter were established. A simulation model was established in PSCAD, which was validated by simulation.Transient analysis and LVRT energy absorbing solutions were proposed if grid faults happen. Then the analytical formulas of stator and rotor’s transient flux and current were presented by the method of Laplace transform when stator voltage drops to zero. Besides, electromagnetic transient characteristics of DFIG faced with symmetrical grid voltage drop were analyzed. Reducing the rotor fault current to protect the DFIG-side converter and avoiding the DC bus overvoltage to protect the capacitor are the key issues of LVRT. The value of crowbar resistance was derived from absoring reactive power and avoiding the DC bus overvoltage. LVRT was achieved by employing an external circuit including crowbar and chopper circuit in PSCAD.Moreover, control strategy of converters under the unbalanced grid voltage were established. A mathematical and instantaneous power model of DFIG were established in the premise of positive and negative sequence separation. The negative component of stator flux is the root cause of active power, reactive power and torque pulsation, stator current unbalanced and rotor current distortion when grid is unbalanced. The positive and negative sequence components of the stator current were controlled by the two sets of control systems respectively, which was to suppress the unbalanced three-phase current of stator and grid-side converter, and then simulated in PSCAD.Furthermore, wind farm modeling and reactive power compensation were discussed. The main issues and equivalent methods of wind farm modeling were summarized, as well as the positive impact of wind farm cluster effect on the power system. Compared the complete polymerization and clustering equating methods, the latter has a higher accuracy in the application of wind farms modeling. To make DFIG-side and grid-side converter have reactive power compensation capabilities, reactive power reference values in control strategies of DFIG-side and grid-side converters can be changed. Meanwhile, the basic principle and the function of SVG in the maintenance of the voltage stability between wind farm and grid are also researched, and validated by simulation.