Dynamic Control Of DC-bus Voltage And Optimal Design Of Support Capacity For DFIG System

VSCF(Variable Speed Constant Frequency) doubly fed wind power generationsystem has good dynamic performance, variable speed constant frequency, low capacity ofpower electronic converter, achievement of maximum wind energy extraction, etc.With ofthese advantages, It’s universally used in wind power generation system. The wind speedmutation could cause the change of power generation, thus causing the DC bus voltagefluctuation of the dual PWM(Pulse-Width Modulation) converter. The value of DC sidecapacitance is generally increased, in order to reduce the DC bus voltage fluctuation. Butthe bulk capacitor is large in volume as well as quality, and low in reliability, whichbecomes the bottleneck of the whole system in reliability and service life. To solve thisproblem, this paper studied the dynamic control of DC-bus voltage and the optimal designof support capacity for DFIG(Double Fed Induction Generator) system. This study canguarantee the DC bus voltage stability and accelerate the dynamic response, In the case ofsmaller capacitance values.In this subject, the fundamental principle of DFIG was formulated firstly, themathematical model of DFIG in static three-phase coordinates was established and themathematical model in two-phase synchronous coordinates was deduced by coordinatetransformation theory. The stator flux oriented control strategy is used, in order to realizethe decoupling control of active and reactive power. Since the DC bus voltage iscontrolled by the grid side converter, in order to achieve high performance control ofnetwork-side converter and stability control of DC bus voltage and reducing the filtercapacitor, a dual closed-loop control of DC voltage outer control and current inner controlapplied, and the design of internal and outer loop controller parameters optimized, byestablishing the mathematical model of the gird side PWM converter in two-phasesynchronous speed rotating coordinate system. In order to ensure the system’s dynamicand static performance, the impact of outer voltage loop controller is analyzed capacitorselected, the controller design method and parameter tuning formula is drawn, the supportcapacity is optimized with the DC capacitor design requirements combined. For DC voltage fluctuations caused by wind mutations, to improve the systemdynamic response speed, this paper utilizes a new improved control strategy whichproposes modulation wave compensation of current front feed in the small-signal modelanalysis method. Thereby, the control strategy can eliminate the delay of the current loop,make the responsive system faster. Tt’s established a MATLAB/Simulink simulationmodel of the DFIG system in this paper, with simulating, comparing and studying of theproposed theoretical analysis, the design method and control strategy, it’s verified that thecontrol scheme has the feasibility and correctness.