Research On Improved Direct Power Control Method Of Open Winding BDFG

The global economy is developing rapidly.As the primary condition for maintaining economic development,energy has become the focus of attention.Traditional fossil energy storage is limited and will become exhausted eventually.The negative impact of its excessive development and use will become increasingly prominent,especially the serious problem of environmental pollution and hazard to human body and so on.Governments are trying to do one thing to looking for traditional energy alternatives as soon as possible in recent years.Wind energy,solar energy and tidal energy begin to occupy a place in the world energy structure.Wind energy has attracted the attention of governments and researchers all over the world for its wide distribution,clean and renewable.With the deepening of research,the main models for wind power generation are permanent magnet synchronous generator and doubly fed induction generator at present.However,permanent magnet synchronous generator require for full scale converter,high permanent magnet materials prices and easy demagnetization.Double-fed induction generators have complex structures,poor reliability and difficult control.Brushless double-fed generator(BDFG)as a new type of generator not only has low cost and simple structure,but also is very suitable for application in wind power generation system.Aiming at the power tracking control of BDFG,this paper proposes an improved direct power control method for this kind of generator,and focuses on the following aspects.Firstly,the working mechanism and different operating modes of the brushless doubly-fed generator are expounded,and the power flow direction under different operating modes is analyzed and studied.A mathematical model based on a rotating coordinate system suitable for this kind of generator control is established,and its performance is analyzed through simulation.Secondly,direct power control strategy is improved for BDFG with special rotor structure.The grid-side converter of BDFG system adopts a model prediction direct power control method,and carries on the simulation contrast analysis with the traditional voltage type direct power control.Theoretical analysis of direct power control methods for close and open winding BDFG side converters respectively,and the simulation research and comparative analysis are carried out.Thirdly,a power controller with an approximately constant switching frequency is proposed to replace the traditional hysteresis comparator.The dual-triangular waveform with a phase shift of 180°is used for modulation,and the power of the generator is controlled by using a power loop based on a linear model.The simulation results show that the proposed power controller can make the active power switching frequency almost constant,reduce the power pulsation,and improve the control accuracy.Finally,in view of the advantages of the double-triple level converters applied to the open-winding BDFG direct power control,based on the common DC bus structure,a common-mode voltage suppression method based on the open-winding BDFG direct power control of a double three-level converters is proposed.The simulation results show that the proposed method has significant effect on common-mode voltage suppression.