Direct-drive Wind Power Converter Control Strategy Under Non Ideal Grid Voltage Conditions

With the single wind turbine capacity and the total installed capacity increasing rapidly,the interation between the wind power systems and grid is becoming more complicated.Direct-drive permanent magnet wind power system(D-PMSG)based on full power converter realizes the full decoupling between grid and generators,having been proved to have better performance than doubly-fed structure in the aspect of grid fault operation.However,research on D-PMSG's performance characteristics is not deep enough,and there are a lot of engineering practice problems have not been solved effectively.When the grounding short circuit fault occurs in power system,due to severe power imbalance in the DC bus the DC-link voltage may exceeds the maximum allowed,and the negative sequence voltage can also casue the active/reactive and DC-link voltage of 2 times the power frequency fluctuations.In order to solve the above problems,urgent D-PMSG control strategy research is needed to improve the fault ride-through capability of the wind power system.At first,this paper establishes the two-phase synchronous rotating coordinates mathematical model of PMSGa and the converter under symmetrical grid voltage condition,and builds a complete control system including PMSG,machine-side converter and grid-side converter,which lays the theoretical foundation for the subsequent chapters.Secondly,the mathematical model of grid-side converter is presented.The single current loop vector control based on grid voltage orientation is analyzed,and the reason why this method can not eliminate the negative sequence current and restrain the fluctuation of DC-link voltage is revealed.The control scheme to eliminate the negative sequence current and improve the power quality is studied.It is pointed out that the control scheme is still unable to eliminate the power and voltage fluctuation.And then the overvoltage protection schemes based on energy consumption resistance and based on energy storage system are introduced in detail and the simulation is carried out.Then,an improved power control strategy is proposed,which can priority output reactive power without excessive increase the burden of overvoltage protection circuit and assist grid voltage recover.Control strategy based on the positive and negative sequence rotating coordinate douple current loop vector and control strategy based on PIR controller positive synchronous rotating coordinate single current loop vector are analyzed deeply.Also,the calculation method of current instruction to eliminate negative sequence current or eliminate voltage fluctuation is deduced.The research contents of this paper are in link with the reality and trend of the development of wind power technology in China.The conclusions have important theoretical significance to improve the stable operation of wind power system under grid fault conditions.Finally,a 2KW back-to-back wind power system platform is set up.The main circuit,and hardware and software design methods are introduced in detail.The experimental results verify the correctness of the control algorithm proposed in this paper.