Research On Fault Crossing Strategy Of Wind Farm Connected To Grid Based On MMC-HVDC

With the rapid development of wind power generation,long distance transmission has become a restrictive factor for the development of wind power generation.How to transmit power generated by wind farms to the far distance power grid has become the first problem to be solved.Flexible HVDC technology has become a key research project of power transmission in recent years because of its advantages in long-distance transmission.As the representative of new flexible DC transmission,modular multilevel converter has become the focus of research.However,when fault occurs in the power grid side,the unbalanced power of MMC-HVDC will lead to the rise of DC line voltage,which seriously affects the safe and stable operation of the system.Starting from the development background of wind power generation and MMC-HVDC,this paper first introduces the development of wind power generation and MMC-HVDC technology.Then the mathematical models of two kinds of wind turbines,including double-fed induction generator and permanent magnet synchronous generator,are established,and the control strategy of the controller is designed on the basis of the mathematical model.Secondly,the topology and working principle of MMC converter are analyzed,and the advantages and disadvantages of two main modulation modes,the nearest level approximation modulation and the carrier phase shifted pulse width modulation,are analyzed,and carrier phase shifted pulse width modulation is chosen as the modulation strategy of this simulation model.Then the cause of the capacitance voltage fluctuation of the sub module of the MMC converter is analyzed.The circulation suppression and the submodule capacitance voltage balance control strategy based on the sorting method is proposed to realize the capacitance voltage balance of the sub module.Then the mathematical model of the MMC converter is established.On the basis of the mathematical model,the control strategy of the two side converter is designed in view of the different control requirements of the power grid side converter and the wind farm side converter.A wind farm and MMC-HVDC simulation model was established in PSCAD/EMTDC,and the correctness of the simulation model was verified by simulation.Finally,after analyzing the characteristics of the simulation model of the wind farm connected to power grid with the MMC-HVDC and fault occurs in the power grid side,the traditional fault crossing model is established.After the simulation analysis of the traditional fault crossing model,it is found that the traditional fault crossing model will make the power waste and the MMC-HVDC output power overload which will cause voltage drop of DC line after failure.In view of the shortcomings of the traditional fault model,a comprehensive fault crossing strategy which take a DC energy storage device consisting of a bidirectional DC-DC converter and a super capacitor as the main part and take a traditional fault crossing strategy as the auxiliary is proposed,and the correctness of the comprehensive fault crossing strategy is verified by simulation.