Control Strategy For VSC-HVDC System Based On Offshore DIFG Wind Farms

As a great potential development of new green energy,wind power has been the focus of researched by domestic and foreign scholars since the last century.With the maturity of wind power technology,combined with the increasing scarcity of land resources and abundant offshore wind resources,development and utilization of wind power began to shift from land to sea.However,due to the presence of wind power distance,the output power of the random characteristics,wind farms select the appropriate transmission networking is very important.Flexible HVDC(VSC-HVDC)using full-controlled power electronic devices and Space Vector Pulse pulse width modulation(SVPWM)technology to achieve active power and reactive power decoupling control of independent,does not require inter-station communication,dynamic compensation AC bus voltage reactive power,stable AC bus voltage.In wind power,solar power grid,VSC-HVDC has good prospects.Doubly fed induction generator(DFIG)as the wind power generator mainstream models,the paper first analyzes the working principle and mathematical model of DFIG,according to the operating characteristics of DFIG,regulating instantaneous active and reactive power,the stator flux oriented vector control strategy of generator controller is proposed,and to verify its validity under the control strategy simulation software,the results show that the DFIG can work in the state of constant wind speed and the step changing wind speed,to provide for the realization of DC transmission grid connected guarantee;Introducing the configuration and basic principle of VSC-HVDC system,establishing mathematical model of VSC-HVDC,according to the characteristics of offshore wind power,the corresponding control strategy of two ends VSC-HVDC network system for wind farm is proposed,wind farms side converter station controls output AC voltage magnitude and phase stability,uses given AC voltage control,and overcomes the voltage fluctuations caused under the changing of wind speed effectively by the designing of the compensation amount,so that the wind farms can work in a steady state.Grid side converter station controls DC voltage stability,the inner loop current controller uses feedback linearization to make the control system turn into a linear,achieving decoupling control of dq-axis current and improving the performance of the controller.Simulation results show that the wind farms under the wind speed constant and mutation,the converter station controllers can effectively control the system stable,so that the power output of large fluctuations does not appear;when the three-phase AC mains voltage sags or short circuit fault occurs,VSC-HVDC converter stations can isolate the wind farms and the fault system,thus improving the ability to access non-fault system of wind farms;these studies will be applied to multi-terminal HVDC systems,the DC voltage deviation control strategy based on multi-point DC voltage is adoped,when one converter station fault occurs,the remaining converter station can still work normally,ensuring the effective transmission of wind energy,improving the efficiency of wind power.