Research On The Design And Simulation Of The Control Strategy Of The Converter Within The Wind Power System Based On VSC-HVDC

With the increasing of energy crisis and environment pollution problem,developing and utilizing wind energy has become the focus of discussion over the world.VSC-HVDC is a new type of HVDC topology,which has lots of advantages,such as excellent expansibility,wonderful reliability,high output voltage,low harmonic content and so on.It is recognized to be the best Grid connection scheme for offshore wind farms.However,the study on the low voltage ride through technology of offshore wind farm connected to grid system viaVSC-HVDC is still in the initial stage.In view of this,this article mainly researchs onthe control technology of the Converter for Grid-Connection within the wind power system based on VSC-HVDC,Firstly,this paper analyzes the operation principle,mathematical model and control strategy of the VSC-HVDC system,establishes the overall scheme of VSC-HVDC wind power grid connected system.Accordingto the grid-connected wind power system based on VSC-HVDC,the double closed loop control strategy based on PI controlleris designed.Then,the control scheme of the inner current loop of Converter for Grid-Connection t based on PR controller is put forword to replace the original one.Based on it,the steady state simulation model of15MWintegratingwind farm via VSC-HVDC is established in Matlab/Simulink.Thirdly,the mathematical model of VSC under unbalanced grid is analysed.In order tosuppress the negative sequence current and the DC bus voltage fluctuation,this paper proposes twocontrol schemes based on both PI controller and internal model controller.Simulation results shows that compared withPI control scheme,PR control scheme can achieve better control effect.After analysing the infulanceof asymmetrical grid faults on system operation and the fault characteristics of the integrationsystem,the System levelcontrol scheme of the grid connected DFIG wind farm based on VSC-HVDC system under unbalanced grid situations is established.With the control target of suppressing negative sequence current,the internal loop controller and DC energy releasing circuit is designed in the control system.At last the steady-state and dynamic performance of the proposedcontrollers and control scheme under unbalance operation conditions is shown by the simulation of the 15MWintegrating wind farms via VSC-HVDCin Matlab/Simulink software.The simulation results show that,the control strategy proposed in this paper can ensure the DFIG wind farmbased on VSC-HVDC can keep on working while the gridvolatage is in asymmetric situation and effectively improve the low voltage ride through ablilty of the offshore wind farm.