Research On Control Strategy Of VSC-HVDC Transmission System For New Energy

With the continuous development of renewable energy sources and increasing electric field scale of new energy,technology of grid-connected new energy and long distance power transmission has gained lots of attention.VSC-HVDC has the advantage of flexible power control,well power quality and has no commutation failure.Furthermore,the system can provide reactive power support to the wind farm and alternating current system.Because of this,it has become the effective solution to the grid-connection of wind farms.Not only that,the VSC-HVDC technology plays an irreplaceable advantages in weak grid power supply,city expansion and the electric power market.VSC-HVDC has become an important part of China's smart grid construction.In this paper,it aims at studying the control strategy of VSC-HVDC with DFIG wind farms.It firstly introduces the basic structure and operating principle of VSC-HVDC,and establishes the model in rectangular coordinates and synchronous rotating reference frame.Then put forward the equivalent circuit of the converter and on this basis analyzes scope of application of the traditional control strategy.Secondly,it analyzes the advantage and disadvantage of traditional droop control strategy of DC voltage,and according to the methods of power margin control and secondary frequency modulation of power system,an improved droop control strategy is came up with.This control strategy can effectively refrains from overload of converter,and realize no-deviating regulation of DC votage within limits.Thirdly,A control strategy for DFIG wind farm low voltage ride through based on VSC-HVDC is proposed.Traditional Crowbar circuit will make a short circuit in RSC when the fault is checked in the system.In order to let DFIG provide reactive power support to the system,the strategy make the Crowbar circuit quit running when there is fault in the system.It calculates reactive power limit of the converter in wind farms and VSC-HVDC system.In accordance with the influence of their automatic adjustment,it puts forward the reactive power distribution about the reactive power compensation dosage.Moreover,it present a comprehensive coordinated control strategy of low voltage ride through in advance.This strategy sufficiently takes advantage of the reactive compensation ability of the system,effectively aviods the waste of reactive power and realizes adaptive control.Finally,a VSC-HVDC system with DFIG wind farms simulation model is built under the environment of Matlab/Simulink to verify the effectiveness of the proposed control strategies respectively.