Research Of Continuous Fault Ride Through Control Of DFIG-based Wind Turbine During Commutation Failure Fault In HVDC Transmission System

The line commutated converter based high voltage direct current(LCC-HVDC)transmission system has many advantages,such as large transmission capacity and long transmission distance,good economy,fast and controllable power flow,and advantage of asynchronous interconnection of power grid,etc.Consequently,the LCC-HVDC transmission system has been used widely in large-scale wind power transmission system.However,the LCC-HVDC transmission system brings some new challenges to the safe and stable operation of the sending alternate current(AC)grid.The commutation failure fault is the most frequent fault events in the LCC-HVDC transmission system.When the commutation failure fault occurs,the voltage of the sending AC grid changes continuously and presents the “first reduce then rise”characteristic.Therefore,the grid-connected doubly fed inductance generator(DFIG)-based wind turbine may be tripped by cascading low voltage and high voltage fault,which will threaten the safe and stable operation of the sending AC grid.In order to improve the safety and stability of the sending AC grid during commutation failure fault,this paper studies the continuous fault ride through(FRT)control strategy of the DFIG-based wind turbine for improving the FRT capability of the DFIG when stator voltage changes continuously and realizing the commutation failure FRT of DFIG.The main research contents in this paper are presented as follows,First,the model of wind power HVDC transmission system is built.Based on the model,the voltage features of the sending AC grid during commutation failure fault are analyzed.Meanwhile,the mechanism of voltage fluctuation of the sending AC grid during commutation failure is given and the influence of grid strength on the commutation failure voltage is also revealed.Then,the commutation failure voltage model which presents the voltage features of the sending AC grid during commutation failure is proposed.The influence of commutation failure on the grid-connected DFIG-based wind turbine is also analyzed.Furthermore,the FRT performance of the existing FRT control strategies of DFIG are compared.Finally,considering the dynamic features of stator voltage during commutation failure and the reactive power capability of DFIG,the continuous FRT control strategy of DFIG-based wind turbine is proposed.Moreover,the simulation and experimental platforms are built in this paper.Meanwhile,the proposed continuous FRT control strategy is validated based on the simulation and the experimental platforms.The results show that the proposed continuous FRT control strategy can improve the FRT capability as well as the FRT performance of the DFIG-based wind turbine when the stator voltage changes continuously.The work of this dissertation will provide the theoretical basis for improving the FRT performance of DFIG-based wind turbine when the stator voltage changes continuously.Meanwhile,the safety and stability of the sending-side wind farm and power grid of the LCC-HVDC transmission system will also be enhanced based on the research of this dissertation.