Research On Grid-connected System Of Offshore Doubly-fed Wind Farm Based On VSC-HVDC

With the rapid development of world industry,the demand of electric energy is increasing in the global world,which brings great challenge to power supply.Offshore wind power generation,for its abundant offshore wind resources,has been paid more and more attention in the field of electric power,and the method of connecting offshore wind farm.As we well known,VSC-HVDC(Voltage Source Converter based High Voltage Direct Current)transmission is considered to be an ideal way to connect offshore wind farm due to its flexible control mode and relatively high reliability.Therefore,it is of practical engineering significance to investigate the VSC-HVDC-based offshore wind farm grid-connected systemFirstly,the composition and operation principle of the VSC-HVDC-based offshore doubly-fed wind farm grid-connected system are introduced in detail.Secondly,the mathematical model of DFIG(Double-Fed Induction Generator)and its converter are established.Meanwhile,the vector control technology is applied to relevant control strategy.Then,the mathematical model of VSC-HVDC system is established.The corresponding control strategy is analyzed and the controller of inner and outer loop control in converter are designed.Finally,the control strategy of wind farm voltage source converter and grid side voltage source converter of the VSC-HVDC-based offshore doubly-fed wind farm grid-connected system is designed.According to the proposed thesis,PSCAD/EMTDC is applied to establish the model of the VSC-HVDC-based offshore doubly-fed wind farm grid-connected system.The DFIG system,VSC-HVDC system and the corresponding grid-connected system are simulated and analyzed.The correctness of the control strategy is verified.The voltage sag caused by the fault on the power network side is simulated.The influence of this fault on the whole grid-connected system is analyzed And the method of the introduced DC chopper circuit to enhance the fault ride through ability of the VSC-HVDC-based offshore doubly-fed wind farm grid-connected system is simulated.The simulation results show that the fault ride through ability of the grid-connected system is improved.