| Response to the global energy shortage and environment pollution problem,wind power as a kind of large-scale development new energy,has been and will continue to be gotten a wide range of practical use and research attention.First of all,a simulation model of wind farm composed of a doubly-fed asynchronous generator(DFIG)via VSC-HVDC is built on PSCAD/EMTDC.The wind power part includes the model of wind turbine,DFIG and the back-to-back converter,.The control strategy adopted can realize DFIG variable speed constant frequency control,output reactive power decoupling control and maximum wind energy tracking control;VSC-HVDC adopts bipolar symmetric topology,wind farm side converter(WFVSC)main control The voltage amplitude of the offshore wind power AC system and the grid frequency to stabilize the AC voltage at the exit of the wind farm.Second,The influence of the bipolar VSC-HVDC system on the combined system in the case of unipolar fault blocking is analyzed.By studying the Crowbar protection circuit principle of DFIG and the working characteristics of DFIG before and after the operation of the rotor Crowbar protection circuit,a coordinated fault traversal strategy of a new flexible DC transmission system and wind farm without additional installation equipment is designed.The fault traversal strategy considers both the safety constraints of the wind turbine and the DC system and the optimal power output during the wind farm fault,establishes the wind farm bus voltage and the Crowbar switch signal specification,and uses the power adjustment capability of the DFIG rotor side Crowbar circuit to realize the DC system.Fault crossing under a single-pole short-term fault.Based on the coordinated control of the Crowbar circuit of WFVSC and DFIG,the coordination function of fast pitch control and Crowbar circuit control is added.The simulation results show that the pitch control and Crowbar circuit control work together to limit the DFIG output electromagnetic power while suppressing the rapid increase of its rotational speed,thus improving the stability of the wind power system.The influence of Crowbar circuit exit time on the low voltage ride-through performance of doubly-fed induction wind turbines is studied.The overcurrent generated by the direct exit of the Crowbar circuit after the fault is cleared will have an impact on the stable operation of the system.The slope control of the WFVSC to the AC voltage is designed,so that the Crowbar circuit is delayed and cut according to the AC bus voltage signal of the wind farm.The simulation results show that the slope control of the WFVSC AC voltage can avoid the overcurrent of the DFIG after the fault ends,which improves the stability of the system. |
