| Energy is the material basis of human activities,but as the global economy continues to develop,the problem of the energy crisis becomes more serious.Traditional energy resources are limited in storage and will cause a lot of pollution,which prompts human beings to continuously develop new energy sources.wind energy has abundant reserves in the world.In recent years,wind power industry in China has developed vigorously,but at the same time,the grid has put forward strict requirements for grid-connected operation of wind power.Because flexible HVDC transmission has its own technical advantages,it can isolate the impact of wind farm faults to a certain extent,so it is considered as the ideal grid-connected operation mode of wind farms.The paper first analyzes the system structure and operation principle of VSCHVDC,and designs the corresponding internal and external loop control strategy according to the VSC-HVDC control system structure.Because PI control system is built in two-phase rotating coordinate system,it will produce cross-coupling current and need feed-forward decoupling,which makes the whole control system more complex.Therefore,this paper deduces the mathematical model of VSC-HVDC in two-phase stationary coordinate system,and designs the inner current loop with full-order sliding mode controller.The introduction of sliding mode controller eliminates the decoupling link,which greatly reduces the computational complexity of the algorithm and makes the dynamic characteristics of the system better than the traditional PI control.Secondly,two kinds of faults occurring on the AC side of the receiving converter station are studied: three-phase grounding short-circuit faults and single-phase grounding short-circuit faults.When three-phase grounding short-circuit faults occur in the system,as long as the problem of low voltage crossing is solved,the system can still adopt traditional control.However,when single-phase grounding short-circuit faults occur in the system,because of the influence of negative sequence voltage and negative sequence current,if the traditional control is still adopted,the power of the system will fluctuate twice the frequency,while the traditional phase-locked loop will also be affected.In view of the requirement of power grid that wind turbines operating in interconnection must have certain Fault-Crossing ability,this paper focuses on the problem of unbalanced active power transmission between two terminals caused by voltage sag fault at the receiving converter station,resulting in the increase of DC bus voltage.A hybrid circuit system based on DC unloading circuit and energy storage system is adopted at the DC-side capacitor of VSC-HVDC system,and a coordinated control strategy is adopted for the wind farm grid-connected system and hybrid circuit.Aiming at the problem of negative-sequence components,by analyzing the mathematical model of converter under unbalanced grid voltage,this paper studies the real-time detection methods of positive-sequence components and negative-sequence components,and proposes a phase-locked loop-free sine-cosine signal generator based on T/4 periodic delay method.In order to solve the problem of power pulsation twice when single-phase short-circuit fault occurs in the receiving power grid,a compensation strategy based on sliding mode control is proposed,which takes the second-harmonic power of the system as the state variable under unbalanced grid voltage.Different control results are achieved by setting different control objectives and calculating different power compensation values. |
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