| Along with the development of the economy,the society's demand for energy is increasing day by day.Along with the increasingly serious environmental problems,renewable energy has attracted more attention of people,the demand and requirements for renewable energy are also rising.Wind energy is a member of renewable energy.It has great potential.Converting wind energy into electricity is the main form of wind energy utilization.Most of the wind energy resources are far from the load center.The direct current system is suitable for the large capacity long distance transmission.The direct current system is also suitable for the offshore wind power which needs to be connected to the grid with the cable.In this paper,the problems in the wind farm DC system integration are discussed.The study is based on the wind generator,DC transmission system,wind farm collector system voltage control,wind power DC system fault response,fault ride through control methods.The general contents of this article are as follows:(1)The structure,function and control of the current wind turbines are studied.Doubly fed induction generator(DFIG)and direct driven permanent magnet synchronous generator(DD-PMSG)are included.Based on the structure of the wind turbine,the mathematical model of each part is analyzed.The mathematical model and its control under normal and fault conditions are further analyzed.Its mathematical model is deduced in detail.Methods of control on different occasions are designed.Wind turbines' operating principle and corresponding characteristics are explained.Detailed simulation models of DFIG and DD-PMSG are built on the PSCAD/EMTDC platform.Through theoretical analysis and model simulation,the structure and function of wind turbines are understood.And the control and protection of wind turbines are mastered.(2)Wind farm HVDC integration system based on two-level voltage sourced converter based high voltage direct current transmission(VSC-HVDC)is studied.Aiming at the voltage establishment of isolated islanding and offshore wind farm collector system,the AC voltage control method of the two-level VSC wind farm side converter station for the wind farm collector system is proposed.The AC voltage of wind farm collector system is established effectively.Aiming at the transient response of wind farm two-level VSC-HVDC grid integration system,the transient characteristics are analyzed in a series of common scenes,such as wind speed change,wind farm collector system fault and AC system failure of power grid.A new interactive coordinated control is proposed according to the fault condition of power grid.It is helpful to improve the transient characteristics and fault ride through capability of the system.(3)Wind farm HVDC integration system based on modular multilevel converter based high voltage direct current transmission(MMC-HVDC)is studied.Aiming at the voltage establishment of isolated islanding and offshore wind farm collector system,the AC voltage control method of the MMC-HVDC wind farm side converter station for the wind farm collector system is proposed.MMC voltage direct control method is adopted.It can be used in wind power generation.Simulation results show that the proposed AC voltage control method can effectively provide wind turbines with stable AC voltage.Aiming at the transient response of wind farm two-level VSC-HVDC grid integration system,the transient characteristics are analyzed in a series of common scenes,such as wind speed change,wind farm collector system fault,AC system failure of power grid and DC unipolar grounding fault.The system also has certain fault ride through capability.(4)Wind farm HVDC integration system based on MMC using clamping double sub-module based high voltage direct current(C-MMC-HVDC)is studied.In view of this new type of wind farm grid connected mode,the corresponding control of C-MMC used in wind power DC grid connection is proposed according to the characteristics of the C-MMC topology.Voltage control of collector system in wind farm side converter station(WFC-MMC)and DC voltage control in power grid side converter station(GSC-MMC)are included.Dealing with DC fault self clear capability in C-MMC,the mechanism of C-MMC atresia is further analyzed.The method and essence of fault ride through are explained.Aiming at DC fault of wind farm direct current grid connected system,this paper proposes to realize the DC fault ride through of the wind farm direct current grid connected system through the blocking,restart and Crowbar technology of C-MMC converter station with a series of key switches on and off.Details are given on how to coordinate the C-MMC converter station,the Crowbar technology and the key position switch during the period of DC fault and the recovery period after the failure.The transient stability of wind power direct current grid connected system is effectively improved.Wind power DC integration system is protected.Also the stability of the power system is guaranteed. |
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