Research On Harmonic Transfer Characteristics Of Long-distance Transmission Lines For Large-scale Renewable Energy Consumptio

Half-wavelength AC transmission technology has many unique advantages and operating characteristics,which has great application prospect and value in the construction of global energy Internet and long-distance large-scale new energy delivery.It can be used as a major power transmission channel to transport the rich natural resources in northwest China to the load center in the central and eastern part of China in the form of electric energy,it can greatly increase the consumption capacity of large-scale new energy,and solve the problem of uneven distribution of energy and load demand in China.New energy sources such as photovoltaics and wind power will inevitably become harmonic sources in the process of power generation and grid connection,injecting a large amount of harmonics into the system.When harmonics are transmitted on half-wavelength lines,harmonic amplification may occur under specific circumstances,resulting in system voltage distortion and may even destroy the operating characteristics of half-wavelength transmission lines.In order to solve this problem,this paper studies the harmonic transmission characteristics of halfwavelength transmission lines and analyzes the transmission characteristics of harmonics in half-wavelength AC transmission lines.it provides theoretical and technical support for the planning and construction of the later half-wavelength AC transmission lines and the harmonic suppression of new energy through halfwavelength AC transmission lines,so as to ensure the long-distance and large-capacity safe delivery of wind power,photovoltaic and other new energy.Firstly,the topology of wind power,PV and other new energy sources fed through half-wavelength transmission lines is determined,then the harmonic model of doublyfed induction generator,the harmonic model of wind power grid-connected system converter,and the harmonic model of PV inverter are established,and the harmonic characteristics of wind power and PV grid-connected power generation systems are analyzed according to the established harmonic source models.Secondly,aiming at the harmonic problem injected by wind power and photovoltaic grid connected power generation systems,the harmonic transmission characteristics of standard half wavelength transmission lines are analyzed.Starting from the model of transmission lines,it is clear that a distributed parameter model that takes into account the frequency variation characteristics of transmission lines is required when analyzing harmonic transmission characteristics.Based on the transmission line equation and the harmonic equivalent model of transmission lines,the mathematical model of harmonic transmission on half wavelength transmission lines is theoretically derived;Taking the common harmonics generated by wind power and photovoltaic grid-connected power generation systems as the harmonic source of half wavelength transmission lines,this paper analyzes the integer harmonic transmission characteristics,fractional harmonic transmission characteristics,and harmonic transmission characteristics along the line of half wavelength transmission lines under different operating conditions.Then,considering that there are few engineering lines whose transmission distance is equal to half-wavelength in real transmission lines,the electrical distance of transmission lines is usually compensated to half-wavelength by adding tuning circuit.The structures of three common passive tuning circuits are introduced,the transmission parameter matrices of the three tuning circuits are obtained,and the formulas for calculating the parameters of the relevant components of the tuning circuit are given.as a result,π-type and T-type tuned half-wavelength transmission lines with different tuning distances are constructed.Taking the common harmonics generated by wind power and photovoltaic grid-connected power generation systems as the harmonic sources of half-wavelength transmission lines,the harmonic transfer coefficients and voltage total harmonic distortion(THD)of π-type and T-type tuning circuits transmitting different power under three different tuning distances are analyzed and calculated.finally,the similarities and differences of harmonic transfer characteristics of the two tuning circuits are compared.