Analysis Of Harmonic Impact On Half-wavelength Transmission System With New Energy Sources

Half-wavelength AC transmission technology is a kind of UHV(Ultra-High Voltage)transmission mode with good performance and prominent economic advantages.It can overcome the problem that the load center is too far away from the energy storage center in the world.Considering the concept of the Global Energy Internet and application of half-wavelength AC transmission technology,large-scale new energy sources are likely to be connected to the half-wavelength transmission lines,which would cause some serious harmonic problems.However,there are few studies on them.In view of this situation,this paper investigate the resonance of harmonics in grid-connected system and their transmission characteristics through half-wavelength transmission lines when new energy sources are connected,in order to provide technical basis for the practical application of half-wavelength transmission technology.The details are as follows:In chapter 2,the basic principles of analytical and frequency scanning methods were shown.Meanwhile,the mathematical principles of the modal analysis method was derived and its two measurement indicators were introduced.Then,through doing resonance analysis of one example circuit with these three methods,this paper verified the validity of the resonance analysis program.In addition,the technical advantages of modal analysis method was also confirmed.This chapter provided theoretical supports for the subsequent resonance analysis of grid-connected system.In chapter 3,the grid-connected resonance problem of half-wavelength transmission system with hybrid access of wind farms and photovoltaic plants was studied.First,the structure and equivalent circuit of the grid-connected system were constructed.Following this,the resonance analysis models of the main components in the grid-connected system was established.Second,the modal analysis was done to analyze the resonance of the system under standard conditions.The node participation factors and element sensitivity factors were evaluated for the typical resonance modes.At last,the influences of the component parameters such as half-wavelength transmission line length,collection cable length,output line length and numbers of inverters on the typical resonance mode were analyzed.The results show that the length of half-wavelength transmission line and the length of output line have influences on the low-frequency harmonic resonance of some modes.But the length of collection cable will imapct on all the harmonic resonance of typical modes.As for the number of wind power and photovoltaic inverters,it only influence the harmonic resonance of some modes,without changing the resonance centers.In chapter 4,the frequency dependent model was given and its validity was verified.Based on phasor diagram and simulation analysis,the transmission characteristics of integer harmonics and fractional harmonics on a standard half-wavelength transmission line were studied.Considering that the length of the transmission line is often less than the half-wavelength,it is necessary to install tuning networks.Here,three tuning networks and the corresponding formulas calculating their parameters were given.Then,the transmission characteristics of typical harmonics through half-wavelength transmission line with these tuning networks were analyzed.The results of the chapter are as follows.The integer harmonics do not be amplified after passing through standard half-wavelength transmission line,but some fractional harmonics can be increased through the line.There is no harmonic amplification phenomenon for typical harmonics that pass through the line with T-type or capacitive tunning network.However,they can magnify after propagating through the line with ?-type network,thus the power quality will be endangered.The research results can enrich the theory of half-wavelength transmission technology and provide technical support for the practical application.