| In the context of accelerating the construction of new power systems to help achieve the "double carbon" goal,the energy structure dominated by traditional fossil energy has gradually transformed to an energy structure dominated by renewable energy such as solar energy and wind energy.The power grid also shows the characteristics of high penetration and high power electronics,resulting in problems such as increased harmonic content and increased proportion of high harmonic content in the power system.When harmonics are transmitted on long-distance transmission lines,harmonic resonance amplification will occur,causing harm to lines and other power equipment,so the study of the transmission characteristics of harmonics on transmission lines is especially vital.The simulation model of Modular Multilevel Converter(MMC)inverter-tuned half-wavelength transmission system with new energy grid-connected is built according to the operating theory and control principle of the photovoltaic power generation system,the wind power generation system and the MMC inverter-tuned device,and the simulation validates the accuracy of the control principle of the photovoltaic power generation system,the wind power generation system and the MMC inverter-tuned device,and the first and last voltages of the MMC inverter-tuned half-wavelength transmission line and the waveform distributions of the voltage and current along the line are all satisfied with the half-wavelength transmission features.To address the harmonic problems generated by grid-connected PV and wind power generation,According to the bipolar SPWM algorithm used in the grid-connected inverter for PV power generation,the output harmonic voltage of the grid-connected inverter and the features of the harmonic current injected into the grid are obtained;due to the air gap inhomogeneity of the doubly-fed wind turbine and the modulation mode of the grid-side inverter,the DFIG stator side and converter are equated as harmonic current sources respectively,and then the harmonic source equivalent model of the whole wind farm is modeled.The harmonic analysis of the AC output voltage on the inverter side of the MMC inverter tuner shows that the AC output voltage on the inverter side is generally predominated by the odd harmonic component.For the transmission of harmonics on the MMC frequency-tuned half-wavelength transmission line,the harmonic equivalent π model of the transmission line based on the distribution parameters is given,and the transmission mechanism of harmonics on the transmission line is illustrated,and the equation of harmonic transmission coefficient along the transmission line is derived,and the effect of harmonic number,load and power factor on the harmonic transmission coefficient along the line is taken into account to obtain the transmission law of harmonics along the line on the tuned half-wavelength transmission line.The harmonic impedance curves of the theoretical and simulation models of tuned half-wavelength transmission lines are derived using the frequency scan method,and the differences between the two are analyzed to find out the resonance points according to the harmonic impedance curves and to verify the accuracy of the harmonic voltage and harmonic current transfer laws in tuned half-wavelength transmission lines based on the frequencies corresponding to the resonance points.Affected by the standing wave effect,the amplification of harmonic voltage and harmonic current along the line with various loads occurs when considering MMC frequency-tuned half-wavelength transmission line frequencies of 60 Hz and 75 Hz,respectively,which in turn provides a theoretical basis for transmission line design. |
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