Research On The Harmonic Impedance And Harmonic Interaction In Multi-HVDC Transmission System

With the rapid construction of HVDC system,the number of HVDC system is increasing in our country.When multiple HVDC converter stations are located in adjacent AC grids,they constitute a multi-HVDC system.Due to the close electrical distance of each converter station,there exists the harmonic interaction.In this paper,the harmonic impedance characteristics and harmonic interaction phenomenon of multi-HVDC system are studied.Taking the multi-HVDC system composed of Ning-Dong HVDC and Ning-Zhe HVDC as an example,this paper mainly studied the harmonic impedance of the converter,the influence of each harmonic on each converter station and the effect of AC filters combination switching on harmonic interaction.Firstly,based on the switching function model of thyristor converter,the modulation low of harmonics on the AC and DC sides of the converter is analyzed,and the harmonic analysis method of the converter with frequency and sequence network separation is proposed,and the harmonic impedance calculation model of the AC side and DC side of the converter are further derived.Taking the Ning-Dong HVDC system as an example,build the simulation model and verify the calculation results.The results show that the harmonic impedance calculation model can accurately reflect the equivalent harmonic impedance of the AC and DC sides of the converter in wide frequency band.The concepts of harmonic self-impedance and transfer harmonic impedance are introduced to analyze the influence of each harmonic on each converter station in the multi-HVDC system,especially on the harmonic voltage distortion.Based on the harmonic impedance of each component,the harmonic impedance model of Ning-Dong,Ning-Zhe multi-HVDC system is established.The effects of the harmonics in Yinchuan East Station and Lingzhou Station on the harmonic voltage distortion of each converter stations are analyzed and verified.Finally,the concept of harmonic current influence coefficient is defined to study the influence of the combination switching of AC filters on harmonic current interaction in multi-HVDC system,especially the characteristic harmonics.Combined with the harmonic impedance model of Ning-Dong,Ning-Zhe multi-HVDC system and the impedance characters and combined switching mode of AC filters.The interaction effects of the 11st,13rd,23rd,25th characteristic harmonic currents under the combination switching mode of the AC filters are analyzed.Several typical system operation modes are selected,and the simulation result shows the specific AC filter combination switching methods may cause the harmonic current interaction.