Research On DC Filter Algorithms Of Second Harmonic Suppression

As one of the most important secondary energy sources during the production and life of human beings,electric energy can not be separated from urban daily life or industrial and agricultural production.However,the distribution of energy in our country is relatively concentrated,which compared with regional development of load is extremely unreasonable.Construction of long distance and large capacity HVDC transmission calls for no delay.The converter device of HVDC transmission is a typical nonlinear electronic device,which will generate all kinds of harmonics in the AC and DC side during power transmission including characteristic harmonic and non-characteristic harmonics(especially for the second harmonic).That will not only pollute the power quality of power system,but also impact the normal operation of converter transformer and AC system.In order to suppress DC side harmonic in HVDC transmission system effectively,firstly,the formation mechanism,cause and harm of the DC side harmonics in converter are studied and described.The transfer process of DC side harmonic is described.The measures to eliminate the harmonics in electric power system and the configuration of DC filters in HVDC transmission system are studied,especially for the second harmonic control.Secondly,taking an HVDC transmission project as a typical example,HVDC transmission system simulation model is established by using Matlab.When three-phase voltage is balanced and unbalanced,simulation analysis on characteristics and laws of DC side harmonics is carried out before and after filtering,which can be used to evaluate the performance of original DC filter configuration.Thirdly,aimed on DC side harmonic characteristics,the actual parameters of the Long-Zheng HVDC transmission project are used to determine the design parameters and structure of the Cauer filter.First,attenuation characteristics of the Cauer filter,four kinds of ladder network structures and primary parameters are researched.Also,the design steps of the Cauer filter are detailedly listed and calculation method of component parameters is derived.Optimum structure is made for the designed Cauer filter.The performance of the filter is analyzed from three aspects,which can verify the design scheme and the superiority of filtering effect.Also,its design method is universal.Finally,the sensitivity of Cauer filter is calculated and analyzed.Considering the deviation of filter component parameters in actual working condition,simulation is used to compare the filtering effect by slightly change the filter element parameters.Thus,the high stability of optimized structure Cauer filter designed in this paper is verified.The filtering effect will not be affected by the deviation of the component parameters within a certain range.In this thesis,the DC side harmonic analysis of HVDC transmission system is presented on the nonideal condition,which can provide better direction for DC filter design.It can improve the DC side harmonic pollution and optimize the operation environment of HVDC transmission system by adjusting the traditional filtering scheme.