Research On The Influence Of DC Bias On The Excitation Characteristics And Vibration Of Transformer

In order to meet the needs of larger capacity and more long-range electricity,our country has fully invested construction in UHVDC transmission project during the "Thirteen Five" period.But with its comprehensive promotion,it will bring a series of engineering problems.For example,unipolar operation of the ground DC current will invade the transformer,making the magnetic flux density reached to saturation.Transformer will appear excitation current distortion,loss increasing,partial overheating and enhancing vibration.In this paper,three mechanisms for generating DC components are introduced.Based on the unipolar operation of HVDC transmission lines,The author studied the transformer working mechanism under the DC bias and the effect on the excitation current of the three-phase transformer.The equivalent circuit equation of the transformer is combined with the static Maxwell's equation to obtain the expression of the excitation current.In order to compare the distortion of the excitation current,using PSCAD software to analyze the excitation current of three-phase transformer under three different operating conditions.The influence of different DC flux on the excitation current is mainly studied.The peak-to-peak value of the excitation current and the change of the amplitude of each frequency with the increase of the direct flow rate are analyzed.Then,based on different magnetic fields produced by solenoid,this device would cause the magnetostriction of the silicon steel sheet within it.Afterwards,the measurement of the deformation of silicon steel sheet is conducted through the strain resistance sensor attached on silicon steel sheet.The strain resistance was used to measure the strain property of silicon steel sheet through full bridge circuit and the magnetostriction curve of silicon steel sheet gained.The author studied the law change of magnetostrictive ratio with the increase of magnetic field.After that,the author analyzes the theory of the strain characteristics of the abnormal vibration of the core caused by the neutral component of the DC component.The relationship between the strain frequency distribution and the magnetization of the ferromagnetic material under normal working and DC bias is analyzed by using the laws of thermodynamics and Gibbs free energy laws.The strain test points are fitted into a smooth curve,and then the relationship between the DC flux density and the magnetostrictive ratio of the silicon steel sheet is analyzed by the function iterative algorithm.At last,the author designed a set of vibration test system based on LabVIEW.The test system includes hardware structure,the human-machine interface based on LabVIEW and vibration data test system platform.The vibration signal of the transformer is extracted and displayed by this system,studying the spectral and harmonic ratios of vibration signals at different positions on transformers.The wavelet signal is used to analyze the vibration signal of the transformer and used to decompose the vibration signal on the 6th floor.The result is that the frequency band-energy characteristic distribution of the scale part and the wavelet part.In order to determine the extent of influence by the DC component,the author compare the energy percentage of the core,the upper part of the box and the top of the box in each frequency band on the change rule.