Research On Electromagnetic-mechanical Properties And Time-frequency Characteristics Analysis Of Vibration Signals For DC Biased Transformers

With the construction and development of high voltage direct current(HVDC)system in our country,it has led to more and more DC bias phenomenon of transformer.The current of HVDC grounding flows into transformer neutral point in the AC system by the earth.This will power up the saturation of magnetic core in transformer,abnormal change of exciting current.It can not only produce harmonic wave in grid but also consume too much of core and other components in it and increase the temperature of transformer.Meanwhile,it may intensify the vibration of transformer.When the problem gets more serious,the DC bias can cause transformer structure loosen and mechanical fatigue strength.So it is important to take effective action to research transformer DC bias for the sake of the power system security.For preferably analyzing the bias of transformer working status,preventing the accident caused by bias problem and providing support and reference for the reliable operation of power grid,this thesis researches the influence of electromagnetism and mechanical properties of transformer around DC bias,from the angle of vibration propagation process.Firstly,the vibration equivalent calculation method of transformer structures is studied and the numerical model of electromagnetic-mechanical coupling fields for transformer is carried out for researching the distributions of stress and strain.Secondly,the vibration frequency characteristics of the transformer parts with DC bias are analyzed.Combined with the change rule of mechanical performance of core material under the different electromagnetic conditions,the feature extraction methods are presented for describing bias vibration and saturated state.This thesis involves mathematical deduction,measurement of material parameters,electromagneticmechanical field simulation,vibration experiment design and signal time-frequency analysis,etc.The main contents of this thesis are as follows:(1)The transformer vibration mechanism and process with core and winding as the vibration source are investigated.Aiming at the characteristics of orientation silicon steel sheet forced by magnetostrictive force and considering the non-uniform unsteady magnetic flux changes in the core,the iron column in axial direction,yoke in horizontal and vertical directions are equivalent to elastic rods respectively.Then the stress analysis and the vibration equation is derived.Meanwhile,the windings are equivalent to multifreedom degree vibration system for expressing the vibration mathematical process.According to basic principles of vibration,all structures in the transformer are equivalent to a whole spring damping element so as to describe the transformer vibration process effectively.(2)The numerical model of electromagnetic-mechanical coupling fields for transformer is proposed,including the governing equations,boundary conditions and efficient iteration solution method.Silicon steel sheet strain test experiment platform is developed and magnetostricion is measured under different magnetic induction intensity.The single value curve of measurement is set as the calculation condition of silicon steel for transformer calculation.According to the computed results,the mechanical deformation of transformer core,winding and shell surface are analyzed selectively and the vibration asymmetry phenomenon on transformer is confirmed.(3)The transformer vibration test system is developed in this thesis,the widely used piezoelectric vibration acceleration sensor as the component for transferring mechanical signal into electrical signal.This system can be used for multi-channel signal acquisition,data storage,frequency spectrum analysis,etc.A custom testing transformer is adopted as the research object and the vibration acceleration signal on transformer under DC bias are acquired for analyzing the relation between vibration and DC bias.(4)The vibration time-frequency characteristics and feature extraction method are researched,in order to reflect the bias state of transformer.Using the HHT analysis method to obtain time-frequency laws of displacement signal.On this basis,the fundamental mode energy rate is presented for describing the bias state.On the other hand,based on the nonlinear hysteresis relation of core material in the process of saturation magnetization,the other characteristic value is present.Through practical calculation and analysis,it is a complete way that the two characteristic values are used together for confirming the DC bias state.In addition,according to the odd harmonic growth process and the influence of signal composition during core magnetization,the bias magnetic saturation law is studied.The feature extraction method and the saturated state analysis method in this thesis are mainly based on the composition of displacement signal,not affected by the transformer vibration intensity.Thus,the universal applicability for different types of transformer has been achieved.(5)The fact test for main transformer in substation is built for analyzing the bias vibration.This test is considered in adjustment period of “jin-zhong HVDC system”.The 220 kV transformer distance 16 km from DC grounding electrode as the test object,the vibration on each side of transformer are measured under different DC earth current.And the time-frequency characteristics and bias characteristic values are computed and the results verified the accuracy of the method mentioned above.In terms of the feature of vibration intensity bigger on large capacity transformer,the noise produced by bias vibration is discussed.The noise change around the main transformer under DC bias is measured and analyzed at the end of this thesis.It provides a reference for safe operation of power grid.