Research And Simulation Analysis Of Power Transformer Vibration Characteristics Under DC Bias

The current status of China's energy development is characterized by a consumption and uneven regional distribution.Therefore,the goal strategy of "transmitting electricity from west to east,inter-supply from north to south," is proposed,which makes the operation of large-capacity and long-distance DC transmission lines inevitable.In the single-phase grounding operation mode,as a result,the occurrence of DC bias in the power transformer,such as the magnetic flux density changes in the positive and negative half cycles of the core,the increase of the higher harmonic content,etc.It will affect the stability and reliability of the power transformer to a certain extent,and eventually lead to the power transformer core vibration is increased,local overheating etc.At the same time,it will cause a series of problems such as the power voltage of the power grid is unstable and the wrong protection of the relay.This thesis will analyze the vibration problem caused by the core of the power transformer under the condition of DC bias.The research work is mainly as follows:Firstly,this thesis proposes two main causes of DC bias,and explains the transmission path of power transformer vibration.Based on the theoretical analysis,the action mechanism of DC bias and iron core vibration acceleration formula are deduced,which lays a theoretical foundation for the simulation analysis of power transformer electromagnetic and vibration characteristics in the later period under DC bias.Secondly,this thesis takes a S11-M-180/220 three-phase five-post power transformer as an example,and uses COMSOL finite element simulation software to study the vibration problem of the power transformer under DC bias.Based on the theoretical analysis of the electromagnetic field,the electric,magnetic and structural force fields of the power transformer are coupled by multiple physical fields to calculate.Through calculation,obtain the results of the magnetic flux density,excitation current,harmonic distribution,vibration displacement and stress distribution of the core,during normal operation and when different DC currents flow in power transformer.The simulation results show that: when the DC bias phenomenon occurs,the value of the positive and negative half cycles of the power transformer core magnetic flux is changed;the excitation current waveform is distorted;Odd and even harmonics appear at the same time and the content of higher harmonics increases;the core vibration will increase.Finally,ANSYS simulation software is used to perform modal analysis on the iron core structure of the power transformer,and the modal analysis results of the iron core are solved.Through the analysis and comparison of the three models,the material properties of the core laminated structure are determined.When DC bias occurs,the third-and sixth-order natural frequencies of the power transformer core are likely to cause resonance.After analyzing the influence of structural components on the vibration mode,the natural frequency and the mode shape of the power transformer are solved.It is concluded that when considering the influence of structural parts,the natural frequency of the model is reduced by 29%,so it is necessary to consider the influence of structural parts on the core mode of the power transformer,and it is helpful to guide the structural design of the power transformer.