Transformer DC Bias Analysis And Suppression Measures

When the High voltage direct current system is in the Monopole ground return mode operation, the resulting earth current will lead to the occurrence of DC bias in the transformer. This phenomenon will affect the normal operation of the transformer, the transformer core will had a saturation phenomenon, the excitation current was shifted and distorted, the harmonic content increased, leakage loss increased, the noise increased, vibration intensified, in the same time, cause local overheating, insulation aging and other issues, and affect the transformer and the associated power equipment. Therefore, this paper studies the phenomenon of DC bias and its suppression measures.Firstly, The transformer dc bias causes and mechanism are analyzed in theory, Based on the mathematical model of the magnetic circuit coupling of the transformer, the excitation circuit and excitation current are calculated and analyzed. By studying the J-A hysteresis model of the transformer core, The J-A hysteresis model of the transformer core is studied. Based on MATLAB/Simulink simulation, Simulated single phase transformer. Under the effect of different dc voltage, the analysis is carried out for the bias of transformer, the transformer is studied under different dc bias, excitation current change rule and the distortion situation, as well as the harmonic increase and harmonic amplitude and harmonic distortion rate.Secondly, combining the basic theory of electromagnetic field with the finite element analysis method can used Ansoft Maxwell finite element analysis software to build finite element simulation model of three-phase transformer, three phase three-column type transformer and three-phase transformer five-column ransformer. The two dimensional transient solver is used to analyze the difference of the internal characteristics of the transformer core under DC bias.and study the change conditions of transformer internal electromagnetic field in the invasion of different sizes of dc current transformer winding. Through the analysis of the typical path of core magnetic induction intensity and different positions of the magnetic induction intensity amplitude, quantitative analysis and compare the ability of three kinds of typical transformer against DC magnetic bias, to determine the sensitivity of different structure of transformer DC magnetic bias. this paper introduces the method of reverse current transformer, series capacitance method, neutral small resistance method and some commonly used dc bias control method, from the perspective of economics and applicability, comparative analysis the advantages and disadvantages of several kinds of transformer dc bias suppression measures, and points out that the characteristics of the dc bias suppression system should have.Finally, modeling and simulation results show that, transformer DC bias magnetic excitation current distortion is half of the week showing spire wave, negative half cycle almost flat top wave, harmonic content increased, harmonic distortion rate increased, and the three-phase three column transformer under DC bias are the strongest, three-phase five column transformer second, groups of three phase transformer by DC bias the greatest impact.