Research Of DC Magnetic Bias On 500kV Auto-transformer

Being a part of the electric power tunk network, the 500kV transformer plays a significant role to the reliability of the whole system. However, in the actual running process, the 500kV transformers often appear some problems such as abnormal vibration, which is directly caused by the direct current magnetic bias.The phenomenon of direct current components of flux and exciting current arising in a power transformer is called DC bias magnetization. It is harmful to the characteristics of the transformers on line. It causes the increasing of magnetizing current, harmonics leakage magnetic flux, and thus the losses, which in turn results in the structures overheating and insulation ageing. DC bias phenomena might be resulted from the large transient fluctuations in the earth's magnetic field triggered by solar flares or the interference of DC ground currents in the region of the AC transmission system combined with DC system.The studies of this phenomenon have been paid much attention by industrialized country, however, the mechanism analysis and the analysis in quantities are still in initial stage.The Electric Engineering Department of Shanghai Jiao Tong University did a series of qualitative and quantitative researches on the influence of DC bias on a typical structural three-phase auto-transformer. Throgh the simulation analysis of theoretical model, the demonstration of DC bias on 500kV auto-transformer has been completed. This paper introduces the basic theory of DC bias, and simulates it from both the electrocircuit and the magnetic circuit. Four parts and six chapters are involved in this paper.The first part includes the first two chapters. This part introduces the definition and causation of DC bias, and the research achievements in the world wide. The theoretic baffling problems concerned have been researched. The impact of DC bias magnetization is evaluated. The running states of transformers under diffrent excitations are compared.The second part is Chapter 3. From the systemic aspect, the 500kV auto-transformer is instead of equivalent electrocircuit and magnetic circuit. The simulation model is built. The influence of DC bias on transformer is calculated by MATLAB.The third part is Chapter 4. This chapter builts a 2D model of the 500kV auto-transformer, using finite element analysis software ANSYS to simulate and analyze the transformer iron core. The simulation results proved the principle analysis of DC bias.In the fourth part, which includes Chapter 5 and Chapter 6, several restrictive measures towards DC bias have been analyzed and compared. The main achievements of the dissertation have been concluded.