Study On Magnetization Mechanism And Modeling Methodology Of Power Transformers Under DC Bias

Transformers with neutral grounding will partially be saturated when under single-electrode ground operation of HVDC or Geomagnetically Induced Currents. Then vibration of transformers, harmonic of systems and reactive power will increase significantly. The magnetized mechanism of transformer core and the method of mathematic modeling is important basis of quantitative analysis of dc bias effect.The prevailing J-A theory is extensively used in the arena of magnetic modeling, however, the coexistence of multiple versions with different mathematical expressions often creates difficulty in discriminating the accuracy and effectiveness of the analyzed results by J-A theory. Based on the magnetization mechanism of ferro-magnets, in terms of the fitting formula of the an-hysteretic magnetization curve and the energy conservation equation to be adopted, several queries were raised to the original J-A theory for describing the hysteresis phenomenon, and the irrationality and inaccuracy were thereby elucidated. A correct energy conservation equation was given through physical analysis, and modified mathematical expressions were further deducted to correct the original J-A theory. On acquisition of the model parameters through nonlinear optimization algorithm, simulation studies were carried out regarding various material specimens, and the results of which were compared with the measured ones. The research indicates that, the results given by the proposed modified J-A model coincide with the measured data much better than that by the original J-A model, which therefore presents a more accurate simulation of the ferro-magnets' magnetization process. Then, in consideration of the eddy current loss and additional loss, the energy equation is developed, the modified J-A theory can describe the dynamic magnetized characteristics of magnetic material. A auto-transformer model for DC bias analysis is built based on modified J-A theory in this paper.With regard to the undergoing±660kV HVDC transmission line from Yinchuan to Qingdao, related factors with DC bias impact on the power transformers, quantitative excitation current and quantitative harmonic analysis the main transformer of Kuangzheng substation,30km away from the Qingdao converter station, are carried out based on an established nonlinear system model. The results indicate that, both the odd and even harmonic components are getting much higher under DC bias than that without DC bias, which thereby makes the power transformers to severe harmonic sources.In the end, existing restraining measures and their characteristics are analyzed. Based on the quantitative relationship between transformer output voltage harmonic and neutral DC current, anew method of DC current measurement is proposed, which presents further potential for developing new restraining measures.