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The DC Bias Risk Assessment of Transformer Based on Nonlinear Magnetic Circuit Equations

In the study of DC bias threshold of transformer, there are different opinions at home and abroad. It is of great significance to study a simplified and fast method for transformer DC bias risk assessment. Firstly, the magnetic motive force (MMF) model of single-phase transformer is established, and the concept of equivalent field current and DC bias current is put forward. Secondly, comparing current to magnetic flux based on the correlation of transformer electrical and magnetic, a nonlinear reluctance model is established and a nonlinear magnetic circuit model is built. The DC bias current is calculated by using the linear relation between the magnetic field intensity and the field current. Finally, the simulation model is built according to the magnetic circuit equation. The waveform distortion of the equivalent field current is analyzed under the DC bias state, and the nonlinear function between the bias magnetic flux and the DC bias current is discussed. It is found that the bias magnetic flux reaches saturation, and the DC bias current surges. The transformer has DC bias risk.