Residual Flux Measurement of Power Transformer Based on Transient Current Difference

Power transformer is the main part of the power grid. After some tests and operations of transformers, a certain residual flux density (B-r) is left in the iron core due to the ferromagnetic material hysteresis phenomenon. When the transformer is switched on with no load, B-r will lead to inrush current. Therefore, research on measuring B-r is of great significance for transformer protection. In this article, a B-r measurement method based on applied dc voltage is proposed. First, combined with the magnetic circuit and transient circuit analysis, the relationship between B-r and the extremum value of the transient current difference is obtained. Then, the finite element method (FEM) is used to calculate the transient current under different B-r and test circuit parameters; the suitable test circuit parameters will be determined, and then the empirical formula for calculating B-r is obtained. Finally, the experimental results verify the correctness of the simulation analysis.