Residual ﬂux of the power transformer will accelerate the magnetization saturation of transformer core and generate high transient inrush current. Generally, the peak value of inrush current generated by residual flux in the core can reach 6-8 times of the rated current, which endangers the mechanical stability and insulation strength of the power transformer windings and destroys the normal operation of the power system. Therefore, the study of residual ﬂux in the power transformer has considerable signiﬁcance, especially when the residual ﬂux cannot be directly measured. The point-on-wave control methodology is proposed to control switching for the re-energization of a transformer. However, this method does not accurately measure the residual ﬂux, and the switch-on instant is difﬁcult to control. Another method uses the fluxgate sensors to measure the residual flux in real time, but this method is costly and has errors brought by the additional fluxgate sensors.
To overcome the shortcomings of the existing methods, this paper proposes a method for analyzing and detecting the residual ﬂux, based on the nonlinear magnetizing characteristics of the core. A model of single power transformer core is built to simulate transients in MAGNET software. By analyzing the transient current of the measured coil under applying positive and negative DC voltage to the measuring coil of the toroidal transformer, the direction of residual ﬂux can be determined and the relationship between residual ﬂux density and the magnetizing transient current can be obtained. Finally, the experimental results show that the proposed method can effectively determine the direction and accurately measure magnitude of the residual ﬂux. According to the measured magnitude and direction of the residual ﬂux, the demagnetization data of the toroidal transformer is set to effectively weaken the residual ﬂux and reduce the inrush incurrent.