Quasi-static electric field computation of converter transformer by adaptive time step FEM

An adaptive time-step algorithm has been proposed in this paper based on each step´s temporal local truncation error of C-N (Crank-Nicholson) method. Utilizing the characteristics of polarity reversal voltage, the oscillation of time step at the discontinuity of the derivative of polarity reversal voltage has been avoided successfully by the combination of backward Euler method. The algorithm is verified by a simple model with analytical solution and compared with the traditional fixed time step C-N method. The results show the proposed algorithm needs much less computational costs and achieves much more accuracy. At the end, the proposed algorithm is used to calculate a practical convert transformer under polarity reversal test voltage.