Frequency Adaptive Modeling Method of Transformer Inrush Current Based on the Dynamic Phasor

Abstract

The numerical simulation of inrush current for saturable transformer requires small time steps to avoid numerical overshoot issue, which is inefficient. However, the existing large time-step simulation methods, e.g., frequency adaptive simulation of transients (FAST) cannot yet accurately model it, due to the wide band frequency range. This paper extends the existing theory of FAST, and proposes a multi-timescale transient branch companion model of electrical elements with strong nonlinearity, which is suitable for both small and large timescale electromagnetic transient simulation. Firstly, the dynamic phasor (DP) domain companion model of nonlinear inductor is derived to simulate the flux flowing through the transformer core. A shifting frequency parameter of the DP-based model is then defined to track natural (EMT) or envelope (with large timescale) flux waveform. Then, the relationship between the inrush current and flux is defined by curve fitting method. Finally, integrative simulation of both natural and envelope waveforms of inrush current can be tracked through the curve fitting method. To validate the algorithm, a saturable transformer model is established in this paper and the proposed method is validated in MATLAB.