Alternative Techniques in Simulating Transformer No-Load Characteristics and Inrush Currents by Circuit Models

The paper is devoted to the modeling of a three-phase, three-legged transformer under no-load and inrush current conditions. To simplify the study, transformer simulations are carried out in the ATPDraw environment, which now incorporates a dynamic hysteresis model (DHM). The main distinction of the DHM-based transformer models proposed from existing analogues lies in their ability to replicate transformer behavior over an extended range of excitation voltages. In addition to the use of the DHM, this improvement is achieved by reproducing a variable nature of equivalent air gaps at core joints. Two model versions with variable core gaps differ in whether they account for or disregard magnetic anisotropy of the core steel. Both these experiment-oriented models were fitted accurately to the same measured data by using individual flux–current characteristics of their gaps. In such a way, we have demonstrated the difficulties in distinguishing between effects caused by the core gaps and magnetic anisotropy in practical transformer modeling. By extending the method of variable core gaps to inrush currents modeling, residual fluxes in the core and current peaks were replicated accurately. The results simulated with the topological models presented are in a good agreement with measurements performed on a 300 kVA Yyn 11.350/0.235 kV transformer.