Development of a reversible transformer model for the calculation of inrush currents energizing from higher and lower voltage windings

Transformers are normally energized from the higher voltage winding. In power system restoration, however, they may be energized from the lower voltage winding. Therefore, a reversible transformer model that is able to accurately calculate inrush currents and some other low-frequency transients is required regardless of which winding is energized. Although a reversible transformer model based on a topology-based model that considers magnetic couplings between phases has been proposed, it is difficult to determine the parameters of the model because iron-core and winding design information are not available in most cases. In addition, the approximation of the design information required when such information is not available does not always hold accurately. In this article, a reversible transformer model that can be composed only from available information such as nameplates and test reports, without using the design information (or the approximation), by using mutually coupled magnetic resistances has been developed. As a result of verification using a 500 VA single-phase three-winding transformer, it is indicated that the inrush currents calculated by the developed model are in good agreement with those obtained by the laboratory measurements using the test transformer regardless of which winding is energized.