Investigation of the Effects of DC Bias on Single-Phase Shell Type Transformers Using Frequency-Dependent Reluctance-Based Model

In this article, a reluctance-based model is developed for time-efficient analysis of shell type single-phase power transformers under the direct current (dc) bias conditions. Unlike other reluctance-based transformer models in the literature, the developed one considers frequency dependent nature of the winding resistances. This is important especially for accurate determination of losses and maximum loading capability (MLC) of the transformers under dc bias and non-sinusoidal conditions. In addition to that, by using the developed model, three different MLC determination approaches, which are based on the rated primary side current, rated total loss and rated primary winding loss, are examined for dc bias conditions. These novel examination results clearly point out that among three approaches the rated primary winding loss-based one should be preferred for the safe operation of the transformers in the systems with dc bias.