Derating of distribution transformers under non‐linear loads using a combined analytical‐finite elements approach

Supplying non‐linear loads causes increased losses in transformers which eventually leads to their reduced life spans. Therefore, transformers are derated in order to protect them against premature loss of life. To do this, load losses including ohmic loss and winding eddy current (WEC) loss need to be estimated. This study suggests an improved analytical approach using finite element method (FEM) which includes all material characteristics and geometrical structures in order to calculate WEC loss under non‐sinusoidal load current in each winding individually. By adopting this procedure, harmonic loss factor as a dominant parameter in transformers derating is estimated. By emphasising the region in which the maximum WEC loss occurs, an additional factor ( F MECL ) based on the physical phenomena involved is introduced using FEM. On the basis of the aforementioned concepts, a new relationship is presented for transformer derating. Ultimately, the suggested method and the IEEE standard are applied to derate a distribution transformer and the results are discussed. In addition, the proposed method is validated using a thermal model and its advantages over the IEEE derating method is presented.