Effect of DC biasing in 3-legged 3-phase transformers taking detailed model of off-core path into account

•This paper investigates DC biasing impacts on power transformers.•A new dual circuit model is presented for the off-core flux path including the tank walls.•Parameters of the off-core model are calculated using 3D-FEM.•The tank walls are modeled by two equivalent circuits based on finite difference and surface impedance methods.

Modeling of 3-legged, 3-phase transformers under DC biasing is a challenging topic due to the need to an adequate representation of flux paths beyond the active part including the tank. The main contribution of this paper is to present a new dual circuit model which takes the off-core flux path into consideration. Tank elements including the walls, bottom and cover are considered with a detailed as well as a steady state equivalent circuit. Having a new model, the behavior of a test transformer is studied under DC excitation in terms of magnitude and harmonic content of exciting current as well as reactive and active power flowing through the transformer.