Calculation of off-core inductance in dual-circuit model of transformer

Zero sequence magnetic flux, generated in transformer core in different operation cases, is forced out of the core including the oil gap and the tank. These off-core flux paths can be represented by inductances in a duality transformation based electrical transformer model. These off-core inductances mainly determine the zero sequence impedance that is vital in analyzing a transformer subjected to the GIC event or unbalanced voltages. Estimation of these inductances is one of the challenges in identification of the electric equivalent circuit. The main contribution of this paper is to present an approach for calculation of the mentioned off-core inductances based on 2D-FEM. Since the transformer structure is not symmetric for off-core flux path, 3D-FE analysis is also used to evaluate and improve the presented method. The results calculated for a 3-leg 3-phase transformer have a good agreement with the values obtained from empirical equations typically adopted by manufacturer.