The present paper deals with a new approach for the study of the steady - state and transient behaviour of three-phase transformers. This approach based on magnetic equivalent circuit diagrams, takes into account the nonlinear B-H curve as well as zero-sequence flux. The nonlinear B-H curve is represented by a polynomial representation, based on a set of measurement data. For the numerical simulations, a method considering the currents as state variables has been developed, and extended to ten vector groups. Numerical results compared with test results and with FEM calculations confirm the validity of the present approach.