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 Fourier series, based on a set of measurement data. For the numerical simulations, two methods have been developed, by considering the total magnetic flux respectively the currents as state variables. Numerical results compared with test results and with FEM computations confirm tvalidity of the proposed approach.