The Impact of Off-Plane Flux on Losses and Magnetostriction of Transformer Core Steel

Transformer cores are assembled from several packages of different widths. Furthermore, circular limbs tend to be combined with semicircular yokes. These characteristics mean that the core represents a complex 3-D system. A consequence is balancing off-plane flux (z-flux) perpendicular to the core plane. Measurements of z-induction on model cores show that mere inhomogeneity of stacking may yield significant values of flux density B_z in the normal direction. Close to overlapping regions of corners and T-joints, the order of 10 mT was observed. Losses prove to increase in a nonlinear way with increasing Bz for both alternating magnetization (AM) and rotational magnetization (RM). Effects become significant for B_z = 10 mT, with losses showing increases of order of 30%. The corresponding increases of magnetostriction are much more pronounced, reaching 100%. These increases are clearly supported by the results of Kerr effect studies of domains. They reveal distinct increases of oblique domains. For 1-D AM, they are restricted to inner lancet slopes. For 2-D RM, the latter merge to inner plate domains. For the 3-D case of additional z-flux, the formation of plates is significantly enhanced. Plates can be assumed as a main source of both rising hysteresis losses and increasing magnetostriction.