Effects of magnetic properties and geometry on flux harmonics and losses in 3-phase, 5-limb, split-limb, transformer cores

This paper presents flux harmonics, localised losses and building factors computed in 3-phase transformer cores of different geometries modelled with the B-H characteristics of conventional and high permeability silicon iron. Split and unsplit limbs were considered. Localised harmonics were highest in split cores and cores modelled from high permeability steel, isotropic material produced the lowest flux distortion. The change in flux distribution in split cores caused a large increase in localised harmonics and a consequent loss increase. The building factor was highest in joints because of flux distortion and a marked difference in the variation of overall building factor with flux density was noted between the two materials. Increasing core mass (for the same rating) did not necessarily reduce the localised harmonics and it is shown that a larger core could have a poorer building factor.