Packet-to-Packet Variation of Flux Density in a Three-Phase, Three-Limb Power Transformer Core

The results of an experimental investigation and calculations of the influence of bolt holes and slit edges on the packet to packet variation of flux density in a three-phase, three-limb power transformer core, tested under no load are presented. The overall peak flux density was calculated from the induced voltage in the secondary winding of the core whereas the flux densities in individual packets were calculated from the measured voltage induced across the top and bottom laminations of each packet using a flexible needle probe technique. Three single-phase cores were assembled from the individual packets of the three-phase core. Their effective permeabilities were measured and also calculated from the lamination dimensions and the B-H characteristics of the core material. The experiments and the calculations indicate that the variation in flux density in the three-phase, three-limb core can be attributed to the presence of bolt holes and stress near slit edges of laminations which reduce the effective permeability of the narrow packet considerably.