The spatial variation of localized power loss in two practical transformer T-joints

The power loss and flux distribution in two commercial T-joints have been compared. The simple 45°-90° T-joint and the slightly more complex 45° offset T-joint were chosen for the investigation, as they are the most common joints currently used in power transformers. The localized and overall power loss of each joint was measured using now-established techniques. Each joint was built in turn into a three-limb, three-phase core, employing 15-cm wide, 0.03-cm-thick 3.2% grain-oriented silicon iron laminations. The cores were magnetized at the same overall flux densities. The localized flux distribution in individual laminations in the joints were related to the localized loss variation. Rotational flux was found to occur in both joints and it caused areas of high localized loss. A combination of normal flux between adjacent layers of laminations, rotating flux, and circulating 3rd-harmonic fluxes caused the localized loss to rise to double the average core loss in some regions of the T-joints. For the 15-cm-wide laminations used, the 45° offset joint was 10% more efficient than the 45°-90° T-joint at a flux density of 1.6 T; the mean loss of both joints was more than 25% higher than the loss in the yokes and limbs.