The effects of localised heating on the properties of laminated core material during high voltage impulse

The authors present results of a preliminary investigation into the relationship between the influence of high-voltage impulses and the magnetic-core properties of distribution transformers. Experiments recording domain movement have shown that localized hot-spot heating and imposed impulse voltage can alter the number of domain walls. With an applied alternating field, these changes in domain pattern are rapidly restored to normal. The resulting changes in core losses are very short lived and insufficient in themselves to account for thermal degradation of insulation material leading to failure of the transformer. The B -H loops following a voltage surge do not show any significant changes in loss