Creep stress failure in high voltage transformer interwinding insulation

Three phase transformers have an insulation system which is a complex three dimensional structure of paper and pressboard surrounding the conductors immersed in a large volume of mineral oil. In core form transformers, a key insulation area is the region between adjacent phases which is often reinforced with vertical pressboard barriers. At the higher voltages, these pressboard barriers provide a crucial element in the insulation structure between the phases. The barriers are subjected to large electrical and thermal stresses. The pressboard barriers sit in the time varying electric field pattern generated from the transformer winding coils which are operating with 120deg phase difference. The electric field can be resolved along the surfaces of the pressboard and this is termed creep stress or tangential stress. This electric stress can lead to surface tracking along the oil-pressboard boundary layer. A large scale test facility has been developed to investigate the effects of creep stress with the goal of understanding the onset of surface tracking on the pressboard. This paper details the main features of the test platform and control equipment. Finally, the paper includes some initial results of experiments conducted to detect partial discharge under two temperature conditions.