Effects on tensile strength of transformer insulation paper under accelerated thermal and electrical stress

One of the basic causes of transformer failure is the loss of mechanical strength of insulation paper. This paper investigates the impact of combined thermal and electrical stress on top, middle and bottom layer of transformer insulation paper immersed in insulating oil. This study was carried out in a specially designed test cell. The accelerated thermal stress was selected from 120degC to 160degCand electrical stress from 1.0 pu to 2.5 pu. The tests were performed in stress balance order i.e. for lower temperature, the electric stress was maximum (120degC, 2.5 pu) and for maximum thermal stress, the electric stress was normal i.e. (160degC, 1.0 pu). The paper samples were taken out periodically after 360, 720, 1080 and 1440 hours to assess the deterioration of tensile strength on different layers of paper wrapped on the copper strip. Scanning Electron Microscopy (SEM) study was carried out on the aged paper samples to investigate the extent of failure of paper. The study reveals that top layer of the transformer insulation paper was highly affected under the same stress level and SEM study supports the findings.