The aging characteristics of laminated pressboard and laminated wood in oil cooled power transformers

Presspaper and pressboard products made from pure kraft pulp have been used successfully for decades for the solid insulation of power transformers. Many researchers have investigated the aging behavior of such cellulose products in the past. In the recent years, the transformer industry started to use significantly higher quantities of laminated wood (plywood) products. Yet, manufacturers and users have not thoroughly studied the long-term characteristics of these alternative products in comparison with kraft products. This paper reports and discusses results from accelerated aging tests investigating the behavior of laminated pressboard and laminated wood in mineral oil. The test conditions were chosen to simulate realistically the operation of open breathing transformers from commissioning to end of life, when the cellulose has decomposed to an extent that the apparatus has to be taken out of service. Two different types of laminated wood (low and high density) from two different suppliers were tested in parallel with laminated pressboard at temperatures from 105 to 135 degC, and an aging time of up to 16 months. Regular testing of the condition of the liquid and solid insulation included flexural strength, water and acid contents, degree of polymerization, dissolved gas analysis and furan content. The results show significantly more acids in the case of laminated wood, which contains high quantities of lignin and natural resin compounds. Faster degradation of the mechanical strength associated with cracks and carbon deposits, corrosion and sludge formation was observed with laminated wood. Moreover, the combustible gases composition is different. In the case of laminated wood, hydrogen production and even gas bubble formation was observed