A comparative study of the ageing phenomena in Kraft paper and pressboard used in 500 kV class transformer

Numerous studies have been carried out on the ageing of Kraft paper used in transformer insulation, especially on the production rate of the ageing by-products such as 2-furfural and CO₂, which were then used as the index to infer the status of ageing of the transformer insulation. However, as this study found out, the ageing of pressboard produces the same byproducts at a much higher rate under exactly the same environment, i.e., at the same temperature, with the same moisture and oxygen concentration, etc. Thus the ageing diagnosis of transformer insulation based on the concentration of above byproducts could be misled by the interference of pressboard ageing. A comparative study was carried out to identify the difference between the above two processes, to figure out the scenario when the pressboard ageing can confuse the ageing diagnosis bases on the ageing curve of Kraft paper, and to point out a potential new method to monitor the hot spots occurrence in transformer winding caused by sudden changes such as winding deformation by lightning invasion, or the blocking of oil flow channel by flowing chunk objects. Several other observations in the ageing experiments are discussed.