Thermal-degradation Mechanism of Mineral-oil-immersed Pressboards and Influence of Aging on Pressboard Properties

The effects of thermal degradation and aging on the properties of mineral-oil-immersed pressboards (PBs) were studied from the viewpoint of diagnosis of power transformers. Specifically, thickness, degree of polymerization (DP), and compressive properties of oil-immersed PBs after heat-accelerated degradation in sealed containers were evaluated. The degree of crystallization (Cr) of oil-immersed PBs was then evaluated by X-ray diffraction (XRD). The evaluation results revealed that after thermal aging, DP decreased. Moreover, thickness, compressive elasticity modulus, and hysteresis loss ratio slightly decreased. Conversely, C_r slightly increased. DP of kraft paper is known to decrease after aging. Simultaneously, several compounds with low molecular weight, such as CO, CO₂, and furfurals, are generated, and the tensile strength of kraft paper decreases. It is therefore assumed here that the thickness of PBs decreases very little as they generate low-molecular-weight decomposition compounds and that the decrease in DP causes a slight decrease in the compressive elasticity modulus, E. However, an increase in C_r causes a slight increase in E. As a result, the overall decrease in E is slight.