The effect of thermal aging on surface structure and mechanical strength of oil-impregnated pressboard

Oil-impregnated paper is generally used as insulation material in power transformer, which consists of transformer oil and insulating paper (pressboard). And the pressboard made of cellulose is one of important components in transformer insulating system. Oil-impregnated pressboard stays in the environment with high temperature during normal operation of transformer. It is generally recognized that the hot-spot temperature may be the chief factor determining the lifespan of transformer. Furthermore, the inner temperature of transformer will also influence both electrical and mechanical performances of oil- impregnated pressboard. In this paper, the micro surface appearance and mechanical strength of insulating pressboard under different aging time are studied. The sample preparation includes the following steps: vacuum drying for pressboard and transformer oil, impregnating pressboard with transformer oil, and taking oil-impregnated pressboard into vacuum drying oven for thermal aging. When the aging time of oil-immersed pressboard reaches 100h, 200h, 400h and 800h, the pressboard will be taken out of the oven as specimens respectively. After all the specimens under different aging time are obtained, the micro surface appearance of them was observed by scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM). Then the tensile strength of them was tested by tensile strength tester. The results indicate that long cellulose chain in the surface of pressboard is broken into the shorter one or shed directly with the aging time becoming longer, meanwhile the mechanical strength of pressboard becomes weaker.