The Effect of Moisture on the Creepage Discharge of Transformer Pressboard

This paper addresses the creepage discharge process in moisture-absorbed pressboard under the AC voltage stress. The measurement for partial discharges, dissolved gases and furan compounds have been performed for various types of specimens such as the normal dry specimen, dry pressboard with much moisture in oil and the wet pressboard. The result showed that partial discharge inception voltage, discharge number and accumulated charge quantity increased and the discharge magnitude per pulse decreased in case of specimen with moisture. The partial discharge patterns at the initial stage in the trend with time showed different according to the types of specimens, but after the initial stage the discharge trend in all cases became similar with recurrence pattern of large/small discharges and then small discharges. In case of dry pressboard with much moisture in oil, the generation of gases was less than compared with normal dry specimen, but a lot of furans generated. On the contrary, in case of wet pressboard, large amount of gas generated almost without furan generation. Results indicated a mechanism of creepage discharge in which moisture aggregated in the amorphous region of the pressboard attracted discharges owing to the electric field enhancement, and the degradation and carbonization of pressboards would be accelerated by these discharges.