Effect of ageing on the impulse breakdown strength of oil-impregnated pressboard used in power transformers

This paper presents the lightning impulse voltage test results of service-aged pressboards taken from a failed power transformer after 29 years´ service. Impulse voltage tests have revealed that there is a significant reduction of breakdown voltages, most probably due to the degradation of mechanical strength. The minimum impulse electric strength may reduce approximately by 30% after 29 years´ service. Test results on the service-aged pressboard also indicate that when the moisture content in pressboard is less than 4%, there is no significant correlation between the reductions of the impulse electric strength with increasing moisture content.