117. Impact of a Digital Winding Temperature Indicator on Power Transformer Reliability and Availability

Power transformers are recognized for their excellent reliability. Many of them have been operating for more than 50 years without obvious signs of aging. One reason for their success is the reduced load they carry most of the time. In order to provide reliable service, the substation design normally provides for some redundancy and in normal service, transformer will be loaded below their rated capacity. A transformer may be called upon to carry a severe overload for several hours of days, in the event of an unexpected outage or even with a planned outage for maintenance. Since the earliest days of transformer availability, the winding insulation has been made of cellulose insulation impregnated with insulating oil. Degradation of cellulose is highly dependent on temperature; with time, a deploymerization process takes place and the long chain of cellulose breaks down in smaller segments. The resulting reduction of mechanical strength turns the paper into a brittle material that is exposed to cracking under continuous vibration and occasional electromagnetic shock when the transformer is feeding a short-circuit somewhere on the transmission network. This aging process is irreversible and will define the end of life of the transformer. This aging process is exponentially dependent on temperature, i.e. the aging rate will double for every increase of about 7°C the insulation temperature. While the rated temperature of modern (thermally upgraded) paper is 110°C, it is not uncommon to allow excursions up to 150°C in case of emergency. In this condition, the aging rate is about 40 times higher and additional risks are incurred such as the release of free gas bubbles from the residual moisture in the paper. In order to take full advantage of transformer loading capabilities during emergency conditions without jeopardizing reliability of the unit, it is therefore mandatory to have an accurate knowledge of the insulation temperature. This paper emphasize on the impact of a digital winding temperature indicator on power transforme reliability and availability.