Heat itself does not degrade the insulation paper, however, it acts as the driving force for these reactions accelerating the degradation process. Therefore, hydrolysis and oxidation facilitate the thermal degradation of cellulose. In the presence of water, oxygen, and acids, heat accelerates the aging process. An increase of 6-8°C in temperature can reduce the life of the insulation paper by as much as 50%.
At temperatures above 110°C the glycosidic bonds in the cellulose break forming products such as water, carbon dioxide, carbon monoxide, hydrogen, and methane. What remains are substances such as sludge and acids, promoting hydrolysis.
Heat in a transformer occurs due to power losses in the core and winding structure. The temperature in the transformer varies over time as the load changes and the ambient temperature varies over time. Temperature is also not uniform within the transformer. The temperature is highest in hotspots, nearest the winding and core structure.
The insulation at hotspots are at higher temperatures and will release moisture into the oil, which has a higher moisture solubility in higher temperatures. The moisture will migrate to cooler areas near the bottom of the transformer tank where the cooler insulation will absorb it. The higher the temperature gradient between the hotspot and cooler bottom of the tank the more water will collect at the bottom. This can lead to high degradation rates in these areas and catastrophic failures.
Monitoring the temperatures in the transformer will provide an understanding of the condition of the insulation.
 Saha, Tapan Kumar, and Purkait, Prithwiraj. Transformer Ageing: Monitoring and Estimation Techniques: Monitoring and Estimation Techniques. 1st ed., Wiley, 2017.
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