DP and life of insulation
Much research has been done into the determination of the life expectancy of transformers and the aging of the paper/pressboard insulation. The insulation used in transformers is cellulose-based, made up of polymer chains. The average number of glucose rings which make up the cellulose molecules is known as the degree of polymerization (DP).
For a new transformer, the DP of the insulation paper considered to be typically around 900. Over its life, the paper ages decreasing the DP. At the end of its life, it is usually around 200. This is the recommended DP for the end of life based on the paper losing 75-80% of its tensile strength. This point is regarded as the end-of-life criterion for the insulation and it is recommended that the transformer is retired. Therefore, the life expectancy of the transformer is directly linked to the life of the insulation.
Knowing this start and endpoint for the DP of the paper, a life estimation can be made. However, this would require knowing the rate of aging of the insulation paper.
Degradation of Insulation
Over time the paper insulation naturally degrades due to three processes:
Hydrolysis is the chemical breakdown of the cellulose in the insulation paper due to its reaction to water.
In the presence of oxygen, oxidative degradation of the cellulose polymer chains creates water which further permits hydrolysis.
Heat acts as the driving force for these reactions accelerating the degradation process.
 Saha, Tapan Kumar, and Purkait, Prithwiraj. Transformer Ageing: Monitoring and Estimation Techniques: Monitoring and Estimation Techniques. 1st ed., Wiley, 2017.
 Lelekakis, Nick, et al. “Ageing Rate of Paper Insulation Used in Power Transformers Part 1: Oil/Paper System with Low Oxygen Concentration.” Ieee Transactions On Dielectrics And Electrical Insulation, vol. 19, no. 6, 2012, pp. 1999–2008.
 Lelekakis, Nick, et al. “Ageing Rate of Paper Insulation Used in Power Transformers Part 2: Oil/Paper System with Medium and High Oxygen Concentration.” Ieee Transactions On Dielectrics And Electrical Insulation, vol. 19, no. 6, 2012, pp. 2009–2018.