Current power transformer (PT) ageing models are approximations to reality based on experimental and theoretical evidence; these models are simplifications of complex interactions inside units, because of that loss-of-life results could be presented in a wide range. In PT decision-making context-investment, replacement or maintenance, this amplitude could lead inadequate actions and, in extreme cases, might compromise technical operation and profitability. Solid insulation failure is the leading cause of end-of-life of PTs, insulation paper is composed by long fibers of cellulose; the average length of these fibers is called degree of polymerization (DP), it is widely accepted that DP value is a good index of PT loss-of-life. Considering the above, this chapter presents a holistic methodology for solid insulation ageing assessing based on all thermal degradation process (oxidation, hydrolysis and pyrolysis) and the influence of paper moisture dynamics. Paper moisture is estimated using as input external variables such as hot-spot temperature, transformer technical data and measurements regarding oil moisture, in order to consider uncertainty in oil moisture growing, arithmetic-Brownian-motion (ABM) algorithms are presented. For illustrating the application of this method, DP value of two units is assessed; available information for this example are hourly profiles of load and ambient temperature and yearly moisture in oil samples for a period over 15 years.

Chapter Contents:

• Abstract
• 4.1 Introduction and preliminary issues
• 4.2 State of the art
• 4.3 Theoretical framework
• 4.3.1 Paper as power transformer solid insulation system
• 4.3.2 Paper degradation process
• 4.3.2.1 Oxidation
• 4.3.2.2 Hydrolysis
• 4.3.2.3 Pyrolysis
• 4.3.3 Degradation accelerators
• 4.3.3.1 Heat
• 4.3.4 Paper humidity
• 4.3.5 Assessing of depolymerization process
• 4.4 Proposed method
• 4.4.1 Problem description
• 4.4.2 Oil moisture estimation
• 4.4.3 New approach for degree or polymerization assessing
• 4.5 Casestudy
• 4.5.1 Results
• 4.6 Conclusions
• References

Inspec keywords: decision making; Brownian motion; failure analysis; power transformer testing; paper; moisture; power transformer insulation; power transformers; ageing; transformer oil; pyrolysis

Other keywords: end-of-life; insulation paper; inadequate actions; thermal ageing; solid insulation failure; theoretical evidence; thermal degradation process; profitability; complex interactions; time 15.0 year; current power transformer ageing models; leading cause; paper moisture dynamics; loss-of-life results; DP value; experimental evidence; long fibers; extreme cases; yearly moisture; transformer technical data; solid insulation ageing; technical operation; average length; oil moisture

Subjects: Transformers and reactors; Reliability; Organic insulation