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Assessing DP value of a power transformer considering thermal ageing and paper moisture

Abstract:

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.