A field study of aging in paper-oil insulation systems

The paper used to insulate the windings of power transformers is mostly made from wood pulp, a cellulosic material. Over decades the paper is slowly attacked by water, oxygen, oil acids, and high temperatures and eventually degrades to the point where it is no longer an effective insulator. The transformer is then likely to fail. Power utilities need to know when a transformer is nearing the end of its useful life in order to plan its replacement. However, a problem with monitoring the condition of the paper within a transformer is that it may be difficult to obtain a sample to test. Furthermore, a particular sample may not accurately reflect the overall paper condition. A power transformer operating in Australia failed in 2010. Thus we had the opportunity to study the paper condition at various points within the transformer and evaluate the validity of the current understanding of paper aging. In this article we discuss the mechanisms of cellulose degradation, and the associated equations, and apply them to the paper insulation in the failed transformer.