Measuring and understanding the ageing of kraft insulating paper in power transformers

In this paper we discuss the application of a number of techniques that are currently being developed for use in life assessment/condition monitoring of transformer insulation, and we illustrate possible applications with some recent data. These include: high performance liquid chromatography (HPLC) of the oil to measure the product concentrations; size exclusion chromatography (SEC) of the paper to measure changes in molecular weight distribution (related to the degree of polymerization, DP) and computer modeling of the degradation process to relate chemical degradation rates to changes in physical properties, such as tensile strength, which is of critical importance in assessing the probability of damage to the paper under fault conditions. We also revisit the use of DP measurement as a means of assessing insulation life and present results that illustrate problems that we have identified in obtaining consistent data. We describe a method, developed from the British and American standards, which enables us to obtain reproducible results with a known variance. The aims of this paper are to present a limited review of techniques that are relatively new to the transformer monitoring field, to provide some recent results from this laboratory to demonstrate their potential application, and to show the need for a range of new tools, if we are to improve our ability to assess the condition of transformers and the remanent life of their insulation.