Practical life-assessment technique for aged transformer insulation

The continued use of power transformers beyond what might be considered their nominal design life is now a matter of considerable economic significance. A prerequisite to such extended use is an informed assessment of the remaining reliable life. A practical life-assessment technique is presented that has been recently developed within a major Australian utility and applied to over 40 service-aged transformers. Central to the process is a determination of the ´insulation age´ of a transformer. Several inputs are used to determine this quantity, including the degree of polymerisation of paper samples taken from in situ units. The end result is a reliability-based estimate of future performance that can be used for life extension, refurbishment or replacement decisions hitherto made on an essentially subjective basis. The paper concentrates on the concepts behind this technique; however, practical examples are given to illustrate the major aspects involved.