Distribution transformer life assessment with ambient temperature rise projections

This article describes the use of an improved top oil temperature rise model of a distribution transformer. The IEEE Standard C57.91-1995 model for transformer thermal calculations is discussed briefly, and a loss of life calculation based on the 10-degree rule is formulated. This loss of life formulation is used with the improved model for top oil temperature rise to assess the impact of ambient temperature change on distribution transformer life. A probabilistic model for ambient temperature rise is used, and an increased ambient temperature of 3.5°C in 100 years is studied. Seven artifact transformers and four loading test beds are considered. The service life calculated for a base case and an increased ambient temperature case shows that there is a reduction of the order of 3-6 years for the cases studied.