Using insulation aging to size transformers in high-ambient temperature, secondary-selective applications

The electrical distribution systems in industrial facilities often incorporate a secondary-selective design where loads are fed from redundant transformers normally loaded to less than 50% of their kVA rating. When transformers are operating in ambient temperatures above 30 °C, IEEE Std C57.91 provides guidelines on how to load the transformers to compensate for the accelerated insulation aging. This guide is not directly applicable to secondary-selective systems since it assumes the transformer is being sized to supply the entire load on a continuous basis. This paper describes an approach for using insulation aging to size the transformer with an example using actual historical temperature data from an oil-production facility in a Middle-East country. The paper will demonstrate that transformers operating in a secondary-selective application do not have to be derated even when operating at ambient temperatures far in excess of the rated 30 °C. The resulting safety and economic benefits of the lower transformer kVA rating cascade throughout the power system.