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 degrees 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 degrees C. The resulting safety and economic benefits of the lower transformer kVA rating cascade throughout the power system.