Thermal Modeling of an Inverted-Type Oil-Immersed Current Transformer

A current transformer (CT) is a series device used in power systems for measurement and protection purposes. Useful operational life of these devices is largely determined by the hottest-spot temperature seen by them during various loading conditions. A unique experimental setup was designed to measure top-oil as well as hot-spot temperature profiles of a medium-voltage oil-immersed current transformer. Thermocouples and fiber optic sensors were embedded in the secondary winding of the current transformer. Temperature profiles were recorded with different externally applied current profiles to cause overloads of up to three times the normal rating. Collected data was used in conjunction with existing power transformer thermal models to estimate parameters for the tested CTs. Predicted temperature profiles based on these parameters agree well with the measured values and could be used for offline monitoring under planned overload conditions.