Development of an Improved Model for Assessment of Hot Spot Temperature of Current Transformers

Current transformers form important components that make up a large portion of capital investments. Failure of a current transformer results in an adverse effect in the operation of transmission networks which causes an increase in the power system operation cost and inability to deliver electricity with absolute reliability. The age of a transformer is the life of its insulation, majorly, paper insulation. Transformer aging can be evaluated using the hot spot temperature which has the effect of reducing the insulation life of transformers. Previous researchers have developed models for assessment of top-oil temperature of current transformers. Such models have the limitation that they do not accurately account for the variation effect in ambient temperature and hence not applicable for an on-line monitoring system. This research paper develops an improved model for assessment of hot spot temperature from the IEEE top-oil rise temperature model by considering the ambient temperature at the first-order characterization using appropriate mathematical notations. The ambient temperature, top oil rise over temperature and winding hot spot rise over temperature were used as input parameters for the development of the improved hot spot temperature model by considering the final temperature state since the time-rate-of change in top-oil temperature is driven by the difference between the exits top-oil temperature for ambient temperature variation. The improved model was then implemented in MATLAB to compute the hot spot temperature for 24-hour load cycle. The result of the improved model shows that the least and highest value of the hot spot temperature are 630C and 105.40C respectively indicating a retardation in the aging process of the transformers. The improved model helps to minimize the risk of failure and to extend the life span of transformers thereby controlling the hot spot temperature rise and top oil temperature.