Transformer hotspot temperature calculation using IEEE loading guide

The life of a transformer depends mainly on the life of its insulation system. Ageing is a process mainly defined by the chemical reaction which primarily depends on temperature and time. The hotspot temperature is usually taken into consideration in investigating the remaining life of transformer. There are two models available in IEEE Loading Guide in calculating the hotspot temperature known as Clause 7 and Annex G methods. This paper presents a comparative study between the two methods, through calculating hotspot temperatures of the four transformers using ONAN, ONAF, OFAF and ODAF cooling modes under conditions of constant load, daily load and ambient temperature profile, and a shortperiod overload. The loss of life is also estimated.