Heat distribution pattern of oil-filled transformer at different hottest spot temperature locations

Sustained high temperature on transformer insulation systems is one of the main causes of its accelerated degeneration. Continuous monitoring of transformer heat distribution could provide useful information on intensity and location estimation for hottest spot temperature (HST) development. Furthermore, faults will aggravate the formation of HST inside the transformer tank. However, there is difficulty in measuring and locating HST in many aged in-service transformer because there are no in-tank sensors installed. Thus HST can be monitored by observing the heat distribution pattern in the transformer. Therefore, this paper aims to investigate the effect of HST on the overall transformer temperature distribution and the heat distribution pattern associated with HST. The focus is on the effect of this heat distribution to the surface of transformer tank for condition monitoring purposes. Simulation of mineral oil-filled distribution transformer (ONAN type) was done by using Finite Element Method Magnetism (FEMM) 4.2 where particular interest was on the transformer cross section. Changes of heat dissipation at the inner and outer transformer body tank were analyzed. The results show that the continuous presence of HST increases oil temperatures as well as inner and outer tank surface temperatures. The existence of HST affects the whole temperature distribution pattern inside the transformer. Thus, the transformer HST can be used as indication of potential inside the transformer. The changes of surface heat near to the HST may also be useful in in-service monitoring the transformer condition.