Comparison of two thermal circuit models for HST calculation of oil-immersed transformers

Oil-immersed transformers are key components in power systems. The inordinate temperature rise in oil-immersed transformers, featured by hot-spot temperature (HST), accelerates thermal degradation in insulation. This phenomenon may result in reduction of service age or thermal breakdowns of transformers. HST is also an important parameter to determine the optimum load ability of transformers. A procedure to calculate HST with parameters, including load current, ambient temperature, and top oil temperature is recommended by IEEE and IEC. The recommended method has been proved to be effective. This paper presents two equivalent thermal circuit models for transformers, revealing the relationship between HST and featured oil temperatures, which are the top oil temperature (TOT) and the bottom oil temperature (BOT). The calculation results of the thermal models are compared with measured data obtained from an ONAN, 100 kVA and 5 kV test transformer equipped with temperature probes.