Oil-immersed transformers are the core equipment of power systems for energy transfer. The thermal characteristic of a transformer immediately affects its operational reliability and service life. Therefore, analyzing the hot spot temperature of transformers is highly important. Currently, researchers worldwide have achieved substantial research results in this field. This paper reviews the research status of the transformer hot spot temperature. First, the calculation method of transformer loss, the forms of heat transfer and its calculation method, the characteristics of the cooling method, and its application occasions are summarized. Second, the methods used to research the transformer hot spot temperature are presented in detail. From the perspective of transformer heat production and heat dissipation, research results on the factors influencing transformer hot spot temperature are summarized. Last, the research direction of the transformer hot spot temperature is indicated.

The broadband dielectric spectroscopy was carried out in the frequency range of 1–10⁷ Hz at the −20–200°C range to investigate the effect of temperature on the dynamic thermal–dielectric behavior of the aluminum (Al)/epoxy composite. The epoxy composites with core-shell–structured Al particles were prepared by solution method. The results show that the dielectric permittivity of the composites increased smoothly with a rise of filler content and reduced with an increase in frequency at room temperature. While the dielectric loss and conductivity still remained at low level owing to the nanoscale alumina insulating shell serving as a barrier layer to control the dielectric loss. The dielectric permittivity, dissipation factor, and conductivity of the composites increased with temperature and exhibited an abrupt rise around the glass transition temperature ( T_g). A large increase in the dissipation factor and conductivity with temperature is attributed to the direct current conduction of thermal-activated charge carriers resulting from pure epoxy above T_g. The observed temperature-dependent dielectric relaxations of the composites indicated a thermally activated behavior of the relaxation time of epoxy chain segments.