Acquisition of the internal temperature of a transformer is an important method to evaluate its operating state and performance. Besides

the operating condition of a transformer

the environmental temperature

wind

sunlight and other environmental parameters can also influence its internal temperature directly. By analyzing the thermal circuit model of transformer tank with inner heat source and the heat transmission process of the transformer

we investigated the ideal thermal circuit model of the transformer when the environmental conditions were stable. Moreover

we constructed a thermal circuit model of the transformer after taking the environmental temperature

solar radiation

natural wind velocity and other environmental parameters and operating states into consideration

and introduced the analysis method of thermal circuit model for the calculation of the hot spot and the top oil temperature. Furthermore

we calculated the top oil temperature and hot spot temperature of the transformer under non-rating operating state

and compared them with the measured values of the internal temperature for the actual transformer during its operating process. The results show that temperature calculated by the constructed thermal circuit model is close to the variation of the measured temperature

the maxmum calculation error is only 2.3 ℃

and the constructed thermal circuit model can effectively reflect the influences of the operating state of the transformer and environmental parameters on the internal temperature. The research results can provide a reference for evaluation of the operating state and performance of power transformers.