Abstract
In certain circumstances, power transformers must be energized and loaded after a long period being out of service, an operational condition known as a cold start. Such a condition can occur after transformer maintenance or when a spare unit enters service, where the liquid temperature is nearly equal to the ambient temperature. It has been observed that when such a cold start is carried out in an arctic environment, especially for ONAN-cooled power transformers, the winding’s hotspot thermal response typically exhibits a temporary temperature overshoot before reaching steady state. The overshoot period can last for a few hours, depending on the ambient temperature, leading to further deterioration of the transformer’s insulating system. Currently, many researchers are seeking a better understanding of the previously mentioned phenomenon through various computational approaches; however, a small number of experimental investigations have been reported to date. Therefore, this contribution aims to experimentally study the thermal response of mineral-oil-filled ONAN power transformers during cold start. In this regard, an experimental setup comprising a section of a winding model and a radiator has been used to conduct experiments at ambient temperatures ranging from 20 ℃ down to −40 ℃. In the final analysis, it has been found that the observed overshoot behavior depends significantly on ambient temperature, with an inverse relationship