Understanding the Performance of Natural Ester Insulating Liquids During Energization Transformer at Subzero Temperature

Transformer energization in extremely cold climates is challenging due to the transition phase and crystallization process, which starts at a temperature higher than the pour point. In addition, the time required to start the freezing process varies depending on the absolute value of the temperature. Therefore, the approval of a liquid for use in cold climates depends not only on a defined temperature, but also on the time the transformer is exposed to the defined subzero temperature. In the current study, a case study is presented where a natural ester filled transformer is exposed to −15°C for approximately 400 hours and then energized for 24 hours. Before and after energization, some liquid samples were taken to observe changes in liquid properties and their effect on liquid quality under this condition. The cooling and heating liquid curves were thermally analyzed to study the liquid thermal time constants as a function of time and location.