Investigating Thermal Performance of Oil-Immersed Power Transformers at Low Ambient Temperatures

Power transformers are considered one of the most critical components in the electrical network. Their sudden outage can paralyze the entire power system, leading to severe financial consequences. The transformer's lifespan is largely influenced by its solid insulation ageing rate. Thus, it is crucial for the insulating liquid (cooling medium) to keep the transformer's internal temperatures, especially in the windings, at an acceptable level. Therefore, the main aim of this work is to study how the reduction in ambient temperature is reflected in the winding's temperature behavior, especially the hotspot temperature. To this end, an experimental setup comprising a transformer winding model with extensive temperature monitoring was built. It has been designed to achieve a thermal and hydraulic environment like that in a power transformer winding operated under oil natural air natural (ONAN) cooling mode. The test rig has been filled with the Nytro 10XN mineral oil. Then, the steady state temperature behavior across the entire setup has been studied for ambient temperatures ranging from 20°C down to −32°C. In the final analysis, the experiments showed that as the ambient temperature went down, the winding steady-state hotspot temperature initially decreased. However, for the coldest ambient temperature, −32 °C, the hotspot temperature increased sharply, reaching almost its initial value attained at the ambient temperature of 20°C. This behavior has been attributed to the inverse non-linear relation between the liquid's temperature and its kinematic viscosity.