Abstract: Based on the structural features and fire risks of the underground transformer room, a full-scale fire numerical model of a 220 kV transformer was constructed, taking into account the influence of ventilation valves and evacuation doors on airflow. The study investigated the full-surface fire combustion characteristics of the transformer room's emergency oil sump. The results indicate that, regardless of whether the ventilation valves and evacuation doors are open or closed, the combustion in the emergency oil sump is air supply-limited and belongs to ventilation-controlled fires. When both the ventilation valves and evacuation doors are open, the peak heat release rate of the fire source reaches 12 MW; when the ventilation valves are closed and the evacuation doors are open, the peak heat release rate of the fire source is 5 MW. Both values fall short of the theoretical peak heat release rate of the designed fire source. The temperature decreases from the indoor ceiling downward, with the highest temperature on the side walls exceeding 700 °C. When the ventilation valves are open, the peak thermal radiation can exceed 300 kW/m², and the average ceiling temperature can exceed 470 °C, both of which are higher than the values when the valves are closed.

Key words: underground transformer, fire characteristics, fire heat release rate, thermal radiation, ventilation control