Abstract: Indoor substations often adopt high-pressure fine water mist systems as their primary fire extinguishing systems, while some projects also use Compressed Air Foam Systems (CAFS) as supplementary fire extinguishing measures. However, the effectiveness of combined use of these two fire extinguishing systems is unclear. To investigate the efficacy of extinguishing transformer fires using a combined fine water mist-Compressed Air Foam System, this paper establishes an 110 kV full-scale indoor transformer fire extinguishing test platform. Experiments were conducted to extinguish top-mounted and three-dimensional fires on transformers using single fine water mist and combined fine water mist-CAFS. Temporal variations in data such as the fire extinguishing process and characteristic temperatures were analyzed. The experimental results indicate that the combined use of fine water mist and CAFS can quickly and effectively extinguish indoor transformer fires, with extinguishing times of 10 s and 8 s for top-mounted and three-dimensional fires, respectively. Compared to the single fine water mist extinguishing system, the combined use of fine water mist and CAFS improves the cooling efficiency of the transformer wall by approximately 50%. After the combined fine water mist-CAFS system was continuously activated for 10 minutes, the key protection area remained fully covered by foam, indicating that fine water mist does not cause compressed air foam to rupture rapidly, and the combined use of fine water mist and CAFS is feasible. The research findings can be used to optimize the design of fire protection systems in indoor or underground substations, ensuring their normal operation.

Key words: water mist-compressed air foam combined system, indoor transformer fire, effectiveness of fire extinguishment, cooling efficiency