Abstract: High-pressure fine water mist systems are commonly used in underground substations. Compressed Air Foam System (CAFS), as a measure to enhance firefighting capabilities, is gradually being applied in the fire suppression process of substations. To evaluate CAFS performance in underground substations, this study established a full-scale 110 kV underground substation fire suppression test platform. By adjusting parameters such as flow rate, foam type, and combustion area, the extinguishing effectiveness of CAFS was investigated. Results indicate that the CAFS extinguishing process comprises three distinct phases: initiation, sustained suppression, and flame extinction. Increasing the foam mixture supply intensity significantly accelerates flame expansion and shortens extinguishing time. When supply intensity rose from 13.3 L/(min·m²) to 26.7 L/(min·m²), extinguishing time decreased from 22 s to 7 s. Foam liquid type had minimal impact on extinguishing effectiveness, and the presence of spreading fires had little effect on extinguishing duration. Simultaneously, wall surface cooling rates were influenced by flow rate. Higher supply intensities resulted in significantly lower wall temperatures upon successful extinguishment. Oxygen concentration increased after CAFS activation. As supply intensity increased, oxygen concentration at the moment of successful extinguishment first rose then fell, indicating no direct correlation between the extinguishing process and oxygen concentration changes. Comprehensive analysis suggests selecting a CAFS supply intensity between 17.8~22.2 L/(min·m²) with 3% AFFF foam type.

Key words: confined space fire, underground substation, compressed air foam, fire extinguishing effect, parameter design