Abstract: To investigate the effectiveness of compressed air foam system (CAFS) in suppressing and extinguishing transformer fires in underground confined spaces, this paper conducted full-scale extinguishment tests at 110 kV for various transformer fire scenarios. It analyzed the variation patterns of parameters such as foam concentrate mass, drainage time, extinguishment process, and characteristic temperatures in different regions of the confined space. The cold spray results indicated that the sprayed foam could rapidly cover the protected area, with the transformer top region experiencing the highest foam coverage, reaching a maximum of 50 kg/（m²·min）. This was primarily attributed to the fact that 3/4 of the nozzles in the CAFS spray pipe were directed towards the top of the transformer. The 25% drainage time of the foam was 220 s. The extinguishment test results demonstrated that CAFS could extinguish transformer fires in different scenarios within confined spaces, with the most unfavorable fire scenario being extinguished in 14 s. CAFS exhibited significant cooling effects on the foam-covered areas but less effective cooling in areas where the foam concentrate could not directly act. It is recommended to adopt additional measures to assist in temperature reduction in practical applications.

Key words: underground substation, confined space fire, transformer fire, compressed air foam, fire safety