Abstract: This research focuses on the fire extinguishing of transformer fires in Ningxia 330 kV substation, and conducts in-depth research on the fire extinguishing efficiency of aqueous film forming foam extinguishing agent (AFFF). In view of the local oil fire caused by serious leakage of transformer, round stainless steel oil pans with diameters of 0.6, 1.0, 1.5 m are used in this test to simulate local oil fire scenarios of different scales. And then we focus on the fire suppression performance of foam water spray under different oil pan sizes, so as to provide scientific basis and effective strategic guidance for the relevant layout design, nozzle flow selection and fire extinguishing agent consumption of foam water spray fire extinguishing system of transformers in Ningxia Power Grid. The results show that the temperature change of the oil pan can be divided into three stages: combustion development, steady combustion and extinction attenuation. When the diameter of the oil pan increases from 0.6 m to 1.5 m, the fire extinguishing time is significantly prolonged, and the fire extinguishing time of the 1.5 m diameter oil pan foam water spray increases by 145% compared with the 0.6 m diameter oil pan. This phenomenon is attributed to the expansion of fuel evaporation surface area and the increase in heat release rate (HRR); The steady-state HRR of 0.6, 1.0, and 1.5 m oil pans reached 200, 800, and 1 200 kW, respectively, but the increase in HRR during the flame intensification stage decreased with increasing oil pan diameter. At the initial stage of foam injection, the flame was temporarily strengthened due to the acceleration of fuel evaporation and air mixing effect, but as the injection continued, the fire was extinguished through surface cooling, oxygen isolation and fuel dilution. The research further establishes the correlation equation between the fire extinguishing time of air foam and the size of oil pan, which provides a theoretical basis for the differential design of fire extinguishing systems for substations with different voltage levels.

Key words: transformer fire, foam water spray, AFFF, oil pan size, fire extinguishing performance