Experimental study and numerical analysis of thermal hazard for fire accident in converter transformers

Converter transformers are widely used in high voltage direct current (HVDC) transmission systems. Due to its large storage capacity and relatively high failure probability of the insulation, pool fires are a common type of accidents for this type of transformers, which could pose a significant threat to the safety of personnels and the integrity of nearby transformers. During normal operation, the temperature of the transformer oil can vary between 80 and 90 degrees C. It is known that the oil temperature can have a significant impact on its burning characteristics. However, most of previous simulations of transformer fires utilized the material properties of transform oil obtained at ambient temperature to determine the heat release rate (HRR). In this study, experimental data (D = 0.2-1.0 m) at an initial oil temperature of 85 degrees C were employed to determine the correlation between HRR and burner diameter, which was subsequently used to validate the numerical model. 14 typical fire scenarios were examined to understand the accident development process of transformer pool fires. The results showed that a merged flame was observed at the inner side for the case when the fire occurred both on the oil tank upper surface and in the oil storage pit. The radiation impact on (and thus the risk of failure of) key transformer components was found to vary considerably across the different fire scenarios. The radiative heat flux at the outlet of the enclosure where the fire is located was found to exceed 5 kW/m2 and the recommended safety distance for firefighting can reach 6.6 m. It was also found that the maximum heat radiative flux received by the high voltage bushing of an adjacent transformer can reach 46.2 kW/m2 in the worst-case scenario, with a probability of escalation to the adjacent transformer approximately 3.50x10- 4. Three distinctive areas (safety area, risk area, and dangerous area) were identified to determine firefighting positions and strategies for transformer substation fires. The present results could not only enhance the understanding of the accidental development of converter transformer fires but also provide valuable guidance in firefighting in various fire accident scenarios.