## 1 Introduction  
The Ultra High-Voltage Direct Current (UHV DC) transmission project plays a crucial role in the modern power system, and the converter transformer, as its core equipment, bears the important tasks of voltage conversion and power transmission [1, 2, 3]. However, due to its high-load operation and complex electromagnetic environment, the converter transformer faces many hazards in actual operation, and fire is one of the most serious risks [4]. Under the influence of external ignition sources or internal failures, the converter transformer may experience structural failure due to temperature rise, leading to more serious fire accidents [5, 6, 7]. Table 1 lists a number of accidents due to converter transformer fires in recent years, which not only caused serious damage to the equipment but also posed a significant threat to the stable operation of the power system [8]. Therefore, it has become imperative to research and develop effective fire retardants to improve the fire safety of converter transformers [9]. At present, the fire protection methods for converter transformers are mainly divided into two categories: active and passive. Among active fire protection, the water spray fire extinguishing system has become one of the most widely used fire extinguishing methods due to its efficient fire extinguishing performance and good environmental adaptability [10]. By atomizing water into tiny droplets, water spray quickly covers the fire source and reduces the temperature of the surrounding environment, thus achieving the effect of extinguishing the fire and preventing re-ignition. On the other hand, passive fire protection methods mainly rely on the fire-insulating properties of materials that can effectively delay the transmission of flame and high temperature to the interior of the equipment, thus protecting the core components of the equipment [11]. In recent years, a large number of studies have been carried out on the fire protection of converter transformers. In terms of fire combustion characteristics, converter transformer fires are generally considered to be insulating oil pool fire types [12], and experiments have been conducted to investigate the effects of various factors (wind speed [13; 14], multiple liquid pools [15], height of enclosure walls, and initial temperature of the liquid pools [16]) on the fire burning rate [17; 18; 19; 20], geometry [17], and the laws of thermal radiation [21; 22]. In water spray fire extinguishing, the ability of fine water mist to extinguish electrical fires has been demonstrated [23], and studies have been carried out on the cooling effect of different fine water mist fire extinguishing systems and thermal radiation blocking [6; 24; 25; 26; 27; 28; 29; 30]. In the study of thermal insulation materials, the failure mechanism of casing blocking structure on the valve side of the converter has been investigated [8; 11], confirming its important role in slowing the spread of fire. In order to carry out in-depth research on converter transformer fire protection, the shortcomings of existing experimental studies can be compensated by numerical simulation methods [32]. A number of studies have been conducted to simulate the development of electrical fires based on CFD (Computational Fluid Dynamics) [33], to evaluate the fire extinguishing effect of fine water mist [34; 35], and to study the thermal insulation effect of materials [8]. Simulation studies not only save experimental costs but also accelerate the development and application of technology and are therefore of great importance in studying fire protection means for converter transformers. Existing experimental research provides more realistic data but is costly and poses safety risks. Additionally, some data are difficult to obtain during experiments. These limitations can be addressed through validated simulations, which reduce costs and provide more comprehensive data.