## 1 Introduction  
The ring main unit (RMU) is considered the primary distribution device in modern power systems [1]. The device also plays a significant role in stabilizing the system by controlling and protecting the power supply [2]. The reliability and stability of power equipment are crucial in outdoor electrical engineering. However, fluctuations in internal temperature and humidity can easily impact the maintenance and operational safety of the system, making reliable equipment crucial. For instance, if the surface temperature of the inner wall of metal equipment drops below the dew point temperature, water droplets will condense on the surface, leading to the corrosion of the metal components and a reduced insulation performance. This increases the risk of equipment arcing or breakdown, potentially resulting in safety accidents [3]. Therefore, preventing the aforementioned condensation phenomenon is crucial for the safety of the power system. According to [4], the occurrence of condensation mainly depends on the relative humidity, temperature, and dew point temperature in the cabinet. Many studies have focused on explorative measures and preventive measures for condensation, combined with the causes, conditions, and hazards of high condensation pressure in cabinets, and proposed some technical measures to prevent condensation [5; 6; 7]. Ventilation holes are suggested to be added around the foundation of the RMU to solve this problem by reducing the humidity in the air [8]. However, increasing only ventilation holes without adding dehumidifiers results in limited dehumidification efficiency. To reduce condensation, cabinets are usually equipped with dehumidifiers. Traditional dehumidifiers are divided into heating dehumidifiers and condensing dehumidifiers, but both dehumidification methods have obvious drawbacks. A method combining electric heating and an anti-condensation controller has been proposed for dehumidification and anti-condensation in 12 kV switchgear. Although the internal surface temperature increases and the relative humidity of the air decreases, condensation cannot be completely prevented. The position of the heater also has a significant impact on the dehumidification [9]; from another perspective, currently condensing dehumidifiers mainly use semiconductor refrigeration technology to reduce the temperature in humid air, and then condense the water vapor into water droplets, which are discharged from the switchgear to achieve the purpose of dehumidification [10]. However, the semiconductor condensation dehumidification method is only applicable to areas with slow environmental temperature changes. In addition, the above two traditional methods did not consider the impact of ventilation. Research based on the projects encountered is conducted, which adopts a different method of dry air dehumidification to enhance anti-condensation. By installing inlet and outlet air ducts on the wall of the cable room of the RMU, the dehumidifier introduces dry air from the inlet duct and discharges the humid air inside the cabinet from the outlet duct, forming a good ventilation cycle to achieve the purpose of dehumidification. The different supply air temperatures and flow rates of dehumidifiers may affect the dehumidification. Using ANSYS Fluent 14 to simulate the flow field and thermal environment inside the cabinet is an effective model. Many studies use software for numerical simulation to accurately calculate the fluid flow and thermal field inside the cabinet. The ANSYS Maxwell 15 has been used to investigate the impact of copper resistivity on power loss in low-voltage switchgear [11]. Numerical simulation was conducted on switchgear to study the condensation phenomenon of humid air in switchgear under different relative humidity and boundary conditions [12].