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With the increasing level of electrical equipment manufacturing, the increasingly miniaturized structure of electrical equipment has been widely used, but with the gradual reduction of insulation levels, the internal insulation level of electrical equipment has been more severely tested. Based on the conservation laws of energy, momentum, mass and species, considering air as a mixture of ideal gases composed of water vapor and dry air, taking into account that the density of air changes relatively little during the flow, treating air as an incompressible fluid, and also considering that water vapor diffusion follows Fick's law, a mathematical model of the temperature and humidity field model of the outdoor terminal box under heat conduction, air convection and water vapor diffusion are constructed by computational fluid dynamics (CFD) technology with using the species transport model. This paper analyzes the effects of temperature, humidity, forced ventilation and internal heat source on the condensation phenomenon. The numerical calculation results show that the air temperature in the outdoor terminal box gradually rises under the condition of heat conduction and heat convection due to the self-heating of the conductor. The distribution of humidity shows that the humidity at the gas inlet is at the highest state, and the humidity at the higher temperature is relatively lower. Introduction Due to the particularity of the climatic environment in the southern region of China, electrical equipment operates in high temperature, high humidity and high salt environments. In recent years, among the defects in the switch-gear of Guangdong Power Grid, nearly 50% of the faults are related to the rust, jamming and aging of secondary components. At present, China Southern Power Grid and Guangdong Power Grid have no clear technical specifications for substation equipment cabinets, so that corresponding defects occur from time to time. In recent years, there have been five consecutive occurrences due to power outages caused by abnormal function of substation equipment cabinets. The safe operation of substation equipment has had a serious impact. There are many factors affecting the defects of electrical equipment. One of the main faults is the condensation of electrical equipment. Dew condensation is a phenomenon in which water vapor condenses on objects of relatively low temperature when it is saturated in the air, and temperature and humidity are both the main factors causing dew condensation [1-3]. The partial discharge phenomenon caused by the water vapor cover the insulating material which is generated by the dew condensation, and the long-term operation may cause insulation breakdown failure in the electrical equipment. At present, research on electrical equipment mainly focuses on temperature fields and gas flow fields, and the influence of humidity fields is rarely considered. For example, Liu Yunpeng and Guo Qin [4] have simulated the distribution of temperature field and humidity field in the switch-gear equipment. Pu Liang et al [5] mainly proposes a CFD algorithm for solving humid air temperature and relative humidity. Li Xianting et al [6] performed the influence of the air temperature and the particle size of the sprayed droplets on the humidity field of the environmental test room of an 315 electrical equipment, but it was not specifically analyzed. Deng Shuhui et al [7] performed a method to study the effect of low roof lateral ventilation on temperature and humidity fields. Temperature, Gas Flow and Humidity Field Model of Electrical Equipment Box Physical Model This paper adopts a new type of moisture-proof terminal box for simulation calculation. Considering the simplified processing of the terminal box, it is simplified to a cabinet of 600mm×600mm×1400mm according to its structure, and the physical model is shown in figure 1. At the same time, considering that the terminal box is in direct contact with the outside air, and there is convective heat transfer between the terminal box wall and the surrounding air. Considering that the bottom of the terminal box is directly in contact with the ground, the bottom humidity is relatively high, and considering that the bottom is slotted, so the humid air inlet is set to the bottom slotted, and the air outlet is placed at the upper slot. Figure 1. Calculation model of the terminal box. The factors affecting the temperature of the terminal box are heat conduction, heat convection, heat radiation and other factors. Considering the relatively low internal temperature of the terminal box, the internal heat radiation of the terminal box is ignored in the simulation calculation. Considering humid air as a mixture of dry air and ideal gas of water-vapor, and also considering that the flow rate of moist air is relatively low and the pressure changes are relatively small, the air is considered as incompressible Newtonian fluid, and considering the effect of gravity field on air and water vapor. The gas turbulence model uses the RANS K-ε model and use the multi-component transport model. Mathematical Model The control equation of the continuity, energy, momentum, composition conservation laws, turbulence energy of the RANS K-ε model and dissipation rate that need to be followed in the physical process of the model is as follows: ∂ρ ∂t + ρ∇ ∙ (u) = 0 (1) ρCp ∂T ∂t + ρCpu ∙ ∇T = ∇(λ∇T) + Q (2) ρ ∂u ∂t + ρ(u ∙ ∇)u = −∇p + F (3) ∂cs ∂t + ∇ ∙ (−Ds∇cs) + u ∙ ∇cs = Sm (4) ρ dk dt = ∂ ∂xi [(μ + μt σk ) ∂k ∂xi ] + GkGb − ρε − YM (5)

Numerical Simulation of the Temperature, Humidity Field and Dew Condensation in Substation Equipment