## 5 Experiment and Analysis of Carbon Particle Adhesion Under Different Flow Velocity  
According to the analysis in Section 2.3, although the forced oil circulation inside the rising seat is relatively small due to the commutation flow, the internal insulation oil is not absolutely stationary, but there is thermal convection. Therefore, the influence of insulation oil flow velocity on particle adhesion characteristics needs to be considered. To this end, this paper simulates the oil flow inside the converter flow rising seat through an insulation oil circulation system and conducts adhesion experiments of carbon particles under different insulation oil flow rates. The applied DC voltage is kept constant at 10 kV, and the corresponding carbon particle adhesion images are shown in Figure 13. The average adhesion area ratio of each group of experiments is calculated as shown in Figure 14.  
The experimental results show that the flow velocity of transformer oil can significantly affect the adhesion degree of carbon particles. For example, when the flow velocity is 0 mm/s, that is, when the transformer oil is stationary, the adhesion area of particles is relatively small, about 0.57%. As the flow velocity increases, the proportion of particle adhesion area rapidly increases, reaching a maximum value of 1.80% when the flow velocity is 6.19 mm/s. When the flow velocity of transformer oil further increases, the proportion of adhesive area begins to slowly decrease. At a flow velocity of 18.40 mm/s, the proportion of adhesive area decreases to 0.73%. Therefore, within a specific low flow velocity range, as the flow velocity of the transformer oil gradually increases, the degree of carbon particle  
Figure 14: The relationship between carbon particle adhesion degree and oil flow velocity. Figure 13: Carbon particle adhesion images with different oil flow velocity. adhesion will rapidly increase with the increase in the transformer oil flow velocity until reaching a peak. Subsequently, as the flow velocity further increases, the adhesion degree of carbon particles will gradually decrease from the peak. When the transformer oil is stationary, the carbon particles adhering to the surface of the epoxy resin mainly come from the transformer oil between the epoxy resin boards, and the limited range of electric field action makes it difficult for external carbon particles to migrate directionally to the vicinity of the epoxy resin board. And when the transformer oil starts to flow, the oil flow will carry a large number of external particles to migrate between the epoxy resin boards, which can be equivalent to an increase in particle concentration. Although the adhesion probability of individual particles does not change, the total number of particles that collide with the solid surface increases at this time. Therefore, an increase in the flow velocity of the transformer oil in the low flow velocity range will lead to an increase in the degree of carbon particle adhesion. As the flow velocity of transformer oil further increases, the series of forces generated by fluid motion on particles also increase, and the influence on particle motion and adhesion state cannot be ignored. At this time, the force situation and critical adhesion conditions of particles will change to a certain extent compared to the Equation (8). For particles under critical adhesion conditions, according to Table 5, when the transformer oil flows horizontally, carbon particles will not only experience the force shown in Figure 10, but also the oil drag force \(F_{Dx}\) along the oil flow direction. For particles adhering to the surface of epoxy resin, \(F_{Dx}\) will be offset by the frictional force \(F_{f}\) between the solid surface and the particles. In addition, due to the viscosity of the flowing transformer oil, the motion of fluid particles in contact with the solid surface will be obstructed. The fluid close to the solid surface will experience shear stress under friction.