| China is in an important period of industrialization and urbanization. Speeding up the construction of UHV power grid is of great significance for ensuring the coordinated development between electric power and society and realizing the sustainable development of electric power industry. Large power transformer is important equipment in the power transmission link. During its manufacturing, transportation, site filling, operation, maintenance and other aspects, a certain amount of bubbles might mix with the insulating oil, which is an important factor that causes the accident of power transformers. A large number of researches have been carried out on the discharge characteristics of bubbles in insulating medium at home and abroad. But most scholars studied the air gap insulation in static state by now. However, due to submerged pump and difference in temperature, insulating oil in transformers is in flowing state rather than in static state. Bubbles will flow with the insulating oil, which haven't been studied deeply so far. This paper analyzes typical oil duct structure of large power transformer, designs physical experimental model which reflects typical oil duct structure, studies force characteristics of bubbles in flowing insulating oil, analyzes bubble deformation under force action and its electric field distribution and researched discharge characteristics and influencing factors of micro bubbles. Detailed tasks are as followed:According to the characteristics of large power transformer internal typical oil channel structure, flow velocity, bubble size and number, oil temperature range and oil duct electric field, select the reasonable range of each conditions and design simulating experiment system and experimental analysis. Use finite element method to simulate and analyze electric field in oil duct.According to IEC insulating material electrical strength experiment method, design bubbles in flowing insulating oil tolerance characteristics experiment plan. Test breakdown voltages in different bubble contents, at different flow speed and at different oil temperatures. Based on the experiment system, carry out a large amount of PD experiment and acquire signals using pause current period. We choose discharge repetition rate and average discharge capacity as characteristic parameter. Research result shows that: Increase in bubble content strengthens PD level; Increase in flow speed can restrain oil PD; Increase in oil temperature will weaken PD level.As for the characteristics of bubbles in flowing insulating oil, we analyze the influence of each factor on micro bubbles' PD characteristic from bubble force analysis and electric field simulation. Research result show that: Bubble content influences PD by increasing bubble amount, which raises PD probability; Oil speed influences PD by promoting bubble decomposition and making bubble move through high electric field area more quickly; Oil temperature influences PD in a complex way, it affects by dissolving bubbles and changing oil viscolity. |
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