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Bubbles affect insulation performance simulation of liquid

Author: CaiDan
Tutor: LiuLie
School: National University of Defense Science and Technology
Course: Physical Electronics
Keywords: Liquid Insulation Bubble theory Field enhancing Bubble motion Bubble deformation Paschen 's law
CLC: TM855
Type: Master's thesis
Year: 2011
Downloads: 49
Quote: 0
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Abstract

Liquid medium with a high breakdown field strength, self-healing performance, low cost and ease of operation and other aspects of advantages, which is widely used as an insulating medium intense electron beam accelerator. However, so far, the liquid medium of the physical and chemical properties of its complexity, is still not mature form similar to the gas breakdown models and mechanisms explained. Bubbles in liquid insulation breakdown theory, be able to explain changes in pressure and conductivity of the liquid dielectric breakdown implications for our study of liquid breakdown mechanism opens a new way of thinking. Based on the bubble in an electric field under the three acts, from the theoretical derivation and simulation and other aspects of air bubbles in a strong electric field and its effect on the behavior of liquid insulation properties were studied. Firstly, a uniform electric field is derived electric field distribution inside and outside the bubble, find the electric field inside the bubble apparent enhancement of air bubbles for transformer oil, field enhancement factor of 1.4. With the Paschen curve, the gas pressure is verified effective way to avoid the breakdown of the bubble. Simulate the presence of a bubble, immersed in transformer oil in the electric field between the parallel plate electrodes, discovered that: when the electrode spacing of 1cm, the voltage across the electrodes is 50kV, the bubble radius is 1mm, plus a minimum of 2.03 atm pressure to avoid bubbles breakdown. Then the bubble motion derived equations based on the use of multi-physics simulation software Comsol mobile grid to simulate the bubble motion in a non-uniform electric field, and the more bubbles into the bridge conducted a preliminary validation. The results showed that: (1) bubble motion consists of three phases: the first phase has just been released when the bubble field gradient force played a decisive role, the electric field strength tend to change slowly regional movement, the movement quickly reached the maximum; second phase of the electric field gradient force bubbles, viscous force and buoyancy rate began to fall under the action and move to a region called a cut-off line; the third stage after the bubble reaches the cutoff line force basic balance between the electrodes move slowly. (2) After the bubble movement to the cut-off line speed is small, and thus the residence time between the electrodes, and easy to form aggregates bridge is formed along a trajectory bubble. Finally, a uniform electric field is derived transformer oil bubble surface force distribution of the electric field, and on this basis, combined with Comsol module simulates the level set under the gas bubbles in the field - liquid interface changes, simulation found that: (1) Bubble tensile deformation along the direction of the electric field lines, and its minor axis ratio and initial bubble radius and the intensity is proportional to the applied electric field, so when the applied electric field appear larger or liquid large bubbles, the bubbles may be pulled off the formation of many small bubbles; (2) bubbles in the liquid within the electric field is enhanced on the one hand, the other hand, since the deformation resulting bubble field direction extending gas passage easier breakdown, breakdown in order to avoid air bubbles the gas pressure, the bubble size and the minimum intensity of the applied electric field proportional.

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CLC: > Industrial Technology > Electrotechnical > High Voltage Engineering > High - voltage insulation technology > Insulation test and inspection
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