Abstract
Bubbles will reduce the electric strength of transformer oil, and even result in the breakdown of the insulation. This paper has studied the breakdown voltages of transformer oil and oil-impregnated pressboard under alternating current (AC) and direct current (DC) voltages. In this paper, three types of electrodes were applied: cylinder-plan electrodes, sphere-plan electrodes, and cone-plan electrodes, and the breakdown voltages were measured in both no bubbles and bubbles. The sphere-sphere electrodes were used to study the breakdown voltage of the oil-impregnated pressboard. The results showed that under the influence of bubble, the breakdown voltage of the cylinder-plan electrode dropped the most, and the breakdown voltage of the cone-plan electrode dropped the least. The bubbles motion was the key factor of the breakdown. The discharge types of the oil-impregnated pressboard were different with bubbles, and under DC, the main discharge type was flashover along the oil-impregnated pressboard, while under AC, the main discharge type was breakdown through the oil-impregnated pressboard.
Highlights
Mineral transformer oil is the main dielectric liquid for engineering application, and the oil and pressboard insulation system is widely used in high voltage electric power equipment manufacturing [1]
The work may be concluded as follows: In alternating current (AC) and direct current (DC), the bubbles decreased the breakdown voltage of the cylinder-plan electrodes the most among the three electrodes systems, and the mean values have dropped more than 60%, whereas the cone-plan electrodes have the least decline
That is say the bubbles have the minimum effect on the non-uniform electric field
Summary
Mineral transformer oil is the main dielectric liquid for engineering application, and the oil and pressboard insulation system is widely used in high voltage electric power equipment manufacturing [1]. High-voltage DC projects are being developed in China [2], and the AC and DC high voltages will be applied to the insulation of the converter transformer. The primary insulation of the converter transformer is oil and pressboard insulation [3]. Bubbles will decrease the electrical strength of the oil-pressboard insulation, and will cause a partial discharge in the oil, resulting in the insulation breaking down due to the accumulation of the bubbles. The bubbles are a key factor that affects the breakdown voltages of the oil-pressboard insulation [4].
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