This paper conducts experimental research and theoretical analysis on the effect of cellulose particle motion on impulse breakdown in transformer oil. In the experiment, the spherically capped electrodes immersed in the bulk of the transformer oil are used to form a quasi-uniform electric field. The breakdown tests are carried out and the breakdown voltages for three cellulose contamination levels oil under four types of impulses, 1.4/50μs, 14/220μs, 80/850 μs and 240/2400 μs, are obtained. The experimental results show that both the impulse waveshape and cellulose contamination level have an effect on the breakdown of transformer oil. Longer impulse duration or higher cellulose contamination level of transformer oil can lead to the decrease of impulse breakdown voltage. Specifically, the effect of the high level cellulose contamination for U50% becomes much more significant under long-duration impulses. To verify the findings above, a numerical model is introduced for the cellulose particle dynamics in the transformer oil under these four kinds of impulses. Notably, the calculation result indicates the particles can accelerate with the action of the impulse, and particles can aggregate in concentrated area of the electric field in the oil gap with the action of the long-duration impulse. It is concluded that the particle motion can play a catalytic role in the impulse breakdown of the oil gap.

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.

Due to the polar nature of natural ester, moisture content which may cause deterioration of electric strength is significantly higher in natural ester than that in mineral insulating oil. While the polarity of cellulose particles is stronger in terms of molecular structure than that of the natural ester, the breakdown voltage would be changed with increasing moisture and the cellulose particles. In light of this, the paper presents experimental studies on the effect of cellulose particle on breakdown voltage in wet FR3 natural ester. The DC breakdown tests are conducted and the breakdown voltages for three particle contamination levels, normal, marginal and high are investigated with five moisture content intervals, i.e., less than 100 mg/L, 100-200 mg/L, 200-300 mg/L, 300-400 mg/L and 400-500 mg/L respectively. The results show that the increase of cellulose particle contamination significantly accelerates the formation of cellulose bridge. Both the cellulose particles and moisture content have notable effects on the DC breakdown voltages. Higher cellulose contamination level in FR3 can lead to the decrease of DC breakdown voltage and more remarkable effect can be observed under the combined effect of the moisture. Analyses based on the relative dielectric constant and DC resistivity of the natural ester indicate that the reduction of the natural ester dielectric strength may be mainly attributed to the sharp decrease of the DC resistivity induced by the cellulose particles and the moisture. Also, the comparison of breakdown results between the DC and AC suggests the combined effect of particle and moisture in the natural ester breakdown behaviors.

This study explores the dynamics of gas bubbles in liquid dielectric theoretically and experimentally. A numerical model is introduced for the bubble motion in the transformer oil under non‐uniform electric field. Moreover, the bubble trajectory is obtained through the force calculation of the bubble under the non‐uniform electric field. The numerical calculations for different bubble sizes and applied voltages are also done. Notably, trajectories calculated indicate the bubble aggregation in concentrated area of the electric field between the needle‐plane electrodes. It is concluded that the bubble can play a catalytic role in the breakdown of the oil gap. An experiment is finally conducted on the bubble motion in transformer oil gap submitted to a high divergent electric field and the suspension and aggregation of bubbles are observed before the oil gap breakdown, which confirms the results of numerical calculation.

Due to rapid temperature rise and insulation moisture, bubbles can generate in the oil-paper insulation of oil-immersed power transformers, which can reduce the dielectric strength of oil-paper insulation and even bring about the risk of insulation breakdown. In order to accurately evaluate the conditions of bubble formation, the physical process of bubble formation in oil-paper insulation is studied in this article. Considering the effects of water vaporization, gas dissolution in oil, and moisture migration on bubble formation, a bubble formation prediction model is constructed. Based on Henry’s law, the dissolution equilibrium of gas in oil is taken into consideration and the upper boundary of the initial temperature of bubble effect (ITBE) envelope is obtained in this model. Based on the moisture equilibrium in oil-paper insulation, the bubble formation process caused by the emergence and rapid evaporation of free water is analyzed, and then, the lower boundary of the ITBE envelope is obtained in this model. The model results show that the moisture content of the pressboard and the presence of free water have a great impact on ITBE. Specifically, the higher the moisture content, the lower the ITBE value. In addition, with the further increase of moisture content, the upper and lower boundaries of the ITBE envelope gradually tend to be the same. However, once free water is present on the surface of cellulose, ITBE decreases rapidly to below 120 °C. The model built in this article is of great significance to study the temperature limit of the power transformer. Hence the ITBE envelope can provide an important theoretical reference for reducing the risk of bubble formation during power transformer operation.

The rapid temperature rise brought by short-term overload may cause the oil-paper insulation of transformer to produce bubbles. The existence of bubbles will reduce the electrical strength of transformer insulation, and may lead to partial discharge or even breakdown of the oil-paper insulation in the transformer. In this paper, the bubble effect of oil-paper insulation in natural ester is studied experimentally. A bubble effect test platform is designed and built according to the temperature conditions of the transformer in operation, and bubbling tests are carried out for natural ester cellulose insulation. Initial temperature of bubble escape (ITBE) is obtained for different winding insulation paper layers, temperature rise rate, and pressboard aging time respectively. The experimental results show that bubbles aggregate in the gap between the paper layers before bubbles escape into oil and ITBE is affected by winding insulation paper layers. Besides, a higher temperature rise rate corresponds to a lower ITBE. With the thermal aging of pressboard, ITBE increases because the number and diameter of pores decrease in fibers of pressboard.