Research Of The Effect Of Bubbles And Cellulose Particles On Impulse Breakdown In Transformer Oil

The transformer in field will be subjected to a variety of impulse overvoltage,which is taken as one of the main reasons for the insulation failure of the transformer.Notably, the existence of bubbles and cellulose particles will reduce the insulation performance of transformer oil. Given this, the effect research of bubbles and cellulose particles on impulse breakdown in transformer oil are conclusive to understanding the mechanism of breakdown and obtaining the breakdown characteristics of transformer oil under the impulses.This thesis mainly studies the effect of bubbles and cellulose particles on impulse breakdown in transformer oil. Here come the major contents.First of all, the effect of the impulse waveform on breakdown characteristic in transformer oil is studied. A set of impulse generator is designed and built based on the solid state switch. Consequently, a variety of impulses with different waveforms can be generated and can meet the requirement of the tests. The breakdown tests are conducted and the breakdown voltage and breakdown time delay for the spherically capped electrodes immersed in the bulk of the transformer oil under four types of impulse, 1.4/50 ?s, 14/220 ?s, 80/850 ?s and 240/2400 ?s, are obtained.Secondly,the effect of the cellulose particles on impulse breakdown in transformer oil is studied. First, the breakdown tests are carried out and the breakdown voltages for three cellulose contamination levels oil under four types of impulse are obtained. Then a relevant experiment is done on cellulose particles motion in transformer oil gap submitted to a divergent electric field, and the suspension and aggregation of cellulose particles are observed under different impulses and AC voltage. The effect mechanism of transformer oil breakdown characteristic is explained under different impulse waveforms and for different cellulose particles levels. A numerical model is introduced for the cellulose particle dynamics in the transformer oil under these four kinds of impulses. The effects on the cellulose particle dynamics of impulse waveform and the particle size are compared.The electric field distribution in the oil gap is calculated in the process of cellulose particles motion under the impulses. The dynamics of the cellulose particle under the impulse reveals the mechanism of the more serious deterioration for the breakdown strength of transformer oil with high contamination level under the long-duration of impulse.Thirdly, the bubble generating mechanism in oil-paper insulation of transformer is explored. A simulated experiment of bubbles generation from the oil-immersed pressboard with the moisture content of 2%, 4% and 6% is conducted in the process of a steep temperature gradient occurs. The starting temperature for the bubble formation and the existence state of the bubble are recorded. The effect of the bubble on breakdown in transformer oil is studied through the breakdown tests under four types of impulse. Additionally, another experiment is performed on bubble motion in transformer oil gap submitted to a divergent electric field and the dynamics of the bubble are observed under different impulses and DC voltage. A numerical model is introduced for the bubble dynamics in the transformer oil under the impulses of 1.4/50 ?s and 240/2400 ?s. The effects on the bubble dynamics of impulse waveform and the bubble size are compared. The electric field distribution in the oil gap is calculated during bubble motion under the impulses. The dynamics of the bubble suggests the mechanism of the more serious deterioration for the breakdown strength of transformer oil under impulse. Analysis and discussion about the effect of the bubble generation and dynamics on the transformer oil breakdown characteristic has been made thoroughly.