Research On The Characteristics Of Discharge In Vegetable Oil-paper Insulation Under Lightning Impulse

Vegetable insulating oil is a kind of liquid dielectric insulation,it can meet the demand of green power grid.It has been widely used in power distribution transformer,the current is continuously to large capacity,high voltage and power transformer in the promotion.Oil-paper insulation is the main insulation form in power transformer,power transformer has large size and complex electric field distribution.Based on power transformer insulation structure design and manufacture and to ensure the safe and stable operation of power system,it is necessary to research on the characteristics of discharge in vegetable oil-paper insulation under lightning impulse,the research content mainly includes:(1)The test platform of vegetable oil-paper insulation under lightning impulse was set up to meet the long gap discharge test,vegetable oil-paper insulation characteristics of lightning impulse breakdown test methods was compared,lightning impulse breakdown characteristic of vegetable insulating oil was carried.The results show that the methods of Rising-voltage,Up-and-Down and Multiple-level method can be used in studying the characteristics of oil paper insulation breakdown,take experimental dimension and data reliability into consideration,Rising-voltage is the optimal method to measure lightning impulse breakdown.Lightning impulse breakdown voltage of the three kinds of vegetable insulating oil and mineral insulating oil will improve along with the increase of oil gap,and under the long oil gap distance,the difference will be bigger.(2)Based on the shadow imaging principle and the streamer development record system,lightning impulse streamer development characteristics of vegetable insulating oil was studied as well to extract the basic shape,stopping length and average velocity.Results show that the basic streamer patterns can be classified into four basic forms: filaments,multi-branched filaments,shrubs,and dendrites.The variation of streamer stopping axial length of the three vegetable oils is roughly the same,and the same applied voltage are higher than mineral oil.The streamer average velocity of three vegetable oils have a similar pattern,and the accelerating voltage of vegetable insulating oils are close to its breakdown voltage,while the accelerating voltage of mineral insulating oil are generally higher than its breakdown voltage.(3)Accumulation effect of lots of lightning impulse on vegetable oil-paper insulation was carried out,accumulation effect of the multiple lightning impulse on V-N characteristics of vegetable oil-paper insulation was analysis and accumulation effect of the multiple lightning impulse on dielectric properties and mechanical properties of vegetable oil-paper insulation was studies as well.Results show that the V-N characteristics of vegetable oil-paper insulation accord with double parameters Weibull probability distribution mathematical model.Lightning impulse accumulation of vegetable oil-paper insulation lead the real component of relative dielectric constant increase,the imaginary part of relative dielectric constant and dielectric loss also have the same change with the increase of cumulative times.SEM results revealed that part cellulose molecules of the vegetable oil impregnated paper have sag and pore fracture,it means that multiple lightning impulse accumulation will damage the vegetable oil-paper insulation.