| Mixed insulation oil is made by mixing mineral oil,vegetable oil and synthetic ester,which can complement the performance advantages of single oil product.Based on the previous research on 2-element mixed insulation oil,our team successfully developed a novel 3-elememt mixed insulating oil that meets the requirements of GB2536-2011,and the formula is 76 vol.%Mineral oil+19 vol.%Soybean oil+5 vol.%Palm oil.This mixed insulation oil makes up for the high dielectric loss and kinematic viscosity of the traditional 2-element mixed insulation oil.Compared with mineral oil,the 3-element mixed insulation oil has higher power frequency breakdown voltage and better fire resistance,and can significantly delay the thermal-aging process of oil-paper insulation.At present,the 3-element mixed insulation oil has been used in 10 k V distribution transformer.In order to support the safe operation and effective maintenance of the mixed oil transformer,this paper focuses on the partial discharge,surface flashover,lightning breakdown and gas production characteristics of the3-element mixed insulation oil-paper composite system.The contents of this paper mainly contain the following aspects:（1）According to the typical defect models of partial discharge,the difference between the 3-element mixed insulation oil-paper composite system and mineral insulation oil-paper composite system were studied in corona discharge and air-gap discharge,through experimental phenomena,discharge patterns and basic discharge parameters.Among the two discharge models,the initial discharge voltage and minimum discharge phase of the 3-element mixed insulation oil-paper composite system are slightly lower than those of mineral insulation oil-paper composite system,while has higher discharge pulse magnitude and number.（2）The surface flashover characteristics of the 3-element mixed insulation oil-paper composite system are studied under needle-plate electrode structure and finger electrode structure.Compared with mineral insulation oil-paper composite system,the three-element mixed insulation oil-paper composite system has higher flashover voltages at different electrode distances,which are 7.06%and 5.93%higher than mineral oil-impregnated pressboard under extremely non-uniform electric field and slightly uneven electric field.Higher breakdown voltage of oil,smaller electric field intensity and more evenly distributed are the main reasons for advantages of the3-element mixed insulation oil-paper composite system.After the same times of surface flashover occurred,the surface damage degree of the 3-element mixed insulation oil-impregnated pressboard is less than that of the mineral insulation oil-impregnated pressboard.（3）The lightning impulse discharge characteristics of the three-element mixed insulation oil-paper composite system were studied under uneven electric field.Compared with mineral insulation oil,the difference on breakdown voltage between the novel three-element insulation oil and mineral insulation oil is small under positive polarity lightning impulse voltage,but its breakdown voltage under negative polarities is significantly lower than mineral insulation oil,which is only 43.04%-71.86%of mineral oil at 5-25 mm oil gap.The discharge propagation velocity of the 3-element insulation oil is less than that of mineral oil.which are 24.81%and 19.35%lower than that of mineral oil under positive and negative polarities at 25mm oil gap,respectively.Two types of insulating oil-impregnated pressboards have small difference on lightning impulse breakdown voltage.As the thickness of the pressboard increases,the advantage on breakdown voltage of the 3-element insulation oil-impregnated pressboard gradually appeared.When the thickness is 1.5 mm,the lightning impulse breakdown voltages of the 3-element insulation oil-impregnated pressboard under positive and negative polarities is 2.52%and 4.13%higher than that of mineral insulating oil-impregnated pressboard.（4）After the discharge fault,the contents of H2 and hydrocarbon gases in the three-element insulation oil is slightly lower than those of mineral insulating oil.The contents of H2 and CH4 are the highest in partial discharge fault.For flashover fault of oil-paper interface and breakdown fault of insulating oil-paper composite system,the contents of H2 and C2H2 are changed significantly before and after faults.Besides,the contents of some hydrocarbon gases,such as CH4 and C2H4,are also various under different discharge faults.The characteristics of dissolved gas in oil are comprehensive analyzed under various typical discharge faults.Taking the percentage contents of C2H2,H2 and C2H4 as the characteristic quantities,the boundary conditions of typical discharge faults are determined,and a triangle diagram diagnostic method for the three-element mixed insulation oil transformer is also proposed. |
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