Surface Charge Accumulation And Breakdown Mechanism Of Oil-impregnated Paper In Valve Side Bushing Of UHV Converter Transformer

Converter transformer functions as the hub connecting the direct current(DC)and alternating current(AC)power system along converter valve.The valve side bushing is subjected to DC,combined AC-DC and polarity reversal electrical field besides AC electrical field,thus suffering severe operation failure to the insulation.This thesis focused on the insulation paper of oil-paper insulation system in valve side bushing,investigated the influence of molecular structure modification(MSM)on the structure of cellulose paper,revealed the mechanism of MSM on surface charge,space charge and trap energy characteristics of the oil-impregnated paper.The main research work and achievements in the dissertation are as follows:(1)The cellulose paper was modified by the MSM set-up,and the chemical structure,element composition and physical morphology of the paper were characterized.It was found that highly polarized C-F bond was introduced to insulation paper by MSM,leading to the alternation of surface morphology.The increase in free volume and the counterbalance to dipole moment in molecular caused the reduction in the permittivity of the paper due to the introduced fluorine element by MSM.The permittivity increased with excessive treatment attribute to the introduced deep trap.(2)It was found that the surface charge dissipation of oil-impregnated paper was accelerated and trap energy density was reduced when the paper was treated by MSM.The generated fluorinated layer would capture carriers under external electrical field,which induced reverse electrical field,thus preventing charge from further injecting into the inside deep trap of the sample,leading to the reduction of total trap energy density,suppressing charge accumulation and causing acceleration of charge dissipation.The surface charge dissipation would slow down with excessive treatment due to the dominant effect of deep trap introduced by MSM.The surface charge dissipation of vegetable oil-impregnated paper was accelerated by MSM treatment,which followed similar tendency towards mineral oil-impregnated paper.The vegetable oil-impregnated paper showed faster surface charge dissipation attribute to the charge characteristics of the insulation oil.The fluorinated layer suppressed the injection of charge,leading to the reduction of space charge in oil-impregnated paper.(3)The influence of MSM on DC breakdown strength and creeping discharge characteristics of the oil-impregnated paper was tested.It was found that the improvements of trap distribution and charge dynamic characteristics contributed to the increase of breakdown strength due to MSM treatment.With increasing the paper layer,the average breakdown strength of single layer paper deceased.Due to the suppression of interface charge accumulation,the breakdown strength was improved under multi-layer papers when treated by MSM.The creeping discharge voltage increased attribute to the generation of fluorinated layer.This dissertation obtained the technical modification method for the cellulose paper applied in valve side bushing of converter transformer,establishing the firm theoretical foundation and providing the vital technical support for the development of HVDC transmission technology in the future.