Charge Accumulation And Flashover Of Oilimpregnated Paper For Valve Side Bushing Of Converter Transformer Under Complicated Operating Conditions

Converter transformer is the vital equipment of high voltage direct current(HVDC)transmission system.Oil-impregnated paper insulation of valve side bushing is subject to the synthetic action of complex field containing DC superimposed harmonic and impulse under actual operating condition,which causes insulation failure easily.For researching charge accumulation and flashover characteristics of oil-impregnated paper under the electric field of DC superimposed harmonic and impulse,charge transport properties of oil-impregnated paper under the circumstance of moisture and temperature rise were analyzed,the mechanism of superimposed electric field on charge accumulation and flashover of oil-impregnated paper was researched.Afterwards the surface modification method of insulating paper was proposed,which is aim to provide theoretical and experimental basis for insulation design of converter transformer.The results of this dissertation are as followed:(1)The charge transport properties and flashover characteristics of oil-impregnated paper were measured in the electric field of DC superimposed harmonic.The results demonstrated that harmonic component has capacity of increasing charge accumulation on the surface of oil-impregnated paper,which makes the electric field distortion of specimen intensified and causes the decrease of flashover voltage with the increase of harmonic amplitude.In addition,it is found that at the initial stage of thermal aging of oil-impregnated paper,the discrepancy of deep trap is more obvious under the electric field of DC superimposed harmonic in different proportion,which aggravates the effect of harmonic on surface charge accumulation.With the aggravation of thermal aging of oil-impregnated paper,the number of carbonyl group in cellulose increases and then the energy level of deep trap increases,which weakens the surface charge accumulation under the effect of harmonic.Besides,it is also found that the density of deep trap of oil-impregnated paper is greatly different under the electric field of DC superimposed harmonic of different times when the moisture content of specimen is low.Hence,the surface charge accumulation is greatly affected by harmonic.With the increase of moisture content,the density of shallow trap of oil-impregnated paper increases,which weakens the effect of harmonic on surface charge accumulation.(2)The charge transport properties and flashover characteristics of oil-impregnated paper under electric field of DC superimposed impulse were mearsured.The results demonstrated that as the spicemen temperature increased,the surface charge dissipation of oil-impregnated paper becomes faster when the DC voltage is implemented,which makes the charge accumulation decreases and leads to the phenomenon that the more homopolar charges are injected to the surface of oil-impregnted paper when the DC superimposed impulse voltage is implemented.Hence the trend that surface charge accumulation is aggravated with the increase of homopolar impulse voltage is more obvious,which means the effect of impulse on charge accumulation is intensified.When the heteropolar impulse amplitude is no larger than the DC voltage,surface charge accumulation increases with the increase of the impulse amplitude.In addition,when the magnitude of the heteropolar impulse is greater than DC voltage,surface charge accumulation tends to rise firstly and then fall with the increase of the impulse amplitude,so that the flashover voltage under the electric field of DC superimposed heteropolar impulse is higher than that under the circumstance of homopolar impulse.Afterwards,the flashover voltage under the electric field of DC superimposed impulse decreases with the increase of the DC voltage.(3)In order to suppress the charge accumulation of oil-impregnated paper,surface molecular structure modification was implemented.The results demonstrated that the hydroxyl groups reduced in the cellulose molecular structure of the modified insulating paper,meanwhile the hydrogen atoms is replaced by fluorine atoms to form carbonfluorine bonds.Besides,the modified oil-impregnated paper has the function of binding electrons.Under the electric field of DC superimposed harmonics and impulse,the electrons are trapped by the fluorinated layer to form a shield,which prevents the charge from being injected into the specimen so as to avoid being captured by the deep trap inside the specimen.Hence,the surface charge dissipates faster and charge accumulation reduces.Moreover,it is found that the energy level distribution of the modified specimens alters little under different complex electric fields,so that the discrepancy of surface charge accumulation is small.In the meantime,the effect of harmonic and impulse on charge accumulation is weakened,which contributes to improving the flashover resistance of oil-impregnated paper.