| The serious accidents of transformer can not only cause their own damage, but alsointerrupt the power supply, causing huge economic losses. Transformer failures causedby internal insulation aging is an important part of the transformer accident. Thepolarization/depolarization current method and frequency domain dielectricspectroscopy based on dielectric response are not only non-destructive electricaldiagnostic technique, and also the research focus of domestic and foreign scholars in thefield of transformer insulating condition assessment. Investigating new method or newcharacteristics parameters for diagnosing the transformer insulation aging and assessingits life non-destructively using dielectric response technique is difficult question to bersolved. The relationship between dielectric properties of transformer oil-paperinsulation and its space charge characteristics, the influence of insulation oil property onthe formation and migration of space charge in the aging process of oil-paper insulation,both are need to be investigated. What’s more, moisture is not only the byproduct ofoil-paper insulation aging, and also can deteriorate the property of the oil-paperinsulation. The influence of moisture on the charge distribution of oil-paper insulation isalso need to be studied. In addition, the space charge characteristics of insulatingmaterials under AC condition have not been widely carried out due to the complexity ofthe testing and the hardware restriction. There are many transformers running under theAC condition. It is a new topic to explore the influence of aging on the AC space chargecharacteristics of oil-paper insulation.In this paper, firstly, the polarization/depolarization current and frequency domaindielectric spectroscopy characteristics of transformer oil-paper insulation sample withdifferent aging degree was measured using the polarization/depolarization current andfrequency domain dielectric spectroscopy testing technology. The time domain andfrequency domain dielectric characteristic parameters which are sensitive to the agingstate of oil-paper insulation sample were investigated. The quantitative relationshipbetween time domain or frequency domain dielectric characteristic parameters and theinsulation aging status was established. The temperature effect on thepolarization/depolarization current and frequency domain dielectric characteristics oftransformer oil-paper insulation was also investigated. Secondly, the DC space chargeSupported by: The National Basic Research Program of China (973Program)(2009CB724505-1). characteristics of oil-paper insulation samples impregnated with oil aged for differenttimes and oil-paper insulation samples with different moisture content were measuredby pulse electro-acoustic (PEA) method. The influence of oil aging and moisture on thespace charge dynamic behavior of oil-paper insulation was analyzed respectively. Lastly,space charge characteristics of oil paper insulation with different aging state under thesinusoidal (50Hz) AC were studied using the improved pulse electro-acoustic method.The dynamic behavior of space charge for oil-paper insulation under sinusoidal ACcondition was analyzed. The main innovative achievements of this paper are asfollowing:(1) The depolarization charge quantity of oil-paper insulation samples calculatedusing their Debye circuit parameters is very sensitive to their aging condition. There isan exponential relation between the “stable depolarization charge quantity” of oil-paperinsulation sample and the degree of polymerization (DP) of paper. The “stabledepolarization charge quantity” can be used to predict the aging condition of oil-paperinsulation.(2) The frequency domain dielectric characteristics parameters of the transformeroil-paper insulation in the low frequency band are sensitive to the insulation aging state.The ε ’rand tanδ values at10-3Hz,10-2Hz and10-1Hz of aged oil impregnatedpressboards have an exponential function with the DP. The frequency domain dielectriccharacteristic quantity (ε ’rand tanδ) of the oil-paper insulation sample in10-3-10-1Hzcan reflect the aging status of oil impregnated insulation paper.(3) The ε ’rand tanδ values at10-3Hz,10-2Hz and10-1Hz of aged oil impregnatedpressboards increase in an exponential way with tepmperature. The“frequency-temperature shift” method could be used to impute the frequency domaindielectric characteristics parameters at different test temperatures to the referencetemperature. In this way, the temperature effect on the frequency domain dielectriccharacteristics parameters can be resolved. In addition, the “frequency-temperature shiftfactor” increases exponentially with temperature.(4) The effect of thermally aged oil and moisture on DC space charge dynamics inoil-paper insulation system had been investigated using the PEA technique respectively.Compared with the space charge dynamics result of oil-paper insulation thermal aging,it is founded that the influence of insulating oil properties and moisture on the DC spacecharge of multi-layer oil-paper insulation is significant. The total amount of charge, thesurface trap energy density and the electric field distortion of the oil-paper sample increase with the oil aging degree. The higher moisture content of the oil-paper sampleis helpful to the charge injection and migration. The higher mobility of the trappedcharges leads to less slow moving charges trapped in the sample with higher moisturecontent.(5) Space charge characteristics of oil-paper insulation with different aging stateunder the sinusoidal AC were studied using the improved PEA system. The amount ofspace charge accumulated in the oil-paper insulation sample under sinusoidal ACcondition is significantly reduced compared with that under DC condition. The chargepolarity is opposite at the electrode position for two phases where the phase differencebetween them is180o. In the process of AC electric field applied, the space chargeaccumulated at the paper-paper interface under the phase90oand270omigrates to theopposite electrode with the aging conducted. |
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