| Due to its superior electrical, chemical and mechanical properties as the insulationmaterial, low density polyethylene (LDPE) is widely used in DC high voltage cablepower transmission equipment, electrical and electronic equipment in aerospace andnuclear industries. However, under long-term effects of such factors as high fieldstrength, a large amount of radiation and strong ultraviolet radiation in aerospace andnuclear industries, the insulation performance of polyethylene materials will declinewith physical or chemical changes (aging) inside them. In recent years, with theadvances of space charge measurement technology, the phenomenon of charge trappingand detrapping has drawn wide attention. It has been demonstrated that the space chargephenomenon may be closely related to the aging taking place in the material. Therefore,revealing the association between the space charge trapping and detrapping andmicrocosmic chemical changes of material aging is the key to deeply understanding theaging mechanism of polymer materials under various conditions. However, bothtrapping and de-trapping in solid dielectrics are complex topics as they arerelated tomicrostructure of the material. Further studies are required before the relationshipbetween morphology and traps can be established. Therefore, it is of importantacademic value and application prospect to study space charge characteristics and trapfeatures of polyethylene materials under the conditions of gamma and ultravioletirradiation, for understanding the assocication between microstructure changes ofmaterial aging and macroscopic electrical properties.On the basis of measuring space charge with an improved pulsed electro-acousticmethod, this paper puts forward a multilevel trap model and calculation method of trapparameters, accelerated aging experiments with LDPE exposed to γ-Ray andultraviolet respectively were carried out to maintain space charge characteristics of,trap parameters of LDPE were extracted with the multilevel trap model, at the sametime, combined with infrared spectroscopic analysis, breakdown field strength anddielectric spectrum, the relationships between micro chemical changes of the materialand trap parameters were analyzed. This paper achieves following results:①An AC space charge measuring system of PEA method was improved. Spacecharge signals and high voltage AC signals were collected simultaneously by usingsequential acquisition model of a digital oscilloscope with long storage depth. Space charge signals of different phases were acquired by recognition zero points of ACsignals. The improved measuring system maintains higher recognition rate of signalphases, shorter measuring time and smaller error. The application of this improved PEAmeasuring system can be extended to occasion with applied voltage of higherfrequencies and arbitrary waveforms.②A space charge trap model based on multilevel trap distribution is proposed andtrap parameters were calculated on the basis of the model, furthermore, trapcharacteristics of LDPE aged with DC electric field was analyzed. The Multilevel trapmodel accurately reflects the trapping and detrapping process. The space charge data ofLDPE aged by DC electric field in air condition and calculation of trap parametersreveal that: DC electrical aging introduces traps of higher level, the trap levels rangefrom0.8to1.1eV, and trap level related to trap density peaks increases with aging time.③Space charge characteristics of LDPE material aged withγ-Ray and ultravioletwere studied. The results show that hetrocharge was found inside LDPE materials thathad been aged byγ-Ray and ultraviolet in air environment, but accumulation and decayof space charge was slowed. Total charge accumulated within LDPE material increasesbecause of shallow traps that are brought in by aging, and the amount of total chargeincreases with aging proceeds.④On the basis of comprehensive analysis of trap parameters of LDPE materialsaged by γ-Ray and ultraviolet, this paper puts forward a characteristic representationmethod of material aging with changes of curve family that is constituted by curves oftrap level and trap density. The bigger the size of the curves (total trap density) is, themore the martial is aged; the shift of curve family (the shift of peak value of trap density)along axis stands for trap distribution of materials that are aged under differentconditions: for materials aged byγ-Ray and ultraviolet in air environment, the peakvalues of their trap density increase as aging of material proceeds, while the trap levelrelating to peak value varies with different aging conditions. |
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