Electrical properties of thermally aged polyethylenes and of polyethylenes containing voltage stabilisers

Sammanfattning: The use of polyethylene (PE) in electrical applications athigh voltages requires a certain resistance of the material topartial discharges (PDs). The space charge behaviour of PE isof great importance when PE is used as insulator inhigh-voltage DC cables. The long service lifetime ofhigh-voltage equipment makes the long-term properties of PEvery important. In this thesis the PD resistance was assessedby PD lifetime measurements and the space charge behaviour bypulsed electro-acoustic (PEA) measurements. The long-termproperties were assessed by accelerated ageing at elevatedtemperatures.The PD lifetime of a wide range of thermoplastic andcrosslinked PEs - including compounds containing aromaticvoltage stabilisers (fluoranthene, naphthalene, pyrene,o-terphenyl, vinylnaphthalene and anthracene) - were studied.The PD lifetime was essentially independent of molar mass,degree of crosslinking, crystallinity and crystal thickness. Avery brittle, monodisperse, low molar mass sample deviated byshowing a reduced PD lifetime. The added voltage stabilisershad no effect on the PD lifetime.Films of a range of PEs with different degrees ofcrystallinity were aged in air at 100ºC, 115ºC and130ºC for periods up to 28 500 hours, and subsequentlytheir PD lifetime and space charge behaviour were measured. Thefilms were characterised by stress-strain measurements,differential scanning calorimetry, infrared spectroscopy andsize exclusion chromatography.Mere oxidation was not sufficient to cause a decrease in PDlifetime, unless the carbonyl content exceeded a certainthreshold value above which the PD lifetime decreased rapidlywith increasing concentration of carbonyl groups. It issuggested that the carbonylic groups enhance radical formationand thereby accelerate surface erosion, causing early PDbreakdown. The PD lifetime and the space charge behaviour werenot influenced by the effective antioxidant concentration.Oxidation facilitated charge injection and hetero-chargeformation, and the presence of polar groups in the polymermolecules decreased the mobility of the charges. The resultsalso indicate that the transport of the charges occurredprimarily in the non-crystalline components and that thepresence of large crystallites reduced the "migration" rate ofthe charges. Charge packages were formed at the cathode for oneoxidised high-voltage PE grade. It is suggested that thesepackages are due to charge separation and faster migration ofthe negative charges than of the positive charges. It is alsosuggested that volatile species cause the charge packagegeneration.Keywords:polyethylene, voltage stabilisers, thermaloxidation, carbonyl group content, carbonyl containing modelmaterials, partial discharges, corona, space charges, pulsedelectro-acoustic method (PEA).