Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Power utilities are expected to keep the cost of electricity as low as possible. They are also expected to be environmentally friendly and, amongst other things, not to produce unacceptable audible noise. When the electric field on a conductor is high enough corona is produced and this is accompanied by audible noise. Air pressure, which is directly related to altitude, has an effect on the voltage at which corona will start. It is more difficult to ionise the air at sea level (high air pressure) than at high altitude (low air pressure). Altitude does not only affect the corona inception voltage, but also the intensity of the audible noise. A thorough scan of literature revealed that there is very little evidence of prior research work on the effect of air density on corona under fair weather (dry) conditions. In South Africa, transmission lines are built at altitudes higher than 1800 m above sea level. The cost of a 400 kV line is in the region of R2M per km. It is important to predict the noise levels under a proposed line accurately, before it is energised. This research indicated that the altitude correction for conductor corona audible noise, under dry conditions, might be steeper than the general accepted correction of 1 dB/300m. This correction, however, appears to be valid for heavy rain conditions. Under heavy rain conditions the corona is mainly determined by the water droplets, whereas under dry conditions the condition of the conductor plays the biggest role. The air density therefore has a bigger effect on the corona performance under dry conditions. The implication of a steeper altitude correction for dry conditions is that too low noise levels will be predicted for a higher altitude, which could lead to complaints. On the other hand, predictions for lower altitudes will be too conservative. / AFRIKAANSE OPSOMMING: Daar word van kragvoorsieners verwag om die koste van elektrisiteit so laag as moontlik te hou. Hulle is verder onder druk om omgewingsvriendelik te wees en om onder andere nie onaanvaarbare hoorbare geraas te veroorsaak nie. Wanneer die elektriese veld op ‘n geleier hoog genoeg is, kan korona ontstaan wat dan hoorbare geraas veroorsaak. Die lugdruk, en daarom die hoogte bo seevlak, beïnvloed die spanning waarby ‘n geleier in korona sal gaan. Dit is moeiliker om die lug te ioniseer by seevlak (hoë lugdruk) as hoog bo seevlak (lae lugdruk). Die hoogte bo seevlak beïnvloed daarom nie net die spanning waarby korona sal begin nie maar ook die intensiteit van die hoorbare geraas. Dit wil voorkom of die effek van hoogte bo seevlak, op hoorbare geraas, a.g.v. geleier korona tot op datum baie skraps nagevors is. Baie min kon in die literatuur gevind word op die effek onder droë toestande. In Suid-Afrika is dit nodig om transmissielyne op hoogtes van 1800 m en hoër te bou. So ‘n lyn (400 kV) kos in die omgewing van R2M per km. Dit is daarom van uiterste belang om die geraasvlakke wat ‘n beplande lyn sal veroorsaak, akkuraat te bepaal, voordat so ‘n lyn aangeskakel word. Hierdie navorsing het gewys dat die effek van hoogte bo seespieël op hoorbare korona geraas onder droë toestande groter kan wees as wat algemene aanvaar word. Die helling van die korreksiefaktor vir hoogte bo seevlak blyk steiler as 1 dB/300 m te wees vir droë toestande. Die implikasie hiervan is dat geraas voorspellings vir hoër hoogtes bo seespieël te laag sal wees en die vir lae hoogtes te konserwatief kan wees. Die navorsing stem egter saam met die korreksiefaktor van 1 dB/300 m onder swaar reën toestande. Dit wil voorkom of die invloed van die waterdruppels op geleier korona groter is as lugdruk. Onder droë toestande speel die toestand van die geleier 'n groter rol en is die effek van lugdruk groter.