Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Prospects of up-rating existing high voltage direct current (HVDC) transmission schemes, as well as the conversion of existing alternating current (AC) to direct current (DC) lines and the development of new HVDC schemes in sub-Saharan Africa, have led to renewed interest in DC research. The radio interference (RI), audible noise (AN) and corona loss (CL) performance of HVDC transmission lines are critical factors when assessing the reliability of the line design. The RI performance is especially important when considering the successful transmission of the carrier signal of the power line carrier (PLC) system. The PLC system is the main form of communication between teleprotection devices on the Cahora Bassa HVDC scheme. The aim of the dissertation is to devise modelling as well as metrological techniques to characterise DC conductor corona. A particle-in-cell (PIC) computational code is developed to gain a better understanding of the physical processes that occur during corona events. The numerical code makes use of the charge simulation method (CSM) and nite element method (FEM) to solve for the Laplace and Poisson eld equations. Higher-order basis functions are implemented to obtain a more accurate solution to the Poisson equation. The computational tool yields insight into the mathematical models for the various ionization, attachment and electron avalanche processes that give rise to corona currents. Together with a designed and developed electrometer-type circuit, the numerical code assists the visualisation of the space charge particle dynamics that form in the electrode gap during corona events. The metrological techniques consider the wideband time domain (TD) as well as the frequency domain (FD) information of the measured corona pulses in the presence of noise. These are then compared to the narrowband CISPR standard measurements centred around 500kHz. The importance of impedance matching when attempting to derive a wideband excitation function is investigated. The TD measurements are quite distinct from the well-published FD measurements, and consider the pulse shape, pulse spectrum and pulse repetition rates. The use of three possible conductor corona test methods to study direct current conductor RI performance under both positive and negative polarities is investigated at high altitude in this dissertation. These include a small corona cage, a short test line and the Eskom Megawatt Park large outdoor corona cage. Derived wideband and narrowband monopolar DC RI excitation functions at 500kHz are consolidated with existing radio noise (RN) measurement protocols and prediction methods. The use of a corona cage to derive excitation functions for monopolar RI predictions is explored and it is shown that a small corona cage, due to the build-up of space charge in the small distance between the electrodes, cannot be used to predict the RI levels on HVDC transmission lines accurately. As a consequence of the physics, computational modelling and both frequency and time domain measurements, it is now possible to explain why a small cage system prevents the accurate RI prediction on transmission lines. The large outdoor corona cage and short test line RI performance predictions agree with existing empirical prediction formulas. / AFRIKAANSE OPSOMMING: Vooruitsigte van die opgradering van bestaande hoogspanningsgelykstroom transmissielyn skemas, asook die omkering van bestaande wisselstroom na gelykstroom lyne en die ontwikkeling van nuwe hoogspanningsgelykstroom skemas in sub-Sahara Afrika, het gelei to hernude belangstelling in gelykstroomnavorsing. Die korona-werkverrigting van hoogspanningsgelykstroom oorshoofselyne in terme van radiosteuring, hoorbare-geraas en koronaverliese is kritiese faktore om in aanmerking te neem wanneer die betroubaarheid van die lynontwerp geëvalueer word. Die radiosteuring-werkverrigting is veral van belang tot die suksesvolle oordrag van die kraglyndragolf draersein wat die hoof kommunikasievorm tussen beskermingstoerusting op die Cahora Bassa transmissielyn skema is. Die doel van hierdie proefskrif is om modellering- sowel as meettegnieke te ontwerp om gelykstroomgeleierkorona te karakteriseer. 'n Partikel-in-sel numeriese kode is ontwikkel om 'n beter begrip te verkry van die siese prosesse gedurende koronagebeure. Die numeriese kode maak gebruik van die lading-simulasiemetode, sowel as die eindige element metode om die Laplace en Poisson veldvergelykings op te los. Hoër-orde basisfunksies is geimplimenteer om 'n meer akkurate oplossing vir die Poisson vergelyking te verkry. Die numeriese kode bied insig tot die wiskundige modelle vir die verskeie ionisasie-, aanhegtings- en lawineprosesse wat lei tot koronastrome in die area om die hoogspanningsgeleier. Die numeriese kode, saam met 'n elektro-meter wat ontwerp en ontwikkel is, dra by tot die begrip van die ruimtelading partikeldinamika wat onstaan in die elektrodegaping gedurende koronagebeure. Die meettegnieke neem die wyeband tydgebied- en frekwensiegebiedinformasie van die koronapulse in ag in die teenwoordigheid van geraas. Dit word dan vergelyk met die nouband CISPR meetstandaard vir 'n frekwensie van 500kHz. Die belangrikheid van impedansie-aanpassing vir wyeband metings met die doel om opwekkingsfunksies af te lei, word ondersoek. Die tydgebiedmetings verskil van die algemene frekwensiegebiedmetings, en ondersoek die pulsvorm, -spektrum en -herhalingskoers. Die gebruik van drie moontlike koronageleier-toetsmetodes om gelykstroom radiosteurings-werkverrigting vir positiewe en negatiewe polariteite te bestudeer by hoë vlakke bo seespieël word ondersoek in die proefskrif. Dit sluit in 'n klein koronakou, 'n kort toetslyn en die Eskom Megawatt Park groot buitelug-koronakou. Afgeleide wye- en nouband monopolêre gelykstroom radiosteuring opwekkingsfunksies by 500kHz word gekonsolideer met bestaande radioruis metingsprotokolle en voorspellingsmetodes. Die gebruik van 'n koronakou om opwekkingsfunksies af te lei vir monopolêre radiosteuringvoorspellings is ondersoek en daar is gevind dat 'n klein koronakou nie gebruik kan word om radiosteuringvlakke op hoogspanningsgelykstroom transmissielyne akkuraat te voorspel nie. Dit is as gevolg van die opbou van ruimtelading in die klein elektrodegaping. Met behulp van die sika, numeriese modellering en beide die frekwensie- en tydgebiedmetings, is dit nou moontlik om te verklaar waarom die klein koronakou die akkurate radiosteuringvoorspellings op transmissielyne onmoontlik maak. Die groot buitelug-koronakou en kort toetslyn radiosteuringvoorspellings stem ooreen met bestaande empiriese voorspellings formules.

High voltage (EHV and UHV) transmission facilitates transfer of large amount of power over long distances. However, due to the inherent geometry, the line and substation hardware of EHV and UHV class generate high electric fields, which results in local ionisation of air called corona discharges. Apart from producing audible noise in the form of frying or hissing sound, corona produces significant electromagnetic interferences in the radio range. The limit for this corona generated Radio Interference (RI) has been stipulated by international standards, which are strictly to be followed. In line and substation hardware, corona control rings are generally employed to limit or avoid corona. Standard dimensions of corona rings are not available for EHV and UHV class. In most of the cases, their design is based on either a trial and error method or based on empirical extrapolation. Only in certain specific cases, the dimensioning of the rings is carried out using electric field calculations. In any of these approaches, the unavoidable surface abrasions, which can lead to corona, are not considered. There are also efforts to account for nominal surface irregularity by using a surface roughness factor, which is highly heuristic. In order to address this practically relevant problem, the present work was taken up. The intended exercise requires accurate field computation and a suitable criterion for checking corona onset. For the first part, the Surface Charge Simulation Method is adopted with newly proposed sub-modelling technique. The surface of the toroid is discretised into curvilinear patches with linear approximation for the surface charge density. Owing to its high accuracy, Galerkin’s method of moments formulation is employed. The problem of singularity encountered in the numerical approach is handled using a method based on Duffy’s transformation. The developed codes have also been validated with standard geometries. After a survey of relevant literature the ‘Critical Avalanche Criteria’ is chosen for its simplicity and applicability to the problem. Through a detailed simulation, the effect of avalanche space charge in reducing the corona onset voltage is found to be around 1.5% and hence it is not considered further. For utilities not interested in a detailed calculation procedure for dimensioning of corona rings, design curves are developed for circular corona rings of both 400 kV and 765 kV class with surface roughness factor in the range 0.8 – 1. In the second part of the work, a methodology for dimensioning is developed wherein the inevitable surface abrasion in the form of minute protrusions can be accounted. It is first shown that even though considerable field intensification occurs at the protrusions, such localised modification need not lead to corona. It is shown that by varying the minor radius of the corona ring, it is possible to get a design where the prescribed surface abrasion does not lead to corona onset. In summary, the present work has successfully developed a reliable methodology for the design of corona rings with prescribed surface abrasions. It involved development of an efficient field computation technique for handling minute surface protrusions and use of appropriate criteria for assessing corona inception. It has also provided design curves for EHV and UHV class corona rings with surface roughness factor specified in the range 0.8 – 1.0.