Wet snow accumulating and shedding from overhead transmission lines can lead to a number of serviceability, safety and mechanical reliability issues. An innovative and inexpensive method to reproduce wet snow accretions on a cable in a controlled environment is explained. Wet snow sleeves were experimentally reproduced by using this technique to study their shedding mechanism. A numerical modeling technique using nonlinear finite element analysis is proposed to evaluate the dynamic response of an overhead cable subjected to any snow-shedding scenario. A time function is associated to the mass and weight of each snow element, which enables its virtual removal from the model at the time prescribed by the user. The response of a single span of overhead ground wire subjected to total and partial snow shedding scenarios is evaluated.

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No description available.

This thesis documents a research study of High Voltage transmission line faults induced by fire. Conductor to conductor and conductor to ground flashovers have been experienced by electricity utilities around the world under conditions of veld and sugar cane fires. These types of faults are unpredictable and negatively impact line reliability and quality of supply. This is a crucial problem when the revenue of the industry is sensitive to voltage dips. Electricity utilities have taken a preventative approach, like clearing vegetation from the line servitude in order to decrease the frequency of line faults. There has also been a drive to collaborate with sugarcane farmers in order to have harvesting fires planned with utilities. Some success has been achieved with these initiatives however there still remains a large number of faults. The focus of this study is on the mechanism of fire-induced flashover. Previou s work has displayed the existence of two theories. The first theory suggests that flashover is due to the reduction in air insulation strength caused by a reduced air density that results from the thermal effect of the fire. The second theory suggests that small particles present in the fire cause electric field distortions that induce flashover. This study is focused on a theory , which indicates that flashover is induced due to an enhanced electric field which is a result of the conductive properties of the flames present in the air gap (the flame conductivity theory). The effects of particles and a reduced air density is said to support this mechanism that is the primary reason for flashover. This thesis present s a summary of the literature where firstly an understanding of air insulation behavior is displayed. Thereafter specific interest is given to the effect of fire and flames wherein the physics of flames are discussed. This then leads to the description of the flame conductivity theory. Chapter 4 deals with a simulative investigation into the effect a conducting flame has on the electric field distribution. This is looked at with a varying flame conductivity and gap length in mind. The simulations specifically cover the 275 kV and 400 kV line configurations. The simulative investigation results in a mapping of electric field enhancement against conductivity values and gap sizes. Thus a flashover probability is assessed by using the two factor flashover criteria when analyzing the electric field stresses. The objective of the experimental work in this study is to obtain insight on how the flame geometry and orientation affects flashover and the dependence of flashover on gap size. Tests involving a fire beneath a conductor were carried out for different gap sizes . Experimentation with particles above a flame was also conducted. It was concluded that flame structure does have an impact on flashover since a flame with sharp edges is more likely to cause flashover. Particles have a reducing effect on air insulation strength. This is mainly due to the fact that the particle reduces the effective air-gap size. No significant effect over and above this is noticed . For gaps spanned by clean Liquid Petroleum Gas (LPG) flames flashover voltage increases as gap-length increases with some degree of nonlinearity. Flame resistances and conductivity were approximated from measured currents and voltages. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

The overhead transmission line is a fundamental component in the power supply system as it links electricity supply to the various points on the electrical network. Failure of the transmission overhead line will result in interruption of supply and depending on the network configuration may result in long term outages. It is therefore essential that the overhead transmission line asset is inspected and maintained regularly to prevent premature failure. Newer approaches to maintenance management are required to improve the overhead transmission lines performance and reduce the cost and risk associated with the asset. Asset management is seen as the process that can be adopted to enhance overall management of the overhead transmission line. The review of maintenance practices of various Utility’s and that of a pilot site made up of selected lines within Eskom’s North East Transmission Grid revealed numerous shortcomings in the current practices largely due to the application of traditional (non-holistic) methods. This situation supports the development of asset management plans which will cater for improvement in performance, reduction in the risk and cost and achieving service level targets. This research has used asset management principles to design a guideline in the form of a flowchart for effective maintenance management for overhead transmission lines. The key benefits/advantages of the maintenance management guideline are as follows: It is closed loop and process driven. Decision making is more scientific because it requires the use of historical performance data, detailed asset condition information and encourages quantitative analysis. Promotes defect and condition assessment tracking via the condition database. Rather than focusing mainly on defect management, the asset manager will be directed towards the performance specifications and the condition database to establish individual action plans which can be prioritized against short, medium and long term improvement plans per specific asset. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2010.

The South African economy is an emerging market and as such there is a continued and growing need for the efficient supply of cost effective electricity. The capital investment involved in the design, construction, installation and commissioning of overhead transmission line networks are high and so too are the subsequent maintenance and operation costs, incurred over their life cycle periods. The need to improve the electrical operating efficiency of existing and future electrical transmission networks, through the reduction of electrical losses, focused and motivated the research in this particular area. The results and findings produced by this research study show that the magnetic induction produced by the steel core in ACSR (Aluminium conductor, steel reinforced) conductors cause in increase in the ac power losses, associated ac-dc resistance ratio and the effective ac resistance of the conductor, whilst the conductor is energised during normal operation. More specifically, the key parameters that cause this increase in the effective ac resistance of the conductor, as a result of the magnetic induction produced by the steel core, are those of hysterisis and eddy current power losses in the steel core and an added power loss caused by the non-uniform redistribution of current in the layers of aluminum wires, due to the ‘transformer effect’. Therefore the addition of the conductor dc resistance value to the component resistances produced by the current redistribution and magnetic hysterisis & eddy current power losses, form the total effective ac conductor resistance. This is contrary to standard practice where assumption is made that the conductor ac and dc resistance values are equal. The factors which influence the magnetic induction, include amongst others; the ferromagnetic properties of the steel core, the physical construction of the conductor, the conductor operating/core temperature and the load current. In order to calculate the effective ac-resistance of multi-layer ACSR conductors a computer simulation program was developed, which was largely based on determining the impact of varying these key factors, by evaluating its effect on the ac resistance of the conductor. It was found through manipulation of these factors that the total effective ac resistance of the conductor could be reduced and significantly so with higher load currents. The conductor sample used in this research study is commonly known as TERN ACSR conductor in the South African market and it was shown that with practical changes in lay ratios or lay lengths, one is able to reduce the total effective ac resistance of the conductor and associated power losses. Several software simulation exercises were performed using the developed software simulation program, to ultimately produce a set of optimised lay-lengths (lay-ratios) for the TERN ACSR conductor, with the intention that these simulated parameters would be employed in the production of actual conductor samples. The intention going forward after the planned production trial runs would be to test these conductor samples to verify compliance, in meeting both electrical and mechanical performance requirements. It should be noted that the planned production trials and relevant conductor-testing processes did not form part of the scope of this research report but are processes that have been planned for in the near future. Although testing to IEC 61089 are post processes that are planned for outside of this research scope, the specification requirements of IEC61089 were incorporated into the various computer simulation exercises. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009.

Shape memory alloys (SMA’s) are a family of metals that exhibit properties of pseudo-elasticity / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009.

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Thesis (Ph. D.) -- McMaster University, 1997. / Includes bibliographical references (leaves 165-168). Also available via World Wide Web.

Overhead power lines are more reliable and cost effective than underground cables when it comes to power transmission. The overhead power lines are often suspended above ground using timber pole supports. As the placement of each support depends upon topography amongst other things the conditions for most of the supports will be different and each pole can be viewed as a unique object. This requires that each support is dimensioned individually. The purpose of the project is to clarify how the dimensioning of a support should be performed. The main objective for the project is to develop an analytical model and simulation tool which can be used to dimension timber pole supports. Octave, a scientific programming language, is used for the simulation tool and to visualize the analytical model for the timber pole supports. The analytical model is validated by a finite element analysis, which is applied using the program ABAQUS/CAE. From the analytical model nomograms, which are diagrams that relate the parameters of the support, can be constructed. A nomogram is another tool that can be used when dimensioning timber supports. Many relevant design aspects are treated in the standard SS-EN 50341 and serves as the basis for the analytical calculation model. The supports can be divided into three main types; tangential supports, where the conductors continue straight at the support, angle supports, where the conductors make an angle at the support, and terminal supports, which are used at the endpoint of the conductors. Loads taken into account are: self-weight of components, wind, ice, conductor tension and maintenance. The loads are combined using a design equation which takes into account the probability of certain events occurring simultaneously using combination factors. Partial safety factors are also used to take into account possible deviations in loads, material or geometry. The failure modes of interest are rupture due to excessive bending loads and buckling due to excessive axial loads in the support legs. In the analytical calculations the timber is considered to be isotropic in accordance with the standard while the timber is modeled both as isotropic and orthotropic in the finite element analysis. The analytical model assumes that the cross-arm, which is the horizontal part of the support, only acts to distribute the load between the legs of the support without providing any extra stability by linking them together. In the finite element analysis the legs of the support are considered both individually and linked together by a cross-arm to test the validity of the assumption. To compare the results from the analytical model with finite element analysis one support of each type; tangential, angular and terminal is selected. An Octave script is written and applied to the selected supports to calculate their necessary dimensions. Based on the calculated dimensions 3D-models are created using PTC Creo Parametric 3.0 which then are imported into ABAQUS/CAE where the supports are subjected to finite element analysis. In addition to using the analytical model directly expressions that can be used to create nomograms for timber supports are also derived. Comparing results from FEM with the analytical model showed that the tangential and angular supports were over-dimensioned while the terminal supports were under-dimensioned in the analytical model. This means that a smaller dimension could be used for the tangential and angular supports. For the terminal support the analytical model needs to be altered so that failure will not occur. Changing the buckling mode for the terminal support in the analytical solution fixed the under-dimensioning issue. The finite element calculations where a cross-arm was used to link the legs of the support together showed no increased stability for buckling but showed lower bending stresses in the support legs. Using an orthotropic material model for the timber did not effect the calculations greatly as the properties in the length direction of the pole were significantly more important than the properties in the perpendicular directions. Dimensioning using nomograms is less accurate, as it only narrows the results down to a timber pole grade rather than a specific diameter. This decrease in accuracy is not cause for concern as the timber poles are ordered by grade and not a specific diameter. Using nomograms can give quick results when it comes to dimensioning. / Luftledningar är mer tillförlitliga och kostnadseffektiva än markkablar när det kommer till kraftöverföring. För att hålla luftledningar ovan mark används ofta trästolpar. Placeringen av varje stolpe beror bland annat på topografin vilket gör att villkoren för nästan varje stolpe skiljer sig åt. Detta gör att varje stolpe måste dimensioneras individuellt. Projektets syfte är att klargöra på vilket sätt dimensionen för en stolpe ska bestämmas. Det huvudsakliga målet är att utveckla en analytisk modell som kan användas för att dimensionera trästolpar. Octave, ett vetenskapligt programspråk, används för att tillämpa den analytiska modellen på trästolparna. Den analytiska modellen valideras med finit elementanalys, som tillämpas genom användandet av ABAQUS/CAE. Från den analytiska modellen kan också nomogram konstrueras vilket är ett annat sätt att dimensionera stolpar genom att används diagram som relaterar stolparnas parametrar. Många av de relevanta designaspekterna som berör stolpar behandlas i standarden SS-EN 50341 och ligger till grund för den analytiska modellen. Stolparna kan delas in i tre huvudtyper; raklinjestolpar, där ledarna fortsätter rakt vid stolpen, vinkelstolpar, där ledarna avviker med en vinkel, och ändstolpar, som används vid avslut av ledarna. De laster som beaktas är egenvikt hos komponenter, vind, is, spänning i ledare och underhåll. För att kombinera lasterna används en designekvation som tar hänsyn till sannolikheten att vissa händelser sker samtidigt genom att applicera kombinationsfaktorer. Dessutom används partialsäkerhetsfaktorer för att kompensera för eventuella avvikelser i last, material eller geometri. Stolparna dimensioneras mot brott på grund av för hög böjande belastning och knäckning på grund av för hög axialbelastning. I de analytiska beräkningarna antas trä vara ett isotropt material i enlighet med standarden medan det modelleras både som isotropt och ortotropt vid analys med finita elementmetoden. Vidare antas att regeln, den horisontella delen av stolpen, endast fördelar lasten mellan stolpens ben utan att bidra med någon extra stabilitet genom att länka benen samman. För att testa detta antagande modelleras stolpbenen både individuellt och sammanlänkade med en regel vid analys med finita elementmetoden. För att kunna jämföra resultatet från den analytiska modellen med finit elementanalys väljs en stolpe av varje typ; raklinje-, vinkel-, och ändstolpe, ut. Ett skript skrivs i Octave och tillämpas på de utvalda stolparna för att bestämma vilka dimensioner som krävs. Baserat på de beräknade dimensionerna skapas 3D-modeller i PTC Creo Parametric 3.0 vilka sedan importeras till ABAQUS/CAE för analys med finita elementmetoden. Förutom att tillämpa den analytiska modellen direkt härleds uttryck för att kunna generera nomogram. En jämförelse av resultaten från FEM med den analytiska modellen visade att raklinje- och vinkelstolpar blev överdimensionerade medan ändstolpar blev underdimensionerade. Detta innebär att en mindre dimension kan användas för raklinje- och vinkelstolpar. För ändstolpar måste modellen korrigeras för att stolpen ska hålla. Genom att ändra knäckningsfall för ändstolpen i den analytiska modellen löstes problemet med underdimensionering. Beräkningarna i finita elementmetoden där en regel användes för att sammanlänka stolpbenen visade ingen ökad stabilitet för knäckning men böjspänningarna i stolpbenen blev lägre. Att använda en ortotrop materialmodell påverkade inte beräkningarna nämnvärt eftersom egenskaperna i längsriktningen var mycket mer signifikanta än egenskaperna i tvärriktningarna. Vid dimensionering med hjälp av nomogram fås en sämre noggrannhet, eftersom resultatet endast blir en viss stolpklass istället för en specifik diameter. Denna minskade noggrannhet är ingen anledning till oro eftersom trästolparna beställs utifrån klass och inte utifrån specifik diameter. Användandet av nomogram kan ge snabba resultat vid dimensionering.