Browsing by Author "Muftic, Dzevad."

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Design optimisation of bare conductors for overhead line applications.
(2009) Munilall, Anandran.; Ijumba, Nelson Mutatina.; Muftic, Dzevad.
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.
Dynamic characteristics of bare conductors.
(2011) Ojo, Evans Eshiemogie.; Ijumba, Nelson Mutatina.; Muftic, Dzevad.
The dynamic characteristic of transmission line conductors is very important in designing and constructing a new line or upgrading an existing one. This concept is an impediment to line design and construction because it normally determines the tension at which the line is strung and this in respect affects the tower height and the span length. Investigations into the phenomenon of mechanical oscillation of power line conductors have been extensively looked into by many researchers using concepts from mechanics and aerodynamics to try and predict the conductor dynamic behaviour. Findings have shown that precise prediction of conductor windinduced vibration is very difficult i.e. non-linearity. Over the years, various analytical models have been developed by researchers to try and predict the mechanical vibration of transmission line conductors. The first part of this dissertation considers the analysis of the model describing the transverse vibration of a conductor as a long, slender, simply supported beam, isotropic in nature and subjected to a concentrated force. The solution of this beam equation was used to obtain the conductor natural frequencies and mode shapes. Conductor self-damping was obtained by the introduction of both external and internal damping models into the equation of motion for the beam. Next, also using the same beam concept was the application of the finite element method (FEM) for the dynamic analysis of transmission line conductors. A finite element formulation was done to present a weak form of the problem; Galerkin‟s method was then applied to derive the governing equations for the finite element. Assembly of these finite element equations, the equation of motion for the transverse vibration of the conductor is obtained. A one dimensional finite element simulation was done using ABAQUS software to simulate its transverse displacement. The eigenvalues and natural frequencies for the conductors were calculated at three different tensions for two different conductors. The damping behaviour of the conductors was evaluated using the proportional damping (Rayleigh damping) model. The results obtained were then compared with the results from the analytical model and the comparison showed a very good agreement. An electrical equivalent for the conductor was developed based on the concept of mechanicalelectrical analogy, using the discrete simply supported beam model. The developed electrical equivalent circuit was then used to formulate the transfer function for the conductor. Matlab software was used to simulate the free response of the developed transfer function. Finally, the experimental study was conducted to validate both the analytical model and the FEM. Tests were done on a single span conductor using two testing methods i.e. free and force vibration. The test results are valid only for Aeolian vibration. From the test results the conductor‟s natural frequencies and damping were determined. The experimental results, as compared with the analytical results were used to validate the finite element simulation results obtained from the ABAQUS simulation.
Eskom’s proposed strategic research into HVDC transmission.
(EE Publishers., 2005-07) Naidoo, Pat.; Bologna, F. F.; Muftic, Dzevad.; Ijumba, Nelson Mutatina.; Britten, Anthony C.; Pillay, Logan.
Eskom has recently started a large research and development programme for the study of long-distance HVDC transmission in Sub-Saharan African conditions. This paper explains why the research is being done, what its strategic context and technical scope are, how it is being managed, and the progress made so far.
Investigations into the upgrading of transmission lines from HVAC to HVDC.
(2007) Naidoo, Pathmanathan.; Ijumba, Nelson Mutatina.; Muftic, Dzevad.; Britten, Anthony C.
Emanating from the proceedings of CIGRE 2004, a new idea for higher power transmission by recycling and up rating high voltage alternating current transmission lines for high voltage direct current application was presented at the HYDC working group session. To date, there is no known application of the idea. Globally, transmission congestion, power transfer bottlenecks with restricted and limited power transfers and unobtainable servitudes challenge electric power utilities. The literature review shows that since the early sixties, several authors have studied this proposal. However, no applications were done. Admittedly, early HYDC technology was troubled by problems with multi-terminal designs, external insulation breakdown in the presence of DC stress and mercury valve rectifiers struggled with arc backs. To date, power electronic and external insulation technology has grown and matured for confident application both in point to point and multi-terminal application. The economic costs of introducing the DC technology are also more affordable given reducing prices due to higher volume of purchases. With promising developments in insulation and power electronic technology and driven by South Africa's surging growth in the consumption of electrical energy; the subject of upgrading HYAC transmission for HYDC application is revisited. For the research, the emphasis is beyond FACTS and towards a solution that could develop into a new supergrid that could overlay the existing national grid. Thus, the solution is prepared specifically for the case of recycling existing assets for higher power transfers. The working environment is defined by the difficulty in acquiring new powerline servitudes, transmission congestion in complex networks, the need for electrical islands within complex interconnections, and the need for enhanced power system stability and to promote new ancillary services energy management. The focus of this research study was to determine the technical feasibility of upgrading of existing HYAC circuits for HYDC application. It is assumed that the transmission line will remain as is in structure, layout and mechanical design. The changing of external line insulators using live line technology is an accepted modification to the original HYAC line, if required. From the study, we conclude that not all HYAC lines are recommended for upgrade to HYDe. We introduce boundary conditions as a first step towards checking on the suitability of the proposed upgrade from HVAC to HYDC mode. Emanating from this study, the first paper published introduced the initial boundary conditions as being only those lines where the "unused gap" between surge impedance loading and conductor current carrying capability is appreciable and large; generally three to four times surge impedance loading. In the case where the unused gap is the smallest or negligible, then we do nothing. In between, where the unused gap is about two to three times the surge impedance loading, then we can consider active or passive compensation using the HVAC FACTS technology options as proposed by EPRl. Having determined the candidate transmission line configuration for the proposed upgrade to HYDC application, we select the DC operating voltage as based on the voltage withstand capability of external insulation for varying environmental conditions. In addition, the DC voltage will generate allowable electrical fields and corona effects within and outside the transmission servitude. The optimum DC operating voltage would satisfy the conditions of minimum transmission power losses and volt drop for the case of maximum power transfers; within the limits of electrical fields and corona effects.