Browsing by Author "Ijumba, Nelson Mutatina."

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Application of HVDC technology in medium voltage distribution systems.
(2005) Chetty, Somasundran.; Ijumba, Nelson Mutatina.; Venter, Frik.
Approximately 60% of all South Africans do not have access to electricity from the national grid and 80% of the dwellings in the rural areas are not electrified. This is due to the fact that many rural South Africans, similar to other rural markets in the developing world, live in sparsely populated, widely dispersed villages, which cannot be reached within the grid electrification program. HVDC technology provides a viable option to transmit electricity to small distant loads. The objective of the present study is to demonstrate the application of HVDC technology in a medium voltage distribution system, to provide electrical power to Kwa·Ximba, which is a small distant rural area, located in KwaZulu-Natal, South Africa. The proposed system generates electricity from a hydroelectric generation scheme namely Nagle Dam and transmits the excess power to Eskom's Caltoridge-Georgedale sub-transmission network for system enhancement purposes. Extensive technical and economical analyses of the proposed system has been conducted. An HVAC system was also considered for the same purposes in order to make technical and economical comparisons between the use of a HYDC and a HYAC system. In addition, grid extension from Eskom's Catoridge-Georgedale sub-transmission was considered to provide power to Kwa-Ximba without the use of a hydroelectric generation scheme. The proposed networks were therfeore (i) Network A:- Power supply to Kwa-Ximba. and the Catoridge-Georgedale sub-transmission network, from a hydroelectric generation scheme, using HYDC technology, (ii) Network B:- Power supply to Kwa-Ximba, and the Catoridge-Georgedale sub-transmission network, from a hydroelectric generation scheme, using HYAC technology and (iii) Network C:Power supply to Kwa-Ximba by extending Eskom's existing AC CatoridgeGeorgedale sub-transmission network with the hydroelectric generation scheme switched off. It is proposed that Nagle Dam, which is situated adjacent to Kwa-Ximba be used as a hydroelectric generation plant. In order to detennine the most efficient and cost effective use of generator sets, the flow rate, available hydraulic power and available electrical power from the year 2005 to the year 2032 were calculated. The increase in flow rate was based on an annual growth rate of l.5% in water demand. The increase in electrical power demand for Kwa-Ximba was calculated for the next 29 years based on an annual growth rate of 1.8 %. Load flow analyses was conducted on the various power line and busbars that constitute each of the networks, in order to determine the effectiveness of each network. In order to maintain flexibility in power generation, five sets of hydro electrical generators were chosen to give a combined power delivery of 20MW. The first three hydro electrical generators are rated at 5MW each, the fourth set rated at 3MW and the fifth set rated to deliver 2MW, (G I to GS operate 11 KV, 3 phase). The combination of generator sets in use (01 to 05) will vary depending on the electrical power demand in any given year. Analyses of the predicted load flow pattern revealed that in the year 2005, Kwa-Ximba will receive 10.5 MW of power while 8.64 MW of power will be used to enhance the Eskom's Catoridge-Georgedale sub-transmission network, with a 4% spinning reserve. By the year 2014 power supply to the subtransmission network will cease since Kwa-Ximba will be absorbing 12.2 MW of power with a 17.5% spinning reserve. By the year 2032, Kwa-Ximba will absorb 17MW of power with a spinning reserve of 14.63%. The converter stations required for the HYDC transmission network (Network A) will be equipped with YSC and PWM technology and have a true power rating of a 20MW. This wi ll be adequate to supply Kwa-Ximba's power demand right up until the year 2032 when the demand will be 17 MW. Converters will include 10BTs. Two 45 km long, 30 MW, 80 kV triple extruded polymetric HYDe cable will be buried 700mm below natural ground level. The Rectic Master software was used to select an appropriate overhead line for HVAC transmiss ion (Networks B and C). An aluminium, wolf conductor was selected to transmit 20MW of active power. Load flow analyses revealed that the HYDC link contributes positively to network stabil ity by absorbing more reactive power than the HYAC link. The HVDC system absorbed a combined (Kwa-Ximba, Catoridge-Georgdale sub-transmission network) reactive power of 22.04 MY AT, as opposed to the HVAC transmission system where a combined reactive power 1.89 MVAT was absorbed from the connected network. This demonstrated that the HVDC link had the ability to absorb more reactive power from the Catoridge-Georgedale sub-transmission network, therefore contributing positively to the enhancement and stabi lity of the sub-transmission network. Network A contributes more to system stability than Network B. It has also been shown that if Eskom's Catoridge-Georgedale sub-transmission network (Network C) is extended to supply electricity to Kwa-Ximba, this would result in system instability, in the long term. It is evident that Eskom would attain direct benefit from the installation of Newtork A, rather than Networks Band C. The technical and environmental differences noted in the present study, between the HVDC and HVAC systems does not, however, justify the economics to install a HVDe system in order to supply power to Kwa-Ximba. Economical analyses revealed that the implementation of Network A would cost 64% more to install and result in a 75% less annual net profit than Network B. Network B would yield the highest annual net profit for the developer. From the developer's perspective, Network B will be the most feasible network to implement. However, from Eskom's perspective, Network A will be the most bene ficial. Various recommendations have been made by the researcher that would benefit the community of Kwa-Ximba, Eskom and the developer in the long term.
Application of maintenance tools and strategies in integrated risk management of critical physical assets.
(Inderscience Publishers., 2011) Mkandawire, Burnet O'Brien.; Ijumba, Nelson Mutatina.; Whitehead, Howard.
This paper critically analyses various tools, techniques and strategies; and proposes an ‘integrated risk management model’ that utilises advantages of the best combination of tools, techniques and strategies to manage risks thereby optimising operating costs whilst maximising returns on critical assets in high voltage networks; and physical assets in general. We used a triangulation method involving a longitudinal single case study within Malawian power sector, multiple (34) industrial case studies and sample surveys of selected Malawian and South African industries. It was shown that the electric power industry (70%) lacked a clear systemic maintenance and refurbishment risk management model due to the difficulty in determining optimum combination of tools. They also lacked technical skills needed to apply proactive strategies. The core value of tools is in planning of maintenance and refurbishment; and in contextualising, exploring, assessing, treating and monitoring of risks.
Assessment of the electrical performance of the Cahora Bassa HVDC scheme.
(2005) Sithole, Vusi.; Ijumba, Nelson Mutatina.; Britten, Anthony C.
The aim of this study was to assess the electrical performance of the Cahora Bassa HVDC scheme. For practical reasons a database was developed to hold and analyse the performance data. Microsoft® Access 2002 relational database management system was chosen for this work. The principle of simplicity and flexibility were used in the design of the database. The Apollo year 2002 faults data was populated into the database. The database stored the information in a format that enables the user to extract results and the information required by Ciqre, The Ciqre Working Group 14 collects performance information from all the participating HVDC schemes around the world annually. The Apollo converter station's 2002 performance data was compared to other similar HVDC schemes that submitted to Cigre in the year 2002. In addition performance trends were drawn from similar schemes that submitted to Cigre over the past years. The Apollo performance data is quite comparable to other similar schemes in 2002 and over the past years.
Asset management optimization through integrated systems thinking and N-1 contingency capability for refurbishment.
(Institute of Electrical and Electronics Engineers., 2011-09) Ijumba, Nelson Mutatina.; Whitehead, Howard.; Mkandawire, Burnet O'Brien.
This paper presents a systems view of refurbishment systems to evaluate root causes of suboptimal refurbishment. Case studies from ten selected South African and Malawian firms from largest electric power utilities, mining, petrochemical, and processing industries were used to establish causal relationships. Sample surveys of thirty four Malawian firms were used as part of a multimethod or triangulation approach to provide generalizations, validation and reliability. Of the surveyed firms, 66.7%, and of case studied firms 100%, showed that deferred refurbishment was a result of constrained capacity which led to components operating at higher loads, to lack of maintenance windows and to increased failure rates; there was no formal refurbishment model and technical skills base was the weakest asset management link. The study advances a novel way of depicting root causes of suboptimal refurbishment in typically complex dynamic structures using integrated systems thinking approach and applies analytical optimization tools, namely: Linear Programming (LP), metrics and N-1 contingency capability for refurbishment model for drilling deeper into causal typologies portrayed by systems thinking in order to solve optimization problems. A Total Refur-bishment Process model is advanced to replicate refurbishment decision structures for long term sustainability of industries as validated by industries studied.
Bird streamer initiated breakdowns under HVDC conditions.
(2007) Naidoo, Kribashen.; Ijumba, Nelson Mutatina.; Britten, Anthony C.
This dissertation describes the role played by bird streamers in transmission line faults under HVDC conditions. The research was initiated due to the lack of knowledge of these faults under HVDC conditions. An explanation as to what bird streamers faults are, the role they play in the breakdown of air-gaps and a means of preventing bird streamer caused faults from occurring is made Experimental work has been carried out in order to gain an understanding of these faults under HVDC conditions. The overall aim of the experiments was to find a horizontal distance (protected zone) on an HVDC tower top, in order to prevent birds from perching around the centre of the tower. This will lead to a reduction of bird streamer caused faults. A brass rod was used as the artificial streamer in the experiments, under both negative and positive polarity (voltages in the range 0 to 220 kV DC were applied). Later in the experimental phase, a string soaked in a saline solution was used as a more realistic simulation of a bird streamer. QuickField™, an FEM package, was used to simulate the electric field in the region of the live conductor fitting and the tip of the bird streamer, to assess the degree of distortion of the electric field caused by the introduction of the bird streamer. These simulations served as means of verifying the laboratory experiments. This dissertation has proposed a relationship between the breakdown voltage and the protected zone length, based on the air-gap breakdown voltages for both I-String and V-String insulator configurations in the air-gap range 0 to 350 mm. These curves can be used in the design of transmission lines, as a means of reducing bird streamer faults.
Channel characterization for broadband powerline communications.
(2014) Mulangu, Chrispin Tshikomba.; Afullo, Thomas Joachim Odhiambo.; Ijumba, Nelson Mutatina.
The main limiting factor in broadband powerline communications is the presence of impedance discontinuities in the wired channel. This phenomenon is present in both outdoor and indoor powerline communication (PLCs) channels. It has been established that the impedance of the electrical loads and line branching are the main causes of impedance discontinuities in PLC channel networks. Accurate knowledge of the expected impedances of the corresponding discontinuity points would be vital in order to characterize the channel for signal transmission. However, the PLC channel network topologies lead to different branching structures. Additionally, the existence of a myriad of electrical loads, whose noise and impedance vary with frequency, are a motivation for a rigorous design methodology in order to achieve a pragmatic channel model. In order to develop such a channel model, an approach similar to the one applied in radio propagation channel modeling is adopted, where specific attenuation determined at a point is used in predicting the attenuation for the entire power cable length. Therefore, the powerline is modeled with the assumption of a randomly spread multitude of scatterers in the vicinity of the channel with only a sufficient number of impedance discontinuity points. The line is considered as a single homogeneous element with its length divided into a grid of small areas with dimensions that range from 0.5 to 3 mm. Thus, each small area transmits an echo and the forward scattered response gets to the receiver. With this approach, point specific attenuation along the line is proposed and used to derive the channel transfer function. Measurement results show that both the analytical specific attenuation model developed in this work and the channel transfer function are feasible novel ideas in PLC channel network characterization. It is seen from the measurements that the signal attenuation is directly proportional to the number of branches, and this is in line with the findings of previous researchers. A comparison between the measured values and the simulation results of the frequency response shows a very good agreement. The agreement demonstrates applicability of the models in a practical enviroment. Thus we conclude that the models developed do not require knowledge either of the link topology or the cable models but requires an extensive measurement campaign.
DC coronation electroporation.
(2015) Chetty, Nevendra Krishniah.; Davidson, Innocent Ewean.; Chetty, Leon.; Govender, T.; Ijumba, Nelson Mutatina.
Cells are surrounded by a semi-permeable bilayer lipid membrane that acts as a barrier against the entry of foreign molecules. In the fields of molecular biology, biotechnology, and medicine, the ability to breach the cell membrane and introduce molecules into cells for therapeutic purposes is often necessary. Molecules, which are considered foreign to the cell like drugs and extraneous genetic materials, are administered to cells for numerous applications including the treatment and prevention of diseases. There are many accepted methods of facilitating the delivery of molecules to cells. Of all these methods, one important and well-established physical method is electroporation which has been utilised for decades. Electroporation is a widely adopted procedure for the temporary permeabilization of cell membranes due to the application of short electrical pulses. It is a phenomenon resulting from the effects of pulsed electric fields, which induces biochemical and physiological changes to a cell membrane. As a result, some of the molecules that are ordinarily unable to pass through the membrane are thereafter able to gain access to the cell interior via pores that are formed in the membrane. Even though electroporation is fairly safe, there are some drawbacks associated with this method. The traditional method of electroporation requires direct contact of high voltage electrodes and fairly high currents are involved. As a result, the procedure can cause pain, muscle spasms, discomfort, burning and cell and tissue damage. Alternative methods of molecular delivery are therefore being researched, especially non-contact methods such as the use of high voltage plasma and high voltage corona discharge. Successful cell permeabilization with corona discharge ions and plasma has been previously demonstrated. These methods offer the advantage of contact-free treatment with low associated current. In this thesis, the research investigates the delivery of tracer molecules, SYTOX Green, into HeLa cells and the consequential cell destruction by the phenomenon of corona discharge. A high voltage DC, multipoint-to-plane atmospheric-air corona discharge apparatus was designed and constructed to investigate the conditions as well as the characteristics of the corona discharge current pulses that resulted in an acceptable balance between high cell permeabilization and low cell destruction. Firstly, the salient variables that affect molecular delivery and cell destruction were established. Secondly, the variables were optimized to allow for reliable molecular delivery to cells with acceptable levels of cell destruction. Thirdly, the nature and variation of the corona discharge current pulses and its effect on molecular delivery and cell destruction were investigated. Finally, a new method of assessing cell destruction, which combined the measurements of cell viability and cell lysis were used. The variables that were identified, over the course of many experiments, were exposure time to corona discharge, incubation time with SYTOX Green, volume of liquid during exposure, and inter-electrode distance. Further experiments show that when the variables of the experiment are set at optimal values, cell permeabilization is reliable with minimal damage to cells. Once these conditions were obtained and optimised, the effect of different applied voltages on the level of cell permeabilization and the short-term destructive effects on cells were investigated. The general trend is an increase in fluorescence and therefore, molecular delivery, with an increase in applied voltage. Cell destruction also tends to increase with increasing applied voltage. The characteristics of the corona current pulses that were analyzed include amplitudes, repetition rates, widths, and rise-times. The characteristic frequencies of single pulses, obtained from the application of a discrete fast Fourier transform, were also analyzed. For the corona-generating device constructed and the voltages tested, it was found that the only characteristic that varies appreciably with voltage is the pulse repetition rate. A higher pulse repetition rate relates to a greater number of pulses per unit time and therefore, a greater exposure of the cells to the applied electric field. This would, therefore, translate to a higher extent of molecular delivery and a higher accompanying level of cell destruction. This study shows that permeabilization of HeLa cells due to corona discharge can be reliably achieved and the results provide a greater understanding of cell permeabilization due to the influence of corona discharge. It therefore forms an important basis for future research on practical applications that would promote the establishment and acceptance of corona discharge as a procedure for molecular delivery to cells.
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.
Design synthesis of LCC HVDC control systems.
(2011) Chetty, Leon.; Ijumba, Nelson Mutatina.
From the early days of HVDC system applications, the importance of mathematical modelling of the dynamics of Line Commutated Converter (LCC) HVDC systems has been appreciated. There are essentially two methodologies used to develop mathematical models of dynamic systems. One methodology is to define the properties of the system by the “laws of nature” and other well-established relationships. Basic techniques of this methodology involve describing the system’s processes using differential equations. This methodology is called “Deductive Modelling”. The other methodology used to derive mathematical models of a dynamic system is based on experimentation. Input and output signals from the original system are recorded to infer a mathematical model of the system. This methodology is known as “Inductive Modelling”. A review of the current state of the art of modelling LCC HVDC systems indicates that majority of the techniques utilized to develop mathematical models of LCC HVDC systems have used the “Deductive Modelling” approach. This methodology requires accurate knowledge of the ac systems and the dc system and involves complicated mathematics. In practice, it is nearly impossible to obtain accurate knowledge of the ac systems connected to LCC HVDC systems. The main aim of this thesis is to present an “Inductive Modelling” methodology to calculate the plant transfer functions of LCC HVDC systems. Due to the uncertain nature of the effective short circuit ratio of rectifier and inverter converter stations, generic ranges of parametric uncertainties of the developed plant transfer functions were determined. Based on the determined range of HVDC plant parametric uncertainty, Quantitative Feedback Theory (QFT) methodology was used to design the parameters of the LCC HVDC control system. The stability of the start-up and step responses for varying ac system conditions validated the “Inductive Modelling” technique and the QFT design methodology. The thesis presents the following, which are considered to be scientific advancements and contributions to the body of knowledge: · Novel LCC HVDC Step Response (HSR) equations were developed using an “Inductive Modeling” technique. · The range of parametric variations of the LCC HSR equations were determined for various rectifier and inverter ac system effective short circuit ratios. · The LCC HSR equations were used to develop the LCC HVDC plant transfer functions for various rectifier and inverter effective short circuit ratios. · The LCC HVDC plant transfer functions were utilized to design an LCC HVDC control system for varying ac system conditions using Quantitative Feedback Theory (QFT) methodology. The main contributions of this thesis relate to LCC HVDC systems. This thesis does not attempt to advance control theory however this thesis does apply existing classical control theory to LCC HVDC control systems. Index Terms: Line Commutated Converter, HVDC, inductive modelling, power system, transient analysis.
The development of a guideline to assist with compiling asset management plans for transmission lines.
(2010) Mansingh, Sharan.; Ijumba, Nelson Mutatina.; Govender, Saneshan.
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.
Development of a model of the alpha/beta 765kV line for the evaluation of auto-reclosing.
(2009) Van Heerden, Peter Robert.; Zivanovic, Rastko.; Ijumba, Nelson Mutatina.
The intention of this dissertation is to determine, on the Eskom system, if the pre-insertion closing resistors installed on the Alpha-Beta 765 kV line breakers are preventing overvoltages from being caused during auto-reclosing. Other possible solutions of reclosing protection are investigated. It has been shown on two occasions, from actual field data that overvoltages have occurred on the lines after reclosing. High overvoltages on this network could be the cause of the many reactor failures that have occurred. A mathematical model of the Alpha-Beta 765kV system was produced on Matlab/Simulink to simulate the resonance of the line during opening and then the effect on the voltage when reclosing takes place. The effects of installing pre-insertion resistors to reduce overvoltages on reclosing were analysed, as well as looking at controlled reclosing at the optimal voltage across the line breakers. It was shown from the studies, that pre-insertion resistors do limit the overvoltages to within the surge capabilities of the line (10% overvoltage) and that the cause of the previous overvoltages were actually due to insertion resistor operations failure. It was also shown that the method of controlled closing at optimal voltage across the breakers is also a successful method of preventing overvoltages. This dissertation also evaluates a design specification for a switching relay for controlled re-closing of the line.
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.
Effect of ADLash opitical fibre cable on corona onset gradient and electric fields around the shield wire of the Apollo-Cahora Bassa HVDC line.
(2005) Bussy, Eamon Riccardo.; Ijumba, Nelson Mutatina.; Britten, Anthony C.
ADLash® is the trade name for an optical fibre cable attached to a transmission line shield wire using aramid-reinforced bands. The installation of this type of optic fibre cable has been considered for the HVDC line from the Songo hydroelectric scheme at Cahora Bassa in Mozambique, to Apollo substation in South Africa. The impact that installation of this cable will have on the onset of corona is examined. The shield wire with and without ADLash attached is modelled for both the actual line configuration and for corona cage studies. The electric field is calculated using the Method of Images and the Boundary Element Method to predict the electric field enhancements and to estimate the corona onset gradient. Corona onset gradients and phenomena for smooth and stranded conductors are researched to aid the prediction of voltage onset magnitudes for the corona cage. Estimated values are compared with observed values for both AC and DC applied voltages. Different patterns of corona and different corona onset voltages are observed for AC and negative DC applied voltages. The relative permittivity of the ADLash is higher than the surrounding air and this is shown to lead to greatly enhanced electric fields at the air/ dielectric/ shield wire interfaces that are significant enough to cause corona at the rated operating voltage. The corona will bombard the ADLash and probably damage it. Some alternative methods of installing fibre optic cables are reviewed for further research because the use of ADLash cannot be recommended.
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.
Evaluation of overcurrent protection performance and application on the Eskom shunt capacitors during system disturbances.
(2009) Boodhraj, Revana.; Ijumba, Nelson Mutatina.
This dissertation report began as an investigation into an overcurrent relay protection operation on a shunt capacitor bank (SCB) at ESKOM’s Westgate substation. Westgate substation has two SCBs, both of which were in service at the time of the 2007 incident. However, only the overcurrent protection scheme applied on SCB No.2 operated due to an external feeder fault on the Eltro feeder at Westgate substation. In 2004, SCB No.2 had tripped also on an overcurrent relay protection operation for an external fault. The difference identified in the otherwise identical SCBs was the relay technology employed by the overcurrent protection schemes i.e. electromechanical and electronic overcurrent relays were utilised. Therefore an investigation was initiated to determine any difference in the performance and reliability of overcurrent relay technologies in the SCB environment. The purpose of this work is to present the performance of the different technologies of overcurrent relays (electromechanical, electronic and digital) as applied to an ESKOM SCB during system disturbances and to compare their operation and behaviour. MatLAB and DigSILENT simulation packages were used to conduct preliminary fault studies to determine overcurrent relay performance, for a definite time overcurrent setting. These simulation results indicated that the simple electromechanical and electronic overcurrent relay could operate incorrectly in the SCB environment, during system disturbances. Practical laboratory tests were also conducted. This comprised of injecting DigSILENT simulations, comprising of system switching events and external faults, into three technologies of overcurrent relays. These Omicron injection tests found that the Westgate electronic relay would operate incorrectly for certain fault events in the SCB environment. Due to the results observed, further frequency response tests were conducted. These results suggested that the electronic and electromechanical overcurrent relays were susceptible to harmonics i.e. harmonics impact both the pick-up current setting and operating time of electronic and electromechanical overcurrent relays. The digital relay did not exhibit this vulnerability. Finally, recommendations were made to address the incorrect operation of the Westgate electronic relay in its SCB application. These recommendations could be applied in other ESKOM SCB overcurrent protection schemes, to prevent incorrect operation for system disturbances.
The impact of energy efficient lighting on power networks.
(2013) Mansingh, Silesh.; Ijumba, Nelson Mutatina.; Govender, P.
The power utilities in many countries have come under stress as a result of generation capacity deficits, looming fuel costs and in many developing countries the electricity demand is very quickly outstripping the available electricity supply [1]. Since 2006, Eskom has been utilizing large scale residential efficient lighting roll out programmes as a cost effective short to medium term supply security solution while the longer term capital intensive supply side, “Build Programme” was underway [2]. This mostly involved the use of non-linear compact fluorescent lamps (CFLs) to replace linear incandescent lights (ILs) with very little involvement and influence by the utility QOS (quality of supply) engineers that are responsible for ensuring supply quality on the specifically targeted Distribution networks. This dissertation highlights that while CFLs can provide the desired energy or peak power reduction required, they can also have an impact on QOS within specific sensitive networks if the appropriate CFL standards (i.e. IEC 61000-3-2) minimum requirements are not adhered for both power factor and harmonics. These large scale CFL implementations have cost implications and hence metering and verification methods and models have been developed to simulate and quantify the returns from investing in energy efficient lighting initiatives. CFL manufacturers differ on ballast designs which have a direct impact on lamp efficacy, harmonic distortion and power factor which contribute to network quality of supply. It is for this reason that this dissertation raises an awareness of the importance of appropriate CFL standards i.t.o. the impact on quality of supply from a harmonics and power factor perspective. Due to the lack of more recent available or reliable sources of CFL test data, laboratory investigations were used to confirm the harmonics and PF characteristics of a variety of commercially available CFLs in addition to those utilised in previous large scale rollout programmes. The harmonics and power factor performance measured were compared for compliance against the international limits as prescribed in IEC 61000-3-2. Further investigations on international lighting industry standards for CFL’s has revealed gaps pertaining to harmonics and PF limits. This study is concluded by highlighting the impact of CFLs on power networks. It also provides a brief guide for utility power quality engineers by imparting an understanding of their role for the large scale efficient lighting programmes in order to pro-actively contain any possible impact on quality of supply within the regulatory limits as prescribed in the QOS standards.
Impact of HIV/AIDS on education and poverty.
(UN Department of Public Information., 2011) Ijumba, Nelson Mutatina.
South Africa has the largest number of children orphaned or made vulnerable by HIV/AIDS. In the face of AIDS, poverty, and violence, education is a precious right for the children of KwaZulu-Natal, South Africa.
The impact of network changes on power quality and compensation device perfomance.
(2012) Ngcamu, Mbulelo Busani Edmund.; Ijumba, Nelson Mutatina.
This dissertation describes the impact of changing network configuration on power quality and performance of existing compensation devices in the transmission network. The underlying theory was assessed and thereafter the Everest substation network case scenario was selected to study the above due to; the number of reconfigurations it has experienced in the past, increased capacitor bank failures and also due to the harmonics problems experienced. The study involved the installation of harmonic current measuring instruments at Everest 132kV feeders to identify the potential sources of harmonics and to determine the dominant harmonics. A dig Silent Power Factory model was then constructed to perform various simulations in order to determine the impact of the changes done on the Everest network as well as the impact of capacitor switching on the harmonics amplification at Everest 132kV Bus-bars. The study also focussed on analysing the performance history of the capacitor banks at Everest and to determine if high harmonic amplification had an impact on capacitor bank performance. The simulation results revealed that network reconfigurations have negatively impacted power quality at Everest. The results showed that there is also a correlation between the switching of the two 72MVAR capacitor banks and the amplification of the harmonics at Everest. The highest amplification occurred when both capacitor banks were switched in and the resonance point occurred around the 5th harmonic which coincided with the data from field measurements. There was a 61% increase in 5th harmonic impedance amplitude after the Everest network was reconfigured, for the condition when both capacitors are switched in. The lowest amplification occurred when none of the capacitor banks were switched in. Three options were assessed to eliminate the problem of harmonics at Everest, the first one was to prohibit the switching in of both capacitors at Everest and utilise other available means around the Everest network for voltage support. The second option was to change the capacitor size, thus moving the resonance point away from the 5th harmonic. The last option was to install a harmonic filter at Everest to filter out the problematic harmonics. The first option is recommended as it has been successfully tested, can be readily implemented and is much more cost effective compared to the others.
Improved transmission line protection performance concerning high resistance faults.
(2006) Matshidza, Rhulani Daphney.; Zivanovic, Rastko.; Ijumba, Nelson Mutatina.
ESKOM has relied primarily on impedance-based measurement protection relays for the protection of transmission lines. One of the main disadvantages of distance relays is the limited fault resistance measurement capability. High-resistance faults are characterised by low fault currents, therefore the impedance calculated would be much bigger and so the fault will appear to be beyond the protected line. The main aim of the study is to gain clear understanding of the capability of the existing relays used in Eskom Transmission network, to be able to give recommendations on the refinements to the transmission line protection philosophy required to improve future protection performance. Omicron relay tests, showed that the dynamic characteristics of the three selected relays which were tested covers more fault resistance than that of the normal static impedance and also that the effect of DC offset is negligible with regards to fault resistance measurement capability. Normally the relays have built in algorithms which are able to filter nuisance signals. Theoretical case study that compared the most used relays in Eskom Transmission was done and the results are documented. Settings recommendations to improve fault resistance coverage were deduced from the above study. Fault investigation by using digital simulations (Matlab simulations) has proven the lack of capability to operate for some impedance relays in some fault conditions, as the fault resistance sometimes moves fault impedance beyond relay characteristic even when actual . polarization of the relay is considered. Analysis of the protection performance in transmission proved that high resistance faults accounts for at least half of protection equipment performance index (PEPI) incidents. Finally the author made recommendations to improve the protection performance concerning high resistance faults.
Improvement of steady state and voltage stability of a strong network overlayed with higher voltage transmission lines using phase shifting transformers.
(2011) Molapo, Reentseng Majara.; Mbuli, Nhlanhla.; Ijumba, Nelson Mutatina.
This research work deals with the application of the phase shifting transformer in improving the steady state performance and voltage stability of transmission network that has transmission lines at different voltage levels running in parallel to each other. Transmission power system networks are usually developed using lines built at a certain voltage level initially. As power demand requirements increase, building of the new lines at the same voltage level becomes necessary. However, lesser and lesser improvements in transfer capacity are realised when the additional lines are built. This prompts utilities to consider higher voltages for future lines as these have a higher transfer capacity. Utilities usually lay, i.e., they build in parallel, newer, higher voltage transmission lines along side the existing lower voltage ones. Power flow in power system is mainly influenced by impedances of equipment. If the combined impedance of the existing, lower voltage transmission system is relatively less than the impedance of the newer, higher voltage ones, power may primarily flow through it rather than via the newer, parallel higher voltage transmission network. This may lead to a serious underutilisation of the newer infrastructure with a higher transmission capacity. Transmission networks similar to the one described above are common throughout the world. This study was undertaken towards finding solutions to the problem of under utilisation of such transmission lines. The study was performed by first reviewing the literature on the use of phase shifting transformers to redirect power flow in transmission networks throughout the world. This was followed by analysis of the theory on how and what determines the power flow in power networks. Several simulations of varying the phase of the phase shifting transformer were performed on the Cape network, as a case study, to investigate the impact on the power flow distribution and voltage stability performance of the 765 kV and 400 kV transmission lines carrying power to the Western Cape. In this dissertation, it has been demonstrated that a phase shifting transformer can be used to alter the power flow patterns so that power flows are restructured or redistributed, such that power which originally flowed via the low impedance, lower voltage system is transferred to the parallel higher voltage transmission system of lines. It is shown that once the power flows are redistributed, steady state and voltage stability performance of the total system can be enhanced and an increase in its power transfer capacity can be realised.