Research Articles (Electrical Engineering)

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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.
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.
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.
Experimental optimization of nanostructured nickel oxide deposited by spray pyrolysis for solar cells application.
(Research India Publications., 2018) Ukoba, Kingsley Ogheneovo.; Inambao, Freddie Liswaniso.; Eloka-Eboka, Andrew Chukwudum.
This study focused on the experimental optimization of nanostructured nickel oxide (NiO) for solar cell applications. The optimization procedure involved the variation of the precursor concentrations of nickel acetate with attendant measurement of the properties of nickel oxide films. The films were spray deposited on glass substrate. Nickel acetate precursor was used at a substrate temperature of 350 oC. Precursor concentrations were: 0.025 M, 0.05 M, 0.075 M and 0.1 M respectively. The surface morphology revealed nanostructured film with particles densely distributed across the substrate’s surface. The films are homogeneous, smooth, well adherent and devoid of pinholes and cracks. The morphology became grainier as the precursor solution increased. Elemental composition exposes the presence of Ni and O elements in NiO film. Oxygen concentration decreases as precursor solution increases. The film structural property reveals that deposited NiO film has an amorphous structure at 0.025 M while the other concentrations are polycrystalline in nature with cubic structure. X-ray diffractometry (XRD) further reveals that the intensity of NiO films increases with increased molarity. Preferred orientation was along the (1 1 1) peak with minor intensity along the (2 0 0) peak. XRD patterns have peak diffraction at (2θ = 37 o and 43 o) for the (1 1 1) and (2 0 0) planes respectively, and 64 o for the (2 2 0) plane for 0.1 M. Crystallite size was obtained at 63.77 nm maximum. Film thickness increased with increasing precursor concentration from 6.277 μm to 11.57 μm. Film micro strain was observed to have compression for all precursor solutions. Optical studies showed that transmittance decreased with increasing concentration from 80 % to 71 %. Optical band gap energy was between 3.94 eV to 3.38 eV as precursor concentration increased, revealing the effect of varied concentrations on NiO film properties. Optimized results obtained are precursors in the development of low cost, efficient, durable solar cell fabrication for developing countries.
Fabrication of affordable and sustainable solar cells using NiO/TiO2 P-N heterojunction.
(Hindawi., 2018) Ukoba, Kingsley Ogheneovo.; Inambao, Freddie Liswaniso.; Eloka-Eboka, Andrew Chukwudum.
The need for affordable, clean, efficient, and sustainable solar cells informed this study. Metal oxide TiO²/NiO heterojunction solar cells were fabricated using the spray pyrolysis technique. The optoelectronic properties of the heterojunction were determined. The fabricated solar cells exhibit a short-circuit current of 16.8 mA, open-circuit voltage of 350 mV, fill factor of 0.39, and conversion efficiency of 2.30% under 100mW/cm² illumination. This study will help advance the course for the development of low-cost, environmentally friendly, and sustainable solar cell materials from metal oxides.
Influence of concentration on properties of spray deposited nickel oxide films for solar cells.
(Elsevier., 2017) Ukoba, Kingsley Ogheneovo.; Eloka-Eboka, Andrew Chukwudum.; Inambao, Freddie Liswaniso.
Spray pyrolysis technique was used to deposit various concentration of nickel oxide films on glass substrate. The Effect of varying precursor concentration on elemental, morphological and structural properties was investigated on the deposited NiO films. Nickel (II) acetate tetrahydrate precursor was used at substrate temperature of 350 oC. Precursor concentrations were 0.025, 0.05, 0.075 and 0.1 M. Scanning Electron Microscope (SEM) surface morphology revealed nanostructured films with particles densely distributed across substrates surface. Increased in surface grains was observed as the precursor solution increased. Elemental composition of NiO films revealed presence of Ni and O element. There was reduction in oxygen concentration as precursor solution increases. Amorphous structure was observed at concentration of 0.025 M while polycrystalline with cubic structure was observed at higher concentrations. Preferred orientation was along (1 1 1) peak with small intensity along (2 0 0) peak. XRD patterns have peak diffraction at (2θ = 37 o and 43 o) for (1 1 1) and (2 0 0) planes respectively and 64 o for (2 2 0) plane for 0.1 M. Film thickness grew with increase in precursor concentration. Film micro strain was observed to have compression for all precursor solution conspicuously revealing the effect of varied concentration on NiO films properties
Modeling of fabricated NiO/TiO2 P-N heterojunction solar cells.
(Research India Publications., 2018) Ukoba, Kingsley Ogheneovo.; Inambao, Freddie Liswaniso.
This paper reports modelling and theoretical validation of a fabricated NiO/TiO2 P-N heterojunction solar cell. The solar cell equations were modelled and thereafter theoretical validation of the fabricated solar cells was performed. Modelling tools were used to validate the influence of NiO material features such as deposition temperature, voltage and defect densities on the performances of an ITO/TiO2/NiO heterojunction solar cell structure. The working points used included a temperature of 350 oC, illumination of 1000 W/m2 using an AM1.5 lamp, with voltage range of 0 to 1.5 volts. The output gave Voc of 0.1445 V, Jsc of 247.959195E-6 mA/cm2 and FF of 37.87 % and Voc 0.7056 and Jsc 28.366911 mA/cm2 when both contacts were added. This opens a new frontier for modelling of metal oxide based thin film solar cells especially NiO thin film solar cells. These findings enhance the quest to develop affordable and sustainable energy and encourage further research in solar cell technologies in low-income countries.
Modelling of broadband powerline communication channels.
(SAIEE Publications., 2011) Mulangu, Chrispin Tshikomba.; Afullo, Thomas Joachim Odhiambo.; Ijumba, Nelson Mutatina.
This paper develops a new PLC model and investigates the impact of the load, line length, and diameter of the transmission line on the channel transfer function over the frequency range of 1-20 M Hz. The results show that with 42 degrees of freedom, the proposed model leads to an average RM SE value of approximately 5.2 dB. With the same conditions, the Phillips model leads to an average RMSE value of approximately 1.62 dB.
Quantitative Feedback Theory design of line current commutated HVDC control systems.
(SAIEE Publications., 2013) Chetty, Leon.; Ijumba, Nelson Mutatina.
Line Current Commutated (LCC) HVDC systems consists of uncertain plants. These uncertainties are result of changes/disturbances in the ac networks or in the LCC HVDC system itself. Further uncertainties can be introduced due to simplified system modelling techniques. Quantitative Feedback Theory is a frequency-domain technique that utilises the Nichols chart to achieve a robust design over a specified region of uncertainty. The Quantitative Feedback Theory design philosophy was applied to design the LCC HVDC control system parameters. The stable start-up and step responses of the LCC HVDC system, for varying ac system conditions, conclusively validate the Quantitative Feedback Theory design method of the LCC HVDC control system parameters.
Rural electrification using overhead HVDC transmission lines.
(Energy Research Centre., 2012) Chetty, Leon.; Ijumba, Nelson Mutatina.
One of mankind’s greatest modern challenges is poverty alleviation. The provision of electricity can greatly assist in this regard. The tapping of small amounts of power from an HVDC transmission line represents a solution to this problem especially in rural areas. This paper analyses the dynamic characteristics of a parallel-cascaded tapping station. The results obtained clearly indicate that the parallel-cascaded tapping station proves to be a viable solution to tapping small amounts of power from an HVDC transmission line.
Safety of low voltage switchgear assemblies in South Africa – Part 1.
(EE Publishers., 2005) Bonner, Mark James.; Ijumba, Nelson Mutatina.
A technical analysis was undertaken to investigate the safety implications of the introduction of the category Specially Tested Assembly (STA) into the South African standard SANS 1473-1:2003. The testing requirements specified for a Specially Tested Assembly (STA) are compared to the testing requirements specified for a Type-Tested Assembly (TTA), tested in accordance with standard IEC 60439-1:1999. The technical inadequacies and the potential safety risks associated with an assembly that is certified as a STA are exposed, and remedial measures proposed.
Safety of low voltage switchgear STAs in South Africa – Part 2.
(EE Publishers., 2005) Bonner, Mark James.; Ijumba, Nelson Mutatina.
This article is Part 2 of a two part article. Part 1 was published in the October 2005 issue of ENERGIZE. A technical analysis was undertaken to investigate the safety implications of the introduction of the category Specially Tested Assembly (STA) into the South African standard SANS 1473-1:2003.
Solar cells and global warming reduction.
(Research India Publications., 2018) Ukoba, Kingsley Ogheneovo.; Inambao, Freddie Liswaniso.
This study proposes one way of addressing the issue of climate change and pollution using solar cells. The quality of life in developing and low-income countries is on the decline because of air pollution. Energy has a role to play in the quality of life and reduction of air pollution especially in those countries. A reduction in the usage of fossil fuels and biomass in these countries will help decrease the air pollution and emissions generated by such energy sources. About 1 million solar lanterns are capable of reducing greenhouse gas emissions by over 30 000 tons. The role of eco-friendly solar cells in elimination of air pollution cannot be overstated.
Suppression of transients across the tap windings of an auto-transformer.
(EE Publishers., 2005-06) Buthelezi, N. V.; Ijumba, Nelson Mutatina.; Britten, Anthony C.
For the energy transfer between the various voltage levels of the Eskom national grid, auto-transformers are mainly installed. These auto-transformers are interfaced with the distribution network.
System identification of classic HVDC systems.
(SAIEE Publications., 2011) Chetty, Leon.; Ijumba, Nelson Mutatina.
Determining models from observations and studying the models’ properties is essentially the functionality of science. Models attempt to link observations into some pattern. System identification is the art of building mathematical models of dynamic systems based on observed data from the systems. This paper presents a system identification methodology that can be utilized to derive the classic HVDC plant transfer functions. The model development and verification was performed using the PSCAD/EMTDC software. The calculated results illustrated excellent response matching with the system results. The derived HVDC plant transfer functions can be utilized to perform small signal stability studies of HVDC-HVAC interactions and its use can also be extended to facilitate the analytical design of HVDC control systems.

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