Research Articles (Electrical Engineering)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6858

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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.
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.
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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Browsing Research Articles (Electrical Engineering) by Author "Ijumba, Nelson Mutatina."