Browsing by Author "Chetty, Leon."

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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 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.
Effectiveness of business education on entrepreneurial intent.
(2017) Chetty, Leon.; Hoque, Muhammad Ehsanul.
The literature on entrepreneurial intent is extensive. The intention of individuals to set up new businesses has been recognised as an important hypothesis in entrepreneurship research. Self-efficacy is defined in the context of entrepreneurship as the strength of a person’s belief that he or she is capable of successfully performing the various roles and tasks of entrepreneurship. Perhaps the most extensive application of self-efficacy has been in the area of training. Business management training is an essential component of the modern academic curriculum. A variety of business management courses are being proposed, without the provision of conclusive quantitative data and information relating to impact of business management training on participants. This research will focus on the impact of business management training on entrepreneurial intention. The aim of this study is to examine the entrepreneurial intent of MBA candidates compared to interns of a government funded programme. This research utilized a quantitative approach. The Liñán and Chen (2009) validated Entrepreneurial Intent Question was used. The measured set of data will consist of two groups namely: (a) TIA intern (TIA) group and (b) MBA candidates (MBA) group. The eligibility criterion for the sample of ten (10) in the MBA group comprised of university post-graduate students who were taking part in the business management training programme. The sample of sixteen (16) in the TIA group comprised of graduate interns employed by Technology Innovation Agency (TIA). Based on the results of this study it can be concluded that (a) an MBA education does not more positively influence attitude toward entrepreneurial behaviour when compared to a TIA funded skills development internship, (b) an MBA education does more positively influence perceived entrepreneurial behaviour control when compared to a TIA funded skills development internship and (c) an MBA education does more positively influence entrepreneurial intent when compared to a TIA funded skills development internship. This research study recommends that the MBA curriculum need to develop techniques to improve a candidate’s attitude to entrepreneurial behaviour.
Electrospinning of composite silica/PVA nanofibres and testing of their effects on the impedance of an air gap.
(2014) Singh, Yeshiv.; Davidson, Innocent Ewean.; Ramjugernath, Deresh.; Chetty, Leon.
Electrospinning is a quick method of producing long continuous fibres with diameters in the nanometre range. At first, only polymers were electrospun. In recent practices, inorganic materials have also been electrospun by combining them with polymers in the electrospinning solution. A number of experiments dealing with electrospinning and applications of silica nanofibres have been presented by different authors. None of these experiments dealt with any electrical properties of silica nanofibres. The experimentation presented herein deals with the effects of composite silica/PVA nanofibres on the electrical impedance of an air gap. Literature covering electrospinning in general and electrospinning of silica is discussed in detail. For the electrospinning process, parameters affecting the morphology of nanofibres are presented by referring to experiments done by other researchers. For silica, the theory behind formation of a silica sol is discussed. All apparatus used in the experimentation is listed. In particular, the electrospinning setup had to be designed, built and optimized. The design of the setup is presented. Included in the design is the construction of a high voltage DC supply to be used to apply high voltage to the spinneret of the setup. The optimization of the setup and the solution used to electrospin fibres is covered in the discussion of the experimental procedure. The fibres were successfully fabricated with reasonable uniformity and nano-sized diameters. These parameters were checked using a scanning electron microscope. Some images taken of the fibres by the microscope are presented with the calculated average fibre diameters. The rest are in the appendix. The chemical composition of the fibres was confirmed using Fourier transform infra-red, the results of which are discussed. The impedance of an air gap with and without fibres was calculated using values measured by an oscilloscope. Measurements were taken over a range of frequencies to observe the change in the reactive component of the impedance. Based on results obtained, it was concluded that composite silica/PVA nanofibres reduce the impedance of an air gap and are therefore ineffective as an insulating material.
Influence of fire on DC corona current.
(2014) Ilunga, Kayumba Grace.; Davidson, Innocent Ewean.; Chetty, Leon.
Corona discharge is one of the undesirable consequences associated with HVDC transmission. Corona discharges are the visible result of the partial electrical breakdown of ionised air in the non-uniform fields surrounding energised conductors. Corona produces undesirable effects such as corona loss, audible noise, radio interference, ozone and nitric oxides. The performance of high voltage transmission lines is presumably affected by the occurrence of wild forest fires beneath these lines. Fire under transmission line generates heat and increases the temperature of the air surrounding the conductors. The increase in temperature due to the presence of fire decreases the breakdown strength of air insulation that can result on flashovers and interruption of power supply. This thesis presents the experimental results of the investigation conducted on a short test line using a small corona cage. The results of the corona current magnitudes obtained from the investigation on the dominant frequencies of the corona current pulses under the influence of high-temperature gas fire are presented. The propagation of the dominant frequency signals is studied in isolation of other sources of high frequency signals within controlled laboratory conditions. The corona current signature technique, which is the combination of the magnitudes of dominant frequencies and the repetition rate of the corona pulses, was introduced to evaluate the impact of fire on the corona current. Results obtained from the dc corona current investigation are then compared with those obtained from ac corona current investigation and presented.
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.
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.