Presently power systems are being operated under high stress level conditions unforeseen at the moment they were designed. These operating conditions have negatively impacted reliability, controllability and security margins. FACTS devices and HVDC transmissions have emerged as solutions to help power systems to increase the stability margins. VSC-HVDC transmissions are of particular interest since the principal characteristic of this type of transmission is its ability to independently control active power and reactive power. This thesis presents various control strategies to improve damping of electromechanical oscillations, and also enhance transient and voltage stability by using VSC-HVDC transmissions. These control strategies are based of different theory frames, namely, modal analysis, nonlinear control (Lyapunov theory) and model predictive control. In the derivation of the control strategies two models of VSC-HVDC transmissions were also derived. They are Injection Model and Simple Model. Simulations done in the HVDC Light Open Model showed the validity of the derived models of VSC-HVDC transmissions and the effectiveness of the control strategies. Furthermore the thesis presents an analysis of local and remote information used as inputs signals in the control strategies. It also describes an approach to relate modal analysis and the SIME method. This approach allowed the application of SIME method with a reduced number of generators, which were selected based on modal analysis. As a general conclusion it was shown that VSC-HVDC transmissions with an appropriate input signal and control strategy was an effective means to improve the system stability. / QC 20110412

Historically, wagons have been designed according to the American Association of Railroads specifications. These require that wagons be designed to withstand a static load between the couplers of 350 tons. This implies that the structure has a certain stiffness. In order to improve load to tare ratio, there has been talk of reducing the end load specifications. This implies that the stiffness of the wagon will reduce. Using more flexible wagons implies that the freight will probably be exposed to a harsher dynamic environment. There is a trade off between the cost of packaging and the cost of protection devices installed in the vehicle. If handling damage can be prevented then an understanding of the dynamic environment will assist in reducing the packaging requirement. This research looked at the dynamic characteristics of an existing design of wagon using modal analysis. The results from the modal analysis were extended to be inputs to the time domain freight model. Various analytical models of the freight were developed depending on the configuration and dynamic properties. Special consideration was given to a cylinder with its axis transverse to the wagon. The modal model was modified to accommodate the change in mass imposed by the freight. The various sources of dynamic excitation were explored, namely inputs from the coupler and from the bogie. Data from shunting yard simulations were used to generate spectra as input to the wagon model. The objective was to use modal techniques to be able to take individual components, form them into a complete model and make informed decisions about the suitability of a certain configuration for traffic. / Thesis (Ph.D.)-University of Natal, Durban, 2002.

Digital media storytelling and the creation of narrative texts using digital technology is an emerging social process that is being utilized by social workers as a means of engaging in critical reflection. As an emerging practice, little is known about the contributions that these texts make to critical social work knowledge; to this end this thesis considers social worker's use of digital media storytelling as a tool for resisting and remembering and as a tool for critical reflection about their changing field. Six digital media stories are considered in this thesis. The texts are deconstructed using multi-modal analysis informed by internet/digital media research scholarship. The layers produced through this deconstruction are crystalized using critical discourse, narrative and metaphor analysis in order to develop a complex understanding of the multi-modal and multi-vocal meaning making processes inherent in these stories. The analysis reveals the way in which discourses and themes present in the contemporary context of social work practice such as neo-liberalism, managerialism and professionalization, are brought to life in the narratives produced by the social workers, who each tell their stories using different genres, from unique points of view, based on their individual subjective positions. The findings point to the significance of digital media storytelling as an important resources for knowledge production and knowledge dissemination. The analysis further points to the significance of connections between and among these texts as demonstrating the tensions and contradictions that are produced through the workers’ attempts to bring to life the social justice values, goals and objectives of social work to which they are committed in a social climate that is increasingly hostile to such approaches to human service work.

Digital media storytelling and the creation of narrative texts using digital technology is an emerging social process that is being utilized by social workers as a means of engaging in critical reflection. As an emerging practice, little is known about the contributions that these texts make to critical social work knowledge; to this end this thesis considers social worker's use of digital media storytelling as a tool for resisting and remembering and as a tool for critical reflection about their changing field. Six digital media stories are considered in this thesis. The texts are deconstructed using multi-modal analysis informed by internet/digital media research scholarship. The layers produced through this deconstruction are crystalized using critical discourse, narrative and metaphor analysis in order to develop a complex understanding of the multi-modal and multi-vocal meaning making processes inherent in these stories. The analysis reveals the way in which discourses and themes present in the contemporary context of social work practice such as neo-liberalism, managerialism and professionalization, are brought to life in the narratives produced by the social workers, who each tell their stories using different genres, from unique points of view, based on their individual subjective positions. The findings point to the significance of digital media storytelling as an important resources for knowledge production and knowledge dissemination. The analysis further points to the significance of connections between and among these texts as demonstrating the tensions and contradictions that are produced through the workers’ attempts to bring to life the social justice values, goals and objectives of social work to which they are committed in a social climate that is increasingly hostile to such approaches to human service work.

The reversed-field pinch (RFP) is a device for magnetic confinement of fusion plasmas. The main objective of fusion plasma research is to realise cost-effective thermonuclear fusion power plants. The RFP is highly unstable as can be explained by the theory of magnetohydrodynamics (MHD). Feed-back control technology appears to enable a robustly stable RFP operation.  Experimental control and identification of nonaxisymmetric multimode MHD is pursued in this thesis. It is shown that nonparametric multivariate identification methods can be utilised to estimate MHD spectral characteristics from plant-friendly closed-loop operational input-output data. It is also shown that accurate tracking of the radial magnetic field boundary condition is experimentally possible in the RFP. These results appear generically useful as tools in both control and physics research in magnetic confinement fusion. / <p>QC 20120508</p>

Inflatable wings provide an innovative solution to unmanned aerial vehicles requiring small packed volumes, such as those used for military reconnaissance or extra-planetary exploration. There is desire to implement warping actuation forces to change the shape of the wing during flight to allow for greater control of the aircraft. In order to quickly and effectively analyze the effects of wing warping strategies on an inflatable wing, a finite element model is desired. Development of a finite element model which includes woven fabric material properties, internal pressure loading, and external wing loading is presented. Testing was performed to determine material properties of the woven fabric, and to determine wing response to static loadings. The modeling process was validated through comparison of simplified inflatable cylinder models to experimental test data. Wing model response was compared to experimental response, and modeling changes including varying material property models and mesh density studies are presented, along with qualitative wing warping simulations. Finally, experimental and finite element modal analyses were conducted, and comparisons of natural frequencies and mode shapes are presented.

This thesis describes the development of microfluidic (MF) platforms for the study of biological and chemical processes. In particular the thesis is divided into two distinct parts: (i) development of a MF methodology to generate tunable cell-laden microenvironments for detailed studies of cell behavior, and (ii) the design and fabrication of MF reactors for studies of chemical reactions. First, this thesis presented the generation of biopolymer microenvironments for cell studies. In the first project we demonstrated a high-throughput MF system for generating cell-laden agarose microgels with a controllable ratio of two different types of cells. The MF co-encapsulation system was shown to be a robust method for identifying autocrine and/or paracrine dependence of specific cell subpopulations. In the second project we studied the effect of the mechanical properties on the behavior of acute myeloid leukemia (AML2) cancer cells. Cell-laden macroscopic agarose gels were prepared at varying agarose concentrations. A modest range of the elastic modulus of the agarose gels were achieved, ranging from 0.62 kPa to 20.21 kPa at room temperature. We observed a pronounced decrease in cell proliferation in stiffer gels when compared to the gels with lower elastic moduli. The second part of the thesis focuses on the development of MF platforms for studying chemical reactions. In the third project presented in this thesis, we exploited the temperature dependent solubility of CO2 in order to: (i) study the temperature mediated CO2 transfer between the gas and the various liquid phases on short time scales, and (ii) to generate bubbles with a dense layer of colloid particles (armoured bubbles). The fourth project involved the fabrication of a multi-modal MF device with integrated analytical probes. The MF device comprised a pH, temperature, and ATR-FTIR probes for in-situ analysis of chemical reactions in real-time. Furthermore, the MF reactor featured a temperature controlled feedback system capable of maintaining on-chip temperatures at flow rates up to 50 mL/hr.

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