An earthing design guide for single wire earth return (SWER) systems in the Northern Cape region

ThesisThe success of the Reconstruction and Development Programme (RDP) with reference to the electrification of rural areas will be enhanced if cheaper technologies 1 techniques are applied than the three-phase and single-phase (2 wire) networks now in use. Single-wire earth return (SWER) technology has been successfully implemented in other countries as part of their rural electrification plan, and already partially in Eskom. The SWER technology consists of a single overhead high voltage (HV) conductor and the earth is used as the current return path. Savings are possible due to the fact that less material and labour are required to construct the network. Existing SWER lines in Eskom indicated that savings were achieved. Further studies during this thesis, however, indicated that more savings and efficiency would have been possible if the earth electrodes installed had been designed according to the conditions on site (soil resistivities, soil type etc. were not previously rigorously taken into consideration). The success and safe implementation of SWER as a technology by Eskom in the Northern Cape region to electrify rural areas, are mostly dependent on the design of the earth electrodes to be installed at the isolating and distribution transformers, as well as the costs involved. Earthing of SWER systems is different from conventional earthing due to the continuous flow of current in the earth electrode compared with the conventional single-phase (2 wire) and three-phase networks where current will only flow in the earth electrode under fault conditions. This major difference highlights the importance of investigating the earthing practice related to the SWER technology in detail. The completed study addresses the need for a SWER earthing guide for application in the Northern Cape region. Design factors such as earthing materials, costs, thermal resistivity of the soil, ground potential rise as well as the special conditions related to the Northern Cape region, namely high soil resistivities, different soil types and seasonal variations, have for the first time been taken into consideration. As previously mentioned, the feasibility of a SWER scheme is dependent on the earthing costs involved, and this is mainly determined by the previous ground potential rise (GPR) limit of 20V. The ground potential rise limit is a safety limit directly adopted from Australian SWER schemes. SWER as a technology in the Northern Cape region with its special conditions is dependent on the possibility to increase the GPR limit of 20V. The feasibility of increasing the GPR limit formed the rationale behind this research. Studies indicate that the GPR limit can be increased from 20V to 35V in the Northern Cape region without sacrificing safety. The studies include simulations done by using a specialised software package called CDEGS. The success of this research is supported by the feasible, cost-effective safe earth electrodes designed and installed on the Rooiwal SWER scheme. Savings of 87% (R 167 352-00) were achieved by the installation of the SWER earth electrodes designed as part of this thesis. This excludes the additional savings of R 30 000-00 by not using conductive concrete such as mitronite as a standard on all SWER earth electrodes in the Northern Cape region

A theory for spiral wave drift in reaction-diffusion-mechanics systems

Reaction-diffusion mechanics (RDM) systems describe a wide range of practically important phenomena where deformation substantially affects wave and vortex dynamics. Here, we develop the first theory to describe the dynamics of rotating spiral waves in RDM systems, combining response function theory with a mechanical Green's function. This theory explains the mechanically-induced drift of spiral waves as a resonance phenomenon, and it can predict the drift trajectories and the final attractors from measurable characteristics of the system. Theoretical predictions are confirmed by numerical simulations. The results can be applied to cardiac tissue, where the drift of spiral waves is an important factor in determining different types of cardiac arrhythmias

Growth and forms of Laplacian aggregates

The shapes and general morphological properties of aggregates grown following the 'T/ rule (Vsurface ex IEI") [L. Niemeyer, L. Pietronero, and H. J. Wiessmann, Phys. Rev. Lett. 52, 1033 (1984)] have been investigated. Vsurface is the velocity at the interface and E the electric field. The fractal dimension decreases monotonically from its diffusion-limited aggregation value ('T/ = 1) to a number indistinguishable from 1 at 'T/ ~ 4. Simultaneously, the multifractal properties become independent of'T/. An alternative method to generate large clusters, and gain insight into the growth process, is also presented. Various analytical approximations are discussed. PACS number(s): 82.20.Wt, 75.10.Jm, 75.10.Lp, 75.30.DsCICyT (Spain) Project No. MAT90-0544 and the U.S. Department of Energy. L.M.S. is thankful to the NSF for financial support under Grant No. DMR 91-17249. F.G. and E.L. acknowledge support from CICyT (Spain).Publicad

Bias in regression coefficient estimates when assumptions for handling missing data are violated: a simulation study

Background The purpose of this simulation study is to assess the performance of multiple imputation compared to complete case analysis when assumptions of missing data mechanisms are violated. Methods The authors performed a stochastic simulation study to assess the performance of Complete Case (CC) analysis and Multiple Imputation (MI) with different missing data mechanisms (missing completely at random (MCAR), at random (MAR), and not at random (MNAR)). The study focused on the point estimation of regression coefficients and standard errors. Results When data were MAR conditional on Y, CC analysis resulted in biased regression coefficients; they were all underestimated in our scenarios. In these scenarios, analysis after MI gave correct estimates. Yet, in case of MNAR MI yielded biased regression coefficients, while CC analysis performed well.Conclusion The authors demonstrated that MI was only superior to CC analysis in case of MCAR or MAR. In some scenarios CC may be superior over MI. Often it is not feasible to identify the reason why data in a given dataset are missing. Therefore, emphasis should be put on reporting the extent of missing values, the method used to address them, and the assumptions that were made about the mechanism that caused missing data

Bariatric surgery and the liver : mechanisms, benefits, and risks

The prevalence of obesity and metabolic diseases such as type 2 diabetes and nonalcoholic fatty liver disease (NAFLD) has risen dramatically over the past decades. At present, bariatric surgery is the most effective treatment for this global health problem, through effects on food intake, gut hormone secretion, metabolic signaling pathways, and adipose tissue dysfunction. The liver occupies a central role in carbohydrate, protein, and lipid metabolism. Notably, a reduction in hepatic fat content and an improvement in hepatic insulin resistance are among the earliest beneficial effects of bariatric surgery, which has therefore emerged as an attractive treatment option for NAFLD. However, as the scope and popularity of weight loss surgery have expanded, new questions have arisen regarding its safety in patients with liver cirrhosis, the outcome of liver transplantation in patients with a history of bariatric surgery, and over incidental reports of liver failure following surgery. Studies in humans and rodents have also linked bariatric surgery to an increased risk of developing alcohol use disorder, a major risk factor for liver disease. This review integrates data from clinical and translational research to delineate both the beneficial impact of bariatric surgery on the liver and the potential risks involved

Activation of Rac-1 and RhoA contributes to podocyte injury in chronic kidney disease

Rho-family GTPases like RhoA and Rac-1 are potent regulators of cellular signaling that control gene expression, migration and inflammation. Activation of Rho-GTPases has been linked to podocyte dysfunction, a feature of chronic kidney diseases (CKD). We investigated the effect of Rac-1 and Rho kinase (ROCK) inhibition on progressive renal failure in mice and studied the underlying mechanisms in podocytes. SV129 mice were subjected to 5/6-nephrectomy which resulted in arterial hypertension and albuminuria. Subgroups of animals were treated with the Rac-1 inhibitor EHT1846, the ROCK inhibitor SAR407899 and the ACE inhibitor Ramipril. Only Ramipril reduced hypertension. In contrast, all inhibitors markedly attenuated albumin excretion as well as glomerular and tubulo-interstitial damage. The combination of SAR407899 and Ramipril was more effective in preventing albuminuria than Ramipril alone. To study the involved mechanisms, podocytes were cultured from SV129 mice and exposed to static stretch in the Flexcell device. This activated RhoA and Rac-1 and led via TGFβ to apoptosis and a switch of the cells into a more mesenchymal phenotype, as evident from loss of WT-1 and nephrin and induction of α-SMA and fibronectin expression. Rac-1 and ROCK inhibition as well as blockade of TGFβ dramatically attenuated all these responses. This suggests that Rac-1 and RhoA are mediators of podocyte dysfunction in CKD. Inhibition of Rho-GTPases may be a novel approach for the treatment of CKD

The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud: spectroscopy, orbital analysis, formation, and evolution

Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy budget of galaxies and probe a critical phase in the evolution of massive stars prior to core-collapse. It is not known whether core He-burning WR stars (classical WR, cWR) form predominantly through wind-stripping (w-WR) or binary stripping (b-WR). With spectroscopy of WR binaries so-far largely avoided due to its complexity, our study focuses on the 44 WR binaries / binary candidates of the Large Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the basis of radial velocity variations, composite spectra, or high X-ray luminosities. Relying on a diverse spectroscopic database, we aim to derive the physical and orbital parameters of our targets, confronting evolution models of evolved massive stars at sub-solar metallicity, and constraining the impact of binary interaction in forming them. Spectroscopy is performed using the Potsdam Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement is performed using the code Spectangular or the shift-and-add algorithm. Evolutionary status is interpreted using the Binary Population and Spectral Synthesis (BPASS) code, exploring binary interaction and chemically-homogeneous evolution. No obvious dichotomy in the locations of apparently-single and binary WN stars on the Hertzsprung-Russell diagram is apparent. According to commonly used stellar evolution models (BPASS, Geneva), most apparently-single WN stars could not have formed as single stars, implying that they were stripped by an undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing (e.g., during the red supergiant phase) are strongly underestimated in standard stellar evolution models.Comment: accepted to A&A on 10.05.2019; 69 pages (25 main paper + 44 appendix); Corrigendum: Shenar et al. 2020, A&A, 641, 2: An unfortunate typo in the implementation of the "transformed radius" caused errors of up to ~0.5dex in the derived mass-loss rates. This has now been correcte

Spatiotemporal assessment of irrigation performance of the Kou Valley irrigation scheme in burkina faso using satellite remote sensing-derived indicators

Traditional methods based on field campaigns are generally used to assess the performance of irrigation schemes in Burkina Faso, resulting in labor-intensive, time-consuming, and costly processes. Despite their extensive application for such performance assessment, remote sensing (RS)-based approaches remain very much underutilized in Burkina Faso. Using multi-temporal Landsat images within the Python module for the Surface Energy Balance Algorithm for Land model, we investigated the spatiotemporal performance patterns of the Kou Valley irrigation scheme (KVIS) during two consecutive cropping seasons. Four performance indicators (depleted fraction, relative evapotranspiration, uniformity of water consumption, and crop water productivity) for rice, maize, and sweet potato were calculated and compared against standard values. Overall, the performance of the KVIS varied depending on year, crop, and the crop's geographical position in the irrigation scheme. A gradient of spatially varied relative evapotranspiration was observed across the scheme, with the uniformity of water consumption being fair to good. Although rice was the most cultivated, a shift to more sweet potato farming could be adopted to benefit more from irrigation, given the relatively good performance achieved by this crop. Our findings ascertain the potential of such RS-based cost-effective methodologies to serve as basis for improved irrigation water management in decision support tools.Yurtdışı Türkler ve Akraba Toplulukları Başkanlığı (YTB) fon