A South African review of harmonic emission level assessment as per IEC61000-3-6

Large-scale renewable power producing plants are being integrated into South African networks. Network operators need to ensure that Renewable Power Plants (RPP) do not negatively affect the power quality levels of their networks, as harmonics amongst others could become a concern. IEC 61000-3-6 details a method for allocating voltage harmonic emission limits for distorting loads. This method works well for the allocation of emission limits; however it does not address the management of harmonic emissions once a plant is connected to the network. The management of harmonic emissions requires that network operators measure or quantify the emissions from loads and generators to determine compliance. Post-connection quantification of harmonic levels and compliance is a challenge for network operators. The question asked is “How should a network operator measure/quantify the harmonic emissions of a load/generator to establish compliance with the calculated limits as per IEC 61000-3-6”. This paper reviews within a South African context methods of assessing harmonic emission levels and then evaluates these methods by means of field data. Opportunities for improvement are identified and operational requirements discussed

The role of values in school discipline

Despite the fact that the Constitution of South Africa uses language that could be described as “value-language”, our country is experiencing an intense moral crisis. There is an urgent need to establish ways of finding answers to the value crisis in South Africa. Morality has been and is part of education. In this article the relationship between values, education and discipline is addressed from a Biblically-based (in this case, reformational) perspective. The teacher as secondary educator plays an important role in the establishment of values among learners. The Department of Education has made clear its intention to establish values in schools in its Manifesto on Values, Education and Democracy. The values emphasised in this document concur with the ideals of nation-building in the new democratic South Africa. Unfortunately, the absence of discipline and self-discipline among learners and educators implies that these ideals cannot be realised. The main cause of discipline problems can possibly be ascribed to the absence of a value system rooted in a specific life and worldview, for without such a perspective the management of discipline problems can only be symptomatic

A resource-based view on the interactions of university researchers

The high value of collaboration among scientists and of interactions of university researchers with industry is generally acknowledged. In this study we explain the use of different knowledge networks at the individual level from a resource-based perspective. This involves viewing networks as a resource that offers competitive advantages to an individual university researcher in terms of career development. Our results show that networking and career development are strongly related, but it is important to distinguish between different types of networks. Although networks on various levels (faculty, university, scientific, industrial) show strong correlations, we found three significant differences. First, networking within one’s own faculty and with researchers from other universities stimulates careers, while interactions with industry do not. Second, during the course of an academic career a researcher’s scientific network activity first rises, but then declines after about 20 years. Science-industry collaboration, however, continuously increases. Third, the personality trait ‘global innovativeness’ positively influences science-science interactions, but not science-industry interactions.research collaboration, science-industry interaction, individual researcher, resource-based view

On-site residence time in a driven diffusive system: violation and recovery of mean-field

We investigate simple one-dimensional driven diffusive systems with open boundaries. We are interested in the average on-site residence time defined as the time a particle spends on a given site before moving on to the next site. Using mean-field theory, we obtain an analytical expression for the on-site residence times. By comparing the analytic predictions with numerics, we demonstrate that the mean-field significantly underestimates the residence time due to the neglect of time correlations in the local density of particles. The temporal correlations are particularly long-lived near the average shock position, where the density changes abruptly from low to high. By using Domain wall theory (DWT), we obtain highly accurate estimates of the residence time for different boundary conditions. We apply our analytical approach to residence times in a totally asymmetric exclusion process (TASEP), TASEP coupled to Langmuir kinetics (TASEP + LK), and TASEP coupled to mutually interactive LK (TASEP + MILK). The high accuracy of our predictions is verified by comparing these with detailed Monte Carlo simulations

Forensics and Case Studies in Civil Engineering Education: State of the Art

This paper reviews the state of the art in the use of forensic engineering and failure case studies in civil engineering education. The study of engineering failures can offer students valuable insights into associated technical, ethical, and professional issues. Lessons learned from failures have substantially affected civil engineering practice. For the student, study of these cases can help place design and analysis procedures into historical context and reinforce the necessity of lifelong learning. Three approaches for bringing forensics and failure case studies into the civil engineering curriculum are discussed in this paper. These are stand-alone forensic engineering or failure case study courses, capstone design projects, and integration of case studies into the curriculum. Some of the cases have been developed and used in courses at the United States Military Academy and the Univ. of Alabama at Birmingham, as well as at other institutions. Finally, the writers have tried to assemble many of the known sources of material, including books, technical papers, and magazine articles, videos, Web sites, prepared PowerPoint presentations, and television programs

High quality ultrafast transmission electron microscopy using resonant microwave cavities

Ultrashort, low-emittance electron pulses can be created at a high repetition rate by using a TM$_{110}$ deflection cavity to sweep a continuous beam across an aperture. These pulses can be used for time-resolved electron microscopy with atomic spatial and temporal resolution at relatively large average currents. In order to demonstrate this, a cavity has been inserted in a transmission electron microscope, and picosecond pulses have been created. No significant increase of either emittance or energy spread has been measured for these pulses. At a peak current of $814\pm2$ pA, the root-mean-square transverse normalized emittance of the electron pulses is $\varepsilon_{n,x}=(2.7\pm0.1)\cdot 10^{-12}$ m rad in the direction parallel to the streak of the cavity, and $\varepsilon_{n,y}=(2.5\pm0.1)\cdot 10^{-12}$ m rad in the perpendicular direction for pulses with a pulse length of 1.1-1.3 ps. Under the same conditions, the emittance of the continuous beam is $\varepsilon_{n,x}=\varepsilon_{n,y}=(2.5\pm0.1)\cdot 10^{-12}$ m rad. Furthermore, for both the pulsed and the continuous beam a full width at half maximum energy spread of $0.95\pm0.05$ eV has been measured

Making sense of race/ethnicity and gender in televised football: reception research among British students

Most people today watch football by way of the mass media, sites that reproduce and transform ideologies and ideas surrounding racial/ethnic and gender identity. However, still little remains known as to what extent actual football viewers take up or resist these ideas. Drawing on a cultural studies perspective, this study tries to identify the dominant discourses that British television viewers use to assign meaning to race/ethnicity and gender in men’s and women’s football on television. Eleven focus groups of British students (n = 44) were utilized to explore these discourses. Our findings indicate that viewers from various ethnic backgrounds were largely compliant with the hegemonic media discourses about natural physicality in both gender and race/ethnic comparisons. At the same time, multiple negotiated/oppositional discourses were found in relation to women’s football that showed how other social practices contributed to such readings. Limitations and possible areas for future research are discussed

Mechanical cell-matrix feedback explains pairwise and collective endothelial cell behavior in vitro

In vitro cultures of endothelial cells are a widely used model system of the collective behavior of endothelial cells during vasculogenesis and angiogenesis. When seeded in an extracellular matrix, endothelial cells can form blood vessel-like structures, including vascular networks and sprouts. Endothelial morphogenesis depends on a large number of chemical and mechanical factors, including the compliancy of the extracellular matrix, the available growth factors, the adhesion of cells to the extracellular matrix, cell-cell signaling, etc. Although various computational models have been proposed to explain the role of each of these biochemical and biomechanical effects, the understanding of the mechanisms underlying in vitro angiogenesis is still incomplete. Most explanations focus on predicting the whole vascular network or sprout from the underlying cell behavior, and do not check if the same model also correctly captures the intermediate scale: the pairwise cell-cell interactions or single cell responses to ECM mechanics. Here we show, using a hybrid cellular Potts and finite element computational model, that a single set of biologically plausible rules describing (a) the contractile forces that endothelial cells exert on the ECM, (b) the resulting strains in the extracellular matrix, and (c) the cellular response to the strains, suffices for reproducing the behavior of individual endothelial cells and the interactions of endothelial cell pairs in compliant matrices. With the same set of rules, the model also reproduces network formation from scattered cells, and sprouting from endothelial spheroids. Combining the present mechanical model with aspects of previously proposed mechanical and chemical models may lead to a more complete understanding of in vitro angiogenesis.Comment: 25 pages, 6 figures, accepted for publication in PLoS Computational Biolog

Skill-biased technological change and the business cycle

Over the past two decades, technological progress in the United States has been biased towards skilled labor. What does this imply for business cycles? We construct a quarterly skill premium from the CPS and use it to identify skill-biased technology shocks in a VAR with long-run zero and sign restrictions. Hours fall in response to skill-biased technology shocks, indicating that part of the technology-induced fall in hours is due to a compositional shift in labor demand. Investment-specific technology shocks reduce the skill premium, indicating that capital and skill are not complementary in aggregate production