Harmonic Impact of Rectifiers Served by Unbalanced Three-Phase Sources

A converter is a rectifier or inverter which is intended to transfer electrical energy between AC and DC busses. A common industrial converter usually employs the familiar Graetz bridge configuration and usually has a rating in the kilowatt through lower megawatt range. The operation of such a device is nominally in the balanced three-phase mode in which the phase currents are nonsinusoidal. The Fourier series components of these currents, or harmonics, have been studied extensively, but relatively little has been done in the unbalanced operating mode. The principal goal of this research is to examine how unbalance in magnitude and phase of the AC supply alters the frequency spectrum of a line-commutated power converter. The topics considered in this thesis are for cases of small unbalance, infinite inductance in the DC circuit (Ldc = OO), and pure resistance in the DC circuit (Ldc= 0). Also, symmetrical component analysis is made, and reasons for the presence of uncharacteristic harmonics are studie

Forest Islands in a Sea of Urban Habitat

Urban forests are poorly defined as ecological communities. Substantive links between anthropogenic landscape features and forest ecology are lacking. ‘Urbaness’ is commonly defined by human population density or land use classifications, but their use is inconsistent throughout the literature, and rarely is linked with ecological processes. Furthermore, it is unknown whether urban forests are functioning parts of a patchy urban woodland system or isolated islands amidst an ocean of unsuitable habitat. I first used digital satellite imagery and publicly available U.S. National Park data to link urban land use with forest processes. I then linked those land use classifications with the potential for urban forests to regenerate by investigating tree recruitment in the greater Buffalo, NY (U.S.) metropolitan area. If urban forests link with the greater regional forest ecosystem, then tree species richness should resemble the regional forests. However, if the urban forest patches are isolated, they should contain a subset of the regional forest richness with recruitment limited by forest patch size. Heavy urban cover predicted reduced tree and seedling richness and abundance. Moreover, tree seedling richness decreased with increasing urban land use. Tree seedling richness and abundance both declined when invasive species were present, suggesting invasive species may act as a barrier to tree recruitment. Tree recruitment was more strongly linked with forest patch size than the regional species pool, and active dispersal was limited to wind-dispersed species between urban forests. These results suggest that urban forests are isolated forest islands surrounded by an ocean of urban habitat

Who’s writing open access (OA) articles? Characteristics of OA authors at Ph.D.-granting institutions in the United States

The open access (OA) publication movement aims to present research literature to the public at no cost and with no restrictions. While the democratization of access to scholarly literature is a primary focus of the movement, it remains unclear whether OA has uniformly democratized the corpus of freely available research, or whether authors who choose to publish in OA venues represent a particular subset of scholars—those with access to resources enabling them to afford article processing charges (APCs). We investigated the number of OA articles with article processing charges (APC OA) authored by 182,320 scholars with known demographic and institutional characteristics at American research universities across 11 broad fields of study. The results show, in general, that the likelihood for a scholar to author an APC OA article increases with male gender, employment at a prestigious institution (AAU member universities), association with a STEM discipline, greater federal research funding, and more advanced career stage (i.e., higher professorial rank). Participation in APC OA publishing appears to be skewed toward scholars with greater access to resources and job securit

Changing Microarthropod Communities in Front of a Receding Glacier in the High Arctic

This study was carried out at Ny-Ålesund on Spitsbergen in Svalbard (High Arctic). Eight study sites were established along a transect from the fjord to the snout of the glacier. The sites di ered from each other by the type of vegetation cover and soil characteristics. Soil samples were collected and placed in Tullgren funnels. Extracted arthropods were represented by two groups of mites (Mesostigmata and Oribatida) and springtails (Collembola). The pioneer species that occurred first after retreat of the glacier were representatives of the Collembola (Agrenia bidenticulata and Hypogastrura concolor). Later, other springtails appeared including Folsomia alpha, Folsomia quadrioculata, Hypogastrura concolor, Isotoma anglicana, Sminthurinus concolor and the first species of oribatid mites; Camisia foveolata and Tectocepheus velatus velatus. Arthropod communities recorded along the transect were characterized by large variations in both species composition and abundance of individuals. The greater the distance from the glacier snout, the greater the species richness (2 to 22 species). The mean number of species per sample was the lowest at site 8 (1 0.71) (the closest to the glacier) and greatest at site 1 (14 1.41) (furthest from the glacier). The Simpson’s diversity index (D) was distinctly greater at sites 1 (4.61 0.06) and 3 (3.94 0.11) than at other sites, especially site 8 (1.07 0.06). Densities were least in the samples closest to the glacier (30 to 101 individuals; density 3000–10,100 individuals/m2). At the other locations, abundance was highly variable (905 to 7432 individuals; density 90,500–743,200 individuals/m2). The mean abundances were greatest at sites 2 and 3. The great variations in total abundances observed were often due to the presence or absence of one or more dominant species exhibiting extreme abundance variability between sites. The microarthropod community of the High Arctic is composed of heterogeneous circumpolar species, yet on a landscape scale is extremely dependent on local environmental conditions which may be subject to rapid change

AC losses and critical current density of superconducting GdBa2Cu3O7−x

Energy losses occurring in a cylindrical sample of Gd-Ba-Cu-O, subjected to an external AC magnetic field were examined. The loss dependence on the amplitude of the magnetic induction exhibits two stages of flux penetration into the superconductor. Critical current densitities for both stages of penetration were examined and an explanation for such behaviour is proposed. Support for this point of view is obtained by measurements on pulverized sample material. All measurements were performed at a temperature of 4.2 K and in absence of a background field.\ud \ud Analysis of the data provides two critical current densities: an inter-granular critical current density at weak alternating magnetic fields and an intra-granular critical current density at higher magnetic fields. The intra-granular critical current density is at least two orders of magnitude larger than the inter-granular one

A Virtual Classroom Via Dataconferencing: A Multi-Institutional Experience

This paper presents the experience of five faculty from four universities who are co-developing and team teaching courses using the world-wide web (WWW). In particular two novel power engineering courses, Flexible Control of Transmission Systems and Flexible Control of Distribuction Systems, introduce the student to the application of power electronic-based controllers, known specifically as flexible AC transmission systems (FACTS) controllers, to enhance the performance of the transmission and distribution systems in an electric power system. In order to leverage faculty expertise in the various topics included in each course, a unique partnership was formed among five faculty from the following universities: University of Arkansas, Kansas State University, University of Missouri--Rolla and Purdue University. In order to achieve the most effective presentation, in the presence of geographical, cost, time and technology constraints, the co-developers decided to pursue various distance learning strategies to link the four universities together in a live interactive virtual classroom utilizing dataconferencing software and the WWW. In this paper we address the trials and tribulations of our experience. In particular, we discuss the use of video tape technology by mail, the use of live audio and video via an integrated service digital network (ISDN) approach, and lastly, dataconferencing software in conjunction with telephone conferencing. Each of these alternative methodologies are examined in detail and are compared and contrasted from a cost and pedagogical point of view based on our experience

Collaborative Distance Education in Power Engineering

This paper presents a perspective on offering shared-courses, or courses simultaneously offered at two or more universities via various distance educational frameworks. Over a three year duration, two senior/graduate level courses were jointly developed and offered to the students at the University of Missouri--Rolla, the University of Arkansas, Kansas State University, and Purdue University. This paper discusses the various distance educational technologies including two-way audio/visual via ISDN line, video-tapes, and web-based conferencing. Both instructor and student reactions to these mediums are included. Pedagogical methods appropriate for these mediums are outlined. The paper concludes with recommendations and strategies for engineering institutions who would like to course-share with other universities and industry

A Multi-Institutional Cooperative Approach to Power Engineering Education

This paper describes the multi-institutional cooperative effort between the University of Missouri--Rolla, Kansas State University, and the University of Arkansas to develop two new courses in flexible power system control

Northern blotting analysis of microRNAs, their precursors and RNA interference triggers

<p>Abstract</p> <p>Background</p> <p>Numerous microRNAs (miRNAs) have heterogeneous ends resulting from imprecise cleavages by processing nucleases and from various non-templated nucleotide additions. The scale of miRNA end-heterogeneity is best shown by deep sequencing data revealing not only the major miRNA variants but also those that occur in only minute amounts and are unlikely to be of functional importance. All RNA interference (RNAi) technology reagents that are expressed and processed in cells are also exposed to the same machinery generating end-heterogeneity of the released short interfering RNAs (siRNAs) or miRNA mimetics.</p> <p>Results</p> <p>In this study we have analyzed endogenous and exogenous RNAs in the range of 20-70 nt by high-resolution northern blotting. We have validated the results obtained with northern blotting by comparing them with data derived from miRNA deep sequencing; therefore we have demonstrated the usefulness of the northern blotting technique in the investigation of miRNA biogenesis, as well as in the characterization of RNAi technology reagents.</p> <p>Conclusions</p> <p>The conventional northern blotting enhanced to high resolution may be a useful adjunct to other miRNA discovery, detection and characterization methods. It provides quantitative data on distribution of major length variants of abundant endogenous miRNAs, as well as on length heterogeneity of RNAi technology reagents expressed in cells.</p