Thesis (PhD)--University of South Australia, 1999

Thesis (M.S.)--Youngstown State University, 2008. / Includes bibliographical references (leaves 107-109). Also available via the World Wide Web in PDF format.

Thesis (M.S.)--Oregon State University, 2005. / Printout. Includes bibliographical references (leaves 95-97). Also available on the World Wide Web.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1959. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 148-162).

Στις μέρες μας το ενεργειακό πρόβλημα αποτελεί ένα από τα σπουδαιότερα προβλήματα του πλανήτη, το οποίο μαζί με την ραγδαία κλιματική αλλαγή οδηγούν στην ανάγκη για παραγωγή ηλεκτρικής ενέργειας με όσο πιο οικονομικό αλλά και φιλικό προς το περιβάλλον τρόπο. Συνεπώς οι Ανανεώσιμες Πηγές Ενέργειας και κυρίως η αιολική ενέργεια αποκτούν σπουδαίο ρόλο στο στόχο αυτό και αλλάζουν άρδην τόσο τον Ευρωπαϊκό όσο και τον Παγκόσμιο ενεργειακό χάρτη. Στην παρούσα διπλωματική εργασία μελετάμε μια διάταξη ανεμογεννήτριας, η οποία αποτελείται σε πρώτη φάση από μια επαγωγική μηχανή βραχυκυκλωμένου κλωβού (SCIG), από έναν μετατροπέα συχνότητας (back-to-back converter) και από ένα σε σειρά συνδεδεμένο τριφασικό R-L φίλτρο στην ac πλευρά μέχρι το ζυγό διασύνδεσης με το δίκτυο, ενώ στη συνέχεια προσθέτουμε, μετά το R-L φίλτρο, μια συστοιχία πυκνωτών συνδεδεμένων σε αστέρα, ένα τριφασικό R-L φορτίο στο ίδιο σημείο με τη συστοιχία πυκνωτών καθώς και τη γραμμή μεταφοράς για σύνδεση με το δίκτυο. Σκοπός λοιπόν αυτής της εργασίας είναι να μελετήσουμε το πλήρες σύστημα της ανεμογεννήτριας στο d-q στρεφόμενο σύστημα κάθετων αξόνων μέσω του μετασχηματισμού Park. Στη συνέχεια προχωρούμε στον υπολογισμό των εξισώσεων στο χώρο κατάστασης και τελικά προτείνουμε ένα σύστημα ελέγχου το οποίο βασίζεται στους PI ελεγκτές. Τέλος, προσομοιώνουμε το πλήρες σύστημα με το Simulink του MATLAB και στη συνέχεια παρουσιάζουμε τις αποκρίσεις των προσομοιώσεων και εξάγουμε συμπεράσματα. / Nowadays the energy problem is one of the most important global problems, which together with the rapid climate change lead to the need for power generation as more economical and environmentally friendly way. Therefore, the Renewable Energy Sources and especially wind power have a great role in this objective and radically change both the European and the global energy map. In this paper we study a wind turbine device, which consists at first of a squirrel cage induction generators (SCIG), a frequency converter (back-to-back converter) and an three-phase R-L filter on the grid side and then me add, after the filter, a capacitor bank connected in star, a three-phase R-L load at the same point with the capacitor banks and the transmission line for grid connection. Aim of this thesis is to study the complete wind turbine system in the d-q rotating vertical axes system through the Park transformation. Then we proceed to the calculation of the equations in state space and eventually propose a control system based on PI controllers. Finally, we simulate the complete system with the MATLAB/Simulink and then we present simulations of the responses and draw conclusions.

Recent failures of large induction machines due to rotor bar breaks have become a major industrial concern in South Africa. These failures have occurred while applying current condition monitoring methods. This would imply that current theories are either inadequate or badly implemented. This thesis investigates two currently used condition monitoring strategies which focus on monitoring the stator current and axial vibration to detect bar breaks as well as a third method which involves putting destructive shaft voltages to good use and using this signal to diagnose rotor defects. This document begins by familiarizing the reader with two conventional approaches which involve monitoring the stator current and the axial vibration to detect rotor bar breaks. Thereafter the origins of shaft voltage are discussed and its use as a condition monitoring tool is first theoretically derived and then validated by finite element simulations. A thorough discussion of the measurement equipment required is presented and ultimately the performance of these three methods is tested by means of a laboratory measurement as well as two on-site measurements. The results from these measurements suggest that when making proper use of both conventional condition monitoring methods, these methods have a 50% success rate in the detection of rotor bar breaks. The alternative method investigated, which involves monitoring the shaft voltage, has a 75% success rate in the detection of rotor bar breaks. This highlights the use of shaft voltage as a condition monitoring tool. / Dr. S. R. Holm Prof. W. A. Cronje

The purpose of this study is to provide a qualitative, detailed description of individual and social behavior in a free-living population of thirteen-lined ground squirrels. Behavioral differences in relation to various periods of the annual cycle are also evaluated.

We describe a new WWW-based information system called Squirrel which demonstrates an attempt to integrate a relational database system (RDBS) with an information retrieval system (IRS) providing context based access to a SGML document collection. Indexing of the documents is performed on full-text basis whereas search capabilities include ranking of the retrieved documents. A functioning prototype has been implemented and is available on the Internet.

The behavior and ecology of Uinta ground squirrels (Spermophilus armatus) at the Utah State University Forestry Field Station northeast of Logan, Utah were studied from 1964-1971 to determine the role of behavior in population regulation. From 1965 to 1968, data were collected to establish population norms. In 1968, the population density was reduced experimentally to help elucidate regulatory processes. This paper compares the dynamics of the population before and after the population reduction. Before the density reduction, juvenile survival was lower than that of yearlings and adults. The disappearance of juveniles was associated with their tendency to disperse from the natal burrow while still small. The litter size of yearlings was smaller than that of adults. The principal changes following the reduction were an increase in percentage of yearling squirrels breeding and a decrease in dispersal of juveniles. Litter size and proportion of adult females breeding did not change significantly. Dispersal of squirrels from the study area was the principal regulatory factor. Losses during hibernation, presumably to badger predation, also decreased after the reduction. Several conclusions were drawn. The importance of anyone of the complex of factors, which determined the dynamics of the population, varied with place, time, and population density. The value of habitat was affected by population density as well as the physical environment. Density pressures seemed to have more effect on subordinate than on dominant squirrels. Surplus animals from the prime habitat raised the population density in the surrounding area. Dispersing animals probably suffered higher mortality than those which stayed, but this did not imply that dispersal was maladaptive for the individual. (58 pages)

Human population growth and development reduce the area and quality of natural communities and lead to a reduction of populations of the species associated with them. Certain species can be useful indicators or "focal species" for determining the quality of ecosystem remnants and the required management practices. Tree squirrels are good models for studies on the effects of fragmentation because they depend on mature forests. The Big Cypress fox squirrel, (Sciurus niger avicennia), a state-listed Threatened subspecies endemic to south Florida, appears sensitive to habitat fragmentation and fire regime. This research aims to assess the conservation status of the Big Cypress fox squirrel. I documented the current distribution of the fox squirrel by obtaining and mapping occurrence records and through interviews with biologists and other field personnel of public land-managing agencies, and private landowners including golf course managers. Transect sampling was used to survey and sample natural areas and private lands to evaluate the distribution, abundance, and habitat use of fox squirrels. Natural areas and suburban areas appear to support Big Cypress fox squirrels, but individuals are widely distributed and only found in low numbers throughout southwest Florida. The distribution of fox squirrel populations depends on land use and understory height, but not the size of trees. Fire suppression has resulted in a dense understory in large portions of parks and preserve lands, which is unsuitable for fox squirrels.