Modeling and control of brushless doubly-fed bar cage induction machines

Thesis (MEng)--Stellenbosch University, 2022.ENGLISH ABSTRACT: In recent years the requirement for more sustainable sources of energy has increased sig nificantly, with wind energy growing increasingly as a renewable source. Many countries are investing greatly in sustainable growth by going completely renewable. Countries like Iceland had an annual consumption of 60.8 TWh in 2019, of which 79% was produced by renewable energy sources. For this growth to be sustainable, more efficient and eco nomic sources of renewable energy will be required. The Brushless Doubly Fed Induction Generator (BDFIG) has become a focus point due to its variable speed capability and brushless technology. The direct-current-link (DC-link voltage) in the BDFIG systems’ back to back con verter allow for bidirectional power flow of the control winding’s power. Making effective control of the DC-link voltage a necessity. Due to the presence of switching elements in the back-to-back converter, there are harmonics introduced into the utility network. To mitigate this low pass filters such as inductance capacitance (LCL) and resistive inductive (RL) filters are often used. Currently, BDFIGs are not used in sizable wind farms. This is mostly due to their complexity when compared to standard doubly fed induction generators (DFIG’s) and permanent magnet synchronous generators (PMSGs). Thus far they have mostly been used in islanding applications. In this mode of operation they must be controlled to provide stable voltages at constant frequency with varying load conditions and changing wind speeds. To compete with DFIG’s which are already in the market, the BDFIG has a few disadvantages due to its slightly more complex structure, higher cost and larger dimensions. In this thesis, the power control of the grid-connected BDFIG systems in wind turbine applications are presented. Additionally the control of these machines as motors are also investigated and performed. An experimental machine consisting of a bar cage rotor is modeled in detail and controlled in simulation and by experimentation. DC-link volt age control is analyzed whereby the grid-side converter is controlled as a voltage source converter. Vector control is used in all control solutions, with reductions in control com plexity made and analyzed for the control winding side of the machine to reduce cost and improve robustness while maintaining responsiveness and accuracy.AFRIKAANSE OPSOMMING: In die laaste paar jaar het die aanvraag vir meer volhoubare energiebronne aansienlik toegeneem, veral wanneer daar gekyk word na wind energie as energiebron. Baie lande is al klaar besig om grootliks in hernubare energie te belê. Ysland, byvoorbeeld, het ’n jaarlikse verbruik van 60.8 TWh in 2019 gehad, waarvan 79% van die energie geproduseer was deur hernubare bronne. Vir die groei om volhoubaar te wees word meer effektiewe en ekonomiese hernubare energiebronne benodig. Die Brusellose Dubbel Gevoerde Induksie Generator (BDGIG) het, as gevolg van die masjien se veranderlike spoed vermoë en brusellose tegnologie, n fokus punt in die navorsingsveld geword. Die direkte stroom (DS) skakel spanning in die BDGIG sisteme se rug-aan-rug omskakelaar laat tweerigting krag vloei van die beheer winding se krag toe. Dus is effektiewe beheer van die DS-skakel se spanning nodig. As gevolg van die wisselings elemente wat in rug-aan-rug omskakelaars voorkom, kan daar harmonieke in die krag netwerk geïnduseer word. Om hierdie te versag word laag deurlaat filters, soos LCL en RL filters, gereeld gebruik. Tans word BDGIG nie op groot wind plase gebruik nie. Dit is meestal as gevolg van die kompleksiteit daarvan wanneer dit met standaard DGIGe en permanente magneet sinkroon masjinee (PMSMe) vergelyk word. Tot dusver is dit meestal vir eiland wyse toepassing gebruik. In hierdie wyse van werking moet hulle beheer word om stabiele spanning teen n konstante frekwensie met afwisselende lading kondisies en veranderende wind spoed te kan bied. Dit is moeilik vir die BDGIG om met DGIGe wat klaar in die mark is te kompeteer aangesien dit nadele soos n meer komplekse struktuur, hoër kostes en groter dimensies het. In dié tesis word die krag beheer van die krag-netwerk-gekoppelde BDGIG sisteme in wind turbine toepassing voorgestel. Verder word die beheer van die masjiene as motors nagevors en uitgevoer. n Eksperimentele masjien, wat bestaan uit n staafhok rotor, is in detail ontwerp en beheer in simulasie sowel as deur eksperimentering. DC-skakel spanning beheer is ook geanaliseer waardeur die krag-netwerk omskakelaar as spanning bron beheer word. Vektor beheer word in alle kontrole oplossings gebruik, met vermindering in beheer kompleksiteit wat plaasvind en ’n analise van die beheer winding kant van die masjien wat verminderde koste en verbeterde robuustheid vertoon terwyl dit responsiwiteit en akkuraatheid volhou.Master

Southern African pasture and forage science entering the 21st century : past to present

Given the global challenges of mitigating and alleviating land degradation and ensuring food security, particularly through protein production, pasture and forage science together with rangeland science will remain imperative to sustainable livestock production systems. Recognising the scientific principles developed in this discipline the past 50 years and more in Southern Africa, customary to our diverse and unique environmental conditions will continue to guide and identify future research priorities. A journey from the past to the present can assist in avoiding reinvention and repetition of pasture research. This review highlights how research priorities have either remained the same over time or where research has received less or more priority. The review further illustrates that there has been more interest in species adaptation to drought, pasture and forage water relations, alternative pasture and forage species, and the combination of pasture species in the 21st century than in the 20th century. In conclusion, the soil–plant– livestock complex can serve as a conceptual basis for more effective research together with pasture breeding and nutrition. Finally, researching pasture and forage sciences in an inter- and multi-disciplinary manner clearly supports the principles of holism applied by General JC Smuts, one of the first pioneers of pasture and forage sciences.http://www.tandfonline.com/loi/tarf20hb2016Plant Production and Soil Scienc

Institutions and governance of communal rangelands in South Africa

The creation of local institutions with a mandate over land access and control is seen as a prerequisite for successful decentralisation of land tenure and effective local resource management in sub-Saharan Africa. However, with land tenure reform in South Africa currently at a state of legislative impasse, real uncertainty now exists over land rights and governance of rangeland in many communal areas. This paper draws on case study material from Eastern Cape province to illustrate how this ongoing uncertainty has resulted in the operation of a range of traditional authority and civil society institutions in different communal areas with varying degrees of legitimate authority over land administration and highly variable performance in managing rangeland resources. Collective management of rangeland resources seems most difficult in environments where land rights are contested because of the coexistence of traditional leaders and civil society institutions. On this basis an approach to tenure reform is advocated, which vests all powers over local land administration in democratically elected and accountable civil society institutions. Some successful examples of this already exist and might serve to guide policy formation, which must be flexible enough to accommodate collective management approaches that emphasise cooperation both within and between communities.Keywords: common property, land tenure, natural resource management, traditional leadersAfrican Journal of Range & Forage Science 2013, 30(1&2): 77–8

Iron and ferritin accumulate in separate cellular locations in Phaseolus seeds

<p>Abstract</p> <p>Background</p> <p>Iron is an important micronutrient for all living organisms. Almost 25% of the world population is affected by iron deficiency, a leading cause of anemia. In plants, iron deficiency leads to chlorosis and reduced yield. Both animals and plants may suffer from iron deficiency when their diet or environment lacks bioavailable iron. A sustainable way to reduce iron malnutrition in humans is to develop staple crops with increased content of bioavailable iron. Knowledge of where and how iron accumulates in seeds of crop plants will increase the understanding of plant iron metabolism and will assist in the production of staples with increased bioavailable iron.</p> <p>Results</p> <p>Here we reveal the distribution of iron in seeds of three <it>Phaseolus </it>species including thirteen genotypes of <it>P. vulgaris</it>, <it>P. coccineus</it>, and <it>P. lunatus</it>. We showed that high concentrations of iron accumulate in cells surrounding the provascular tissue of <it>P. vulgaris </it>and <it>P. coccineus </it>seeds. Using the Perls' Prussian blue method, we were able to detect iron in the cytoplasm of epidermal cells, cells near the epidermis, and cells surrounding the provascular tissue. In contrast, the protein ferritin that has been suggested as the major iron storage protein in legumes was only detected in the amyloplasts of the seed embryo. Using the non-destructive micro-PIXE (Particle Induced X-ray Emission) technique we show that the tissue in the proximity of the provascular bundles holds up to 500 μg g^-1of iron, depending on the genotype. In contrast to <it>P. vulgaris </it>and <it>P. coccineus</it>, we did not observe iron accumulation in the cells surrounding the provascular tissues of <it>P. lunatus </it>cotyledons. A novel iron-rich genotype, NUA35, with a high concentration of iron both in the seed coat and cotyledons was bred from a cross between an Andean and a Mesoamerican genotype.</p> <p>Conclusions</p> <p>The presented results emphasize the importance of complementing research in model organisms with analysis in crop plants and they suggest that iron distribution criteria should be integrated into selection strategies for bean biofortification.</p

Presidential Address:Ethics, legislation and profit as determinanats of conservation farming.

Keywords: conservation; conservation farming; economics; ethics; farming; grassland society of southern africa; legislation; presidential address; research; south afric

Environmental control of the growth distribution index of vegetational grass plants

One of the ways in which grasses respond to varying conditions is by adjusting the growth rates of their different component parts relative to their overall growth rate. When plants are grown in constant conditions, increasing age brings about a decline in the growth rate of the leaf lamina component and an increase in the growth rate of the root component relative to the plant growth rate. Variations in temperature, photoperiod and in the daily amount of radiant energy received by the plant do, however, influence the distribution pattern of growth within the plant.Keywords: laminae|temperatures|photoperiods|radiant energy|energy|Aberystwyth|plants|distribution patterns|growth rates|grasses|vegetation|growth|growth distribution indices|environmental controls|leave

The relative contribution of overstocking and selective grazing to the degeneration of tall grassveld in Natal

Overstocking has for long been blamed for the deterioration of South African grasslands and, in the Tall Grassveld of Natal, it certainly leads to rapid deterioration of both herbage production and species composition. In this veld type, however, selective grazing also has a profound effect on species composition in that it encourages the encroachment of Aristida junciformis, a species which is largely avoided by animals and therefore contributes little to animal production.Keywords: overstockings|selective grazing|degenerations|tall grasses|veld|KwaZulu Natal Province|grazing|herbage production|species compositions|veld types|encroaching species|grasslands|grasses|tall grassvelds (ill