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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
51

Investigation into the performance of outdoor insulators under high humidity conditions.

06 September 2010 (has links)
performance of high voltage outdoor insulators can be greatly affected by weather conditions. The weather conditions under investigation are cold temperatures coupled with high humidity levels. Weather data from a number of coastal stations around South Africa was analyzed to determine whether surface condensation was likely on outdoor insulators under these weather conditions. A heat transfer equation was used to determine the response of the insulator surface temperature to the environmental temperature. It was found that surface condensation would occur on the insulator surfaces which would lead to sudden, heavy wetting of the surface. Outdoor insulators in coastal environments are often heavily polluted, due to salt spray, and when wet, a conductive layer can form on the insulator surface. This conductive layer can result in appreciable leakage currents flowing on the insulator surface, often leading to premature failure. The finite element method program, Maxwcll, was used to simulate the outdoor insulators both under these polluted, wet conditions and under unpolluted conditions. Both cases were simulated for a silicone rubber, glass cap-and-pin and two EPDM outdoor insulators. The polluted insulators were simulated with varying pollution severities. The results of the simulations are analyzed and the surface resistances of the wet polluted insulators were calculated. An experiment was' set up to mask the environmental weather conditions found which would lead to surface condensation. The insulators under test were placed in a chilled weather chamber which introduced a steam fog to simulate the humidity. The leakage current was measured and recorded for comparison with the simulation results. The results of the weather chamber test showed that surface condensation resulted in more severe wetting than manual wetting. The weather chamber surface resistances calculated were much lower than those calculated by the Maxwcll simulations. This was due to the difference it humid particle temperature in the condensation rate equations used for the Maxwell simulations, and the humid particle temperature of the steam fog used in the weather chamber. Polluted coastal outdoor insulators exposed to the above weather conditions will experience larger than normal leakage currents which will lead to premature failure of the units. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2008.
52

The impact of network changes on power quality and compensation device perfomance.

Ngcamu, Mbulelo Busani Edmund. January 2012 (has links)
This dissertation describes the impact of changing network configuration on power quality and performance of existing compensation devices in the transmission network. The underlying theory was assessed and thereafter the Everest substation network case scenario was selected to study the above due to; the number of reconfigurations it has experienced in the past, increased capacitor bank failures and also due to the harmonics problems experienced. The study involved the installation of harmonic current measuring instruments at Everest 132kV feeders to identify the potential sources of harmonics and to determine the dominant harmonics. A dig Silent Power Factory model was then constructed to perform various simulations in order to determine the impact of the changes done on the Everest network as well as the impact of capacitor switching on the harmonics amplification at Everest 132kV Bus-bars. The study also focussed on analysing the performance history of the capacitor banks at Everest and to determine if high harmonic amplification had an impact on capacitor bank performance. The simulation results revealed that network reconfigurations have negatively impacted power quality at Everest. The results showed that there is also a correlation between the switching of the two 72MVAR capacitor banks and the amplification of the harmonics at Everest. The highest amplification occurred when both capacitor banks were switched in and the resonance point occurred around the 5th harmonic which coincided with the data from field measurements. There was a 61% increase in 5th harmonic impedance amplitude after the Everest network was reconfigured, for the condition when both capacitors are switched in. The lowest amplification occurred when none of the capacitor banks were switched in. Three options were assessed to eliminate the problem of harmonics at Everest, the first one was to prohibit the switching in of both capacitors at Everest and utilise other available means around the Everest network for voltage support. The second option was to change the capacitor size, thus moving the resonance point away from the 5th harmonic. The last option was to install a harmonic filter at Everest to filter out the problematic harmonics. The first option is recommended as it has been successfully tested, can be readily implemented and is much more cost effective compared to the others. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.
53

Physical layer forward error correcetion in DVB-S2 networks.

Naidoo, Theran. January 2012 (has links)
The rapid growth of wireless systems has shown little sign of ceasing, due to increased consumer demand for reliable interactive services. A key component of the development has centered on satellite networks, which allows provision of services in scenarios where terrestrial systems are not viable. The Digital Video Broadcasting-Satellite Second Generation (DVB-S2) standard was developed for use in satellite broadcast applications, the foremost being video broadcasting. Inherent to DVB-S2 is a powerful forward error correction (FEC) module, present in both the Physical and Data Link Layer. Improving the error correcting capability of the FEC is a natural advent in improving the quality of service of the protocol. This is more crucial in real time satellite video broadcast where retransmission of data is not viable, due to high latency. The Physical Layer error correcting capability is implemented in the form of a concatenated BCH-LDPC code. The DVB-S2 standard does not define the decoding structure for the receiver system however many powerful decoding systems have been presented in the literature; the Belief Propagation-Chase concatenated decoder being chief amongst them. The decoder utilizes the concept of soft information transfer between the Chase and Belief Propagation (BP) decoders to provide improved error correcting capability above that of the component decoders. The following dissertation is motivated by the physical layer (PL) FEC scheme, focused on the concatenated Chase-BP decoder. The aim is to generate results based on the BP-Chase decoder in a satellite channel as well as improve the error correcting capability. The BP-Chase decoder has shown to be very powerful however the current literature provides performance results only in AWGN channels. The AWGN channel however is not an accurate representation of a land-mobile satellite (LMS) channel; it does not consider the effect of shadowing, which is prevalent in satellite systems. The development of Markov chain models have allowed for better description of the characteristics of the LMS channel. The outcome being the selection of a Ku band LMS channel model. The selected LMS channel model is composed of 3 states, each generating a different degree of shadowing. The PL system has been simulated using the LMS channel and BP-Chase receiver to provide a more accurate representation of performance of a DVB-S2 network. The effect of shadowing has shown to reduce coding performance by approximately 4dB, measured over several code lengths and decoders, when compared with AWGN performance results. The second body of work aims to improve the error correcting capability of the BP-Chase decoder, concentrating on improving the LDPC decoding module performance. The LDPC system is the basis for the powerful error correcting ability of the concatenated scheme. In attempting to improve the LDPC decoder a reciprocal improvement is expected in the overall decoding performance of the concatenated decoder. There have been several schemes presented which improve BP performance. The BP-Ordered statistics decoder (OSD) was selected through a process of literary review; a novel decoding structure is presented incorporating the BP-OSD decoder into the BP-Chase structure. The result of which is the BP-OSD-Chase decoder. The decoder contains two stages of concatenation; the first stage implements the BPOSD algorithm which decodes the LDPC code and the second stage decodes the BCH code using the Chase algorithm. Simulation results of the novel decoder implementation in the DVBS2 PL show a coding gain of 0.45dB and 0.15dB versus the BP and BP-Chase decoders respectively, across both the AWGN and LMS channel. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.
54

Design of a passive rotor transverse flux rotating machine

Drennan, Duncan Scott 04 1900 (has links)
Pages 6-12 missing. / Thesis (MScEng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: The transverse flux machine (TFM) offers the opportunity of high torque to volume ratios which makes it an excellent candidate for direct wheel drives and low speed generator applications. TFMs have a three dimensional flux path which eliminates iron laminates as a viable core material. Soft magnetic composites have been adopted in these machines due to their isotropic nature. There are three main variants of TFMs, namely, active rotor (with magnets on the rotor), passive rotor (with magnets on the stator), and reluctance (with no magnets). As a relatively recent development in electrical machines, the TFM still has many hurdles facing its adoption in industry. Some of these hurdles are high cogging torque, a difficult construction, and expensive materials. This thesis focuses on the design of a three phase 50 kW passive rotor machine. Finite element simulation is used to determine the optimal configuration, and the final machine is analysed in detail. The construction process and associated problems are also detailed. The completed machine did not perform to the desired specification, but much knowledge was gleaned about the TFM, the construction caveats, and future potential directions. / AFRIKAANSE OPSOMMING: Die transversale-vloed masjien (TFM) met sy hoe draaimoment tot volume verhouding, is 'n uitstekende kandidaat vir direkte wiel aandrywing en lae spoed generator toepassings. Die vioed pad van die masjiene is drie-dimensioneel, wat yster laminasies as kern materiaal elimineer. "Soft magnetic composites" kan gebruik word vir hierdie masjiene as gevolg van hulle isotropiese eienskappe. Daar bestaan drie hoof variante van die TFM, naamlik, die aktiewe rotor (met magnete op die rotor), passiewe rotor (met magnete op to stator), en reluktansie (sonder magnete). Die TFM is 'n redelike nuwe tipe masjien en daar is nog probleme wat opgelos moet word voordat die industrie sal begin om dit te gebruik. Van hierdie probleme is "cogging" draaimoment, 'n moeilike konstruksie en duur materiale. Die fokus van hierie tesis is op die ontwerp van 'n 50 kW drie-fase passiewe rotor masjien. Eindige element simulasie is gebruik om die optimale konfigurasie te kry, en 'n analise is gedoen op die finale masjien. Die konstruksie proses en die probleme wat daarmeer saam gaan is ook beskryf. Die prototipe masjien wat gebou is het nie aan sy oorspronklike spesifikasie voldoen nie, maar baie kennis is opgedoen oor die TFM, die konstruksie proses, en potensiele toekomende toepassings.
55

Insulator pollution monitoring device : development, calibration and field evaluation

Schwardt, Wilhelm Heinrich 04 1900 (has links)
Thesis (MScEng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: The calibration and field evaluation of an Insulator Pollution Monitoring Relay (IPMR) were the main aims of this research programme. A repeatable artificial wetting test method was developed after several modifications were made to the steam system, test chamber and the test routine. The IPMR was successfully calibrated with insulators that were artificially polluted according to the solid layer method. Linear and polynomial relationships were determined after curve-fitting techniques were performed on the results. The calibration showed that the IPMR is capable as a device relating the maximum conductivity during artificial wetting to the ESDD, a severity classification parameter. The IPMR was successfully used in a salt fog chamber to determine if the device is capable to evaluate the severity of an instantaneous pollution event. The IPMR was successfully installed at a natural pollution test site along the Cape west coast. The conductivity measurements with natural wetting showed good correlation to flashovers experienced. A rule of thumb, developed to indicate a possible risk of flashover, was based on observations made on the relationship between humidity and surface conductivity. The measured IPMR data was successfully applied to quantify the site severity according to the conductivity measurement with natural wetting. This calculated severity value could be used in the assessment of flashover probability of high voltage insulators. / AFRIKAANSE OPSOMMING: Die hoofdoel van die tesis was die kalibrasie en veldtoetse van 'n isolatorbesoedelingsmonitor (IPMR). 'n Herhaalbare nagebootste benatting-toetsmetode is ontwikkel na veranderings aan die stoomstelsel, toetsruimte en die toetsproses. Die IPMR is suksesvol gekalibreer met isolators wat besoedel was met 'n nagebootste besoedeling volgens die "solid layer method". Liniêre sowel as kwadratiese verwantskappe is ontwikkel na krommepassings op die resultate uitgevoer was. Die kalibrasie het gewys dat die IPMR in staat is om die maksimum geleidingsvermoë wat d.m.v. nagebootste benatting verkry is, met die ESDD, 'n besoedelingsklassifikasie, kan vergelyk. Die apparaat is ook suksesvol gebruik tydens soutmistoetse om te bepaal of dit in staat is om 'n skielike besoedelingsgebeurtenis te kan meet. Na die afhandeling van laboratorium werk is die apparaat by 'n natuurlike isolator besoedeling-toetsstasie langs die Kaapse weskus geïnstalleer. Die geleidingsvermoë metings met natuurlike benatting het goeie korrelasie getoon met isolator oorvonkings. 'n Skattingsmetode wat ontwikkel is om moontlike oorvonkings te voorspel, is gebaseer op waarnemings wat gemaak is van die humiditeit sowel as die oppervlakte geleidingsvermoë. Die IPMR se geleidingsvermoë metings met natuurlike benatting is aangewend om die besoedelingsgraad van die gebied te bepaal. Die bepaalde besoedelingsgraad kan verder gebruik word om die waarskynlikheid van die oorvonking van isolators vas te stel.
56

Rotor design and performance evaluation of a PM-assisted reluctance synchronous traction machine

Sibande, Sguda Enock 04 1900 (has links)
Thesis (MScEng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: The thesis describes the optimum rotor design and performance of a 110kW Permanent-Magnet assisted (PM-assisted) Reluctance Synchronous traction Machine (RSM) using bonded permanent magnet sheets. Particular attention is given to the performance of the machine drive in the flux-weakening speed region. A detail explanation is given of the finite-element design optimisation, the basic principles of operation and the control-design of the PM-assisted RSM drive. A theoretical torque comparison of the PM-assisted RSM, the standard RSM and the induction machine is also done. The measured and calculated results of the different drives are presented and analysed in detail. It is concluded that the performance of the PM-assisted RSM in terms of torque, voltage and power factor compares favourably well with that of the induction machine in both the constant torque and flux-weakening speed regions. Furthermore, it is shown that the temperature rise of the stator winding of the PMassisted RSM is lower than that of the RSM with both machines at the same load. / AFRIKAANSE OPSOMMING: Die tesis beskryf die optimum rotor ontwerp en vermoë van 'n 110 kW Permanent- Pagnet-ondersteunde (PM-ondersteunde) Reluktansie Sinchroon Masjien (RSM) trekkrag aandryfstelsel. Spesifieke aandag word gegee aan die vermoë van die aandryfstelsel in die vloedverswakking spoedgebied. 'n Volledige verduideliking word gegee van die eindige-element ontwerp optimering, die basiese beginses van werking en die beheer-ontwerp van die PM-ondersteunde RSM aandryfstelsel. 'n Teoretiese vergelyking van die draaiumoment-vergelyking van die PM-ondersteunde RSM, die standard RSM en die induksmasjien word gedoen. Die berekende en gemete resultate van die verskillende aandryfstelsels word in detail aangebied en ge-analiseer. Dit is gevind dat die vermoë van die PM-ondersteunde RSM in terne van draairnoment, spanning en arbeidsfaktor gunstig vergelyk met dit van die induksiemasjien in beide die konstante draairnoment en vloedverswakking spoedgebiede. Verder word getoon dat die temperatuur-styging van die statorwinding van die PM-ondersteunde RSM laer is as die van die RSM, met deide masjiene by die selfde las
57

A reliable telemetry software design for a satellite system

Boshielo, Babudi Turcia 12 1900 (has links)
Thesis (MScEng)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: One of the requirements for satellite telemetry systems is the provision of reliable telemetry data to allow accurate monitoring of the satellite status. This thesis focuses on the design of telemetry software that meets this data reliability requirement. An improved synchronization strategy to allow efficient ground reception of the telemetry data is implemented on SUNSAT's direct link. The data collection and transmission functions are also enhanced by the addition of the necessary redundant information to the data while meeting the real-time requirements of the system. To enhance the software quality a development methodology entailing structured programming practices and modular decomposition is proposed. It is shown that the resulting telemetry software fulfils the functional requirements. / AFRIKAANSE OPSOMMING: Een van die vereistes vir die satelliet telemetrie stelsel is die verskaffing van betroubare telemetrie date om akkurate monitering van die satelliet status te verseker. Hierdie tesis fokus op die ontwerp van die telemetrie sagteware wat hierdie data betroubaarheid bevredig. 'n Verbeterde sinkronisasie strategie is geïmplementeer om meer effektiewe grond ontvangs te verseker van die telemetrie data op SUNSAT se direkte skakel. Die data versameling en transmissie funksies is ook verder verbeter deur die aanvulling van nodige oortollige informasie in die data terwyl die intydse vereistes van die stelsel steeds bevredig word. Om die sagteware kwaliteit te verbeter is 'n ontwikkelings metodiek voorgestel wat gestruktureerde programeering strukture en modulêre oplossings tot gevolg het. Die voltooide telemetrie sagteware het getoon dat dit al die vereistes bevredig.
58

Simulation of the switched reluctance machine under single pulse mode operation

Rasmeni, Sibusiso Wiseman 12 1900 (has links)
Thesis (MScEng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Two distinct simulation methods have been developed to simulate the single and the multi-phase excited current waveforms of the switched reluctance machine (SRM) under single pulse mode operation. These simulation methods are explained and evaluated in this thesis. A non-commercial finite element package that takes the SRM's nonlinear magnetic property into account is used in this regard. The simulation program uses the finite element solution directly during the simulation. Both simulation methods have been investigated in terms of result and total simulation time. In this investigation it is shown how feasible the simulation methods will be with the next generation of fast computers. The factors affecting the simulated current waveforms of the SRM under single pulse mode operation are investigated in detail in this thesis. With these factors taken into account, the measured and simulated multi-phase current waveforms are compared with each other and discussed. / AFRIKAANSE OPSOMMING: Twee verskillende simulasie metodes is ontwikkel om die enkel- en multi-fase stroomgolfvorms van die geskakelde reluktansiemasjien (GRM) onder enkelpulsmodus-werking te simuleer. In hierdie tesis word hierdie simulasie metodes verduidelik en geëvalueer. 'n Nie-kommersiële eindige element pakket wat die nielinieêre magnetiese eienskap van die GRM in ag neem is in hierdie verband gebruik. Die simulasie program gebruik die eindige-element oplossing direk gedurende die simulasie. Beide simulasie metodes is ondersoek in terme van resultaat en simulasietyd. In hierdie ondersoek word getoon hoe uitvoerbaar hierdie tipe simulasie metodes gaan wees met die volgende hoë spoed generasie rekenaars. Die faktore wat die gesimuleerde stroomgolfvorms van die GRM onder enkelpulsmodus-werking beïnvloed word deeglik in hierdie tesis ondersoek. Met hierdie faktore in ag geneem word die gemete en gesimuleerde multi-fase stroomgolfvorms met mekaar vergelyk en bespreek.
59

An evaluation of the lighting conditions for robot vision

Ackermann, Dirk Wouter 12 1900 (has links)
Thesis (MEng) -- Stellenbosch University, 1987. / ENGLISH ABSTRACT: A vision robot, with comparable characteristics currently being used, was designed and built. The response of the robot is evaluated in terms of the lighting conditions it is subjected to, treated as a transfer function with a visual display as input and a decision made as output. The sensitivity for luminance, contrast and detail of the display are given. Successful classification of certain displays are accomplished. The limitations of each part of the robot is evaluated and the result of these limitations on the total response of the robot is pointed out.
60

Evaluation of the constant current angle controlled reluctance synchronous machine drive

Fick, Pieter D. 03 1900 (has links)
Thesis (MScEng)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: This thesis describes. the design and evaluation of a constant current angle controller for a variable speed reluctance synchronous machine (RSM) drive, as an energy efficient high performance drive. An accurate model of the RSM, with the use of finite element analysis, is derived and implemented in simulation software. The current- and speed controllers are designed and evaluated using a complete simulation model of the whole drive. The controller is implemented on a TMS320F240 DSPbased digital controller, which was developed. The dynamic performance of the constant-current-angle control is compared with that of the conventional constant-daxis- current control method. The results obtained from the RSM drive confirm the simulation results. In the comparison of the two control methods it is shown that the constant-current-angle controlled RSM drive is an energy-efficient drive with good dynamic performance. / AFRIKAANSE OPSOMMING: Hierdie tesis beskryf die ontwerp en evaluering van 'n konstante stroomhoek beheerder vir 'n reluktansie sinchroon masjien (RSM) aandryfstelsel vir optimum effektiwiteit en dinamika. 'n Akkurate model van die RSM, met behulp van eindige element analise, is opgestel en geimplimenteer in 'n simulasie pakket. Die stroom- en spoedbeheerders is ontwerp en geëvalueer deur middel van 'n simulasiemodel vat die volledige aandryfstelsel. Die beheerder is geimplimenteer deur gebruik te maak van 'n TMS320F240 DSP-gebaseerde digitale beheerder, wat ontwikkel is. Die konstantestroornhoek beheer is vergelyk met die konvensionele konstante-d-as-stroom beheer metode. Die resultate van die praktiese stelsel korreleer baie goed met die resultate van die simulasie. Deur die vergelyking van die twee metodes is dit bevind dat die konstante-stroomhoek beheerde RSM aandryfstelsel 'n energie effektiewe aandryfstelsel met baie goeie dinamiese vermoë is.

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