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A bi-directional, direct conversion converter for use in household renewable energy systemsSchutte, Adriaan Nicholaas 03 1900 (has links)
Thesis (MScIng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / A bi-directional, direct conversion switch-mode converter is proposed for use in distributed household renewable energy systems. The converter is intended as the central interface between the household energy system’s low voltage Direct Current bus and the high voltage Alternating Current bus. The low voltage DC bus is connected to renewable generation and storage devices, while the high voltage AC bus is connected to the user’s equipment and the mains grid.
The converter overcomes the inherent reverse-duty cycle problem associated with bi-directional converters by using a combination step-up / step-down half-bridge converter on the high voltage side of a high frequency transformer. The low voltage side of the transformer is driven by a full bridge inverter that acts as a rectifier during reverse mode.
In order to control the flow of power in both directions the converter implements Average Current Mode Control. A method is developed to determine the transfer functions of common switch-mode converters by inspection alone. This method is applied to the proposed converter, and both current and voltage mode control loops are designed with the frequency response method. The control system is implemented using a Digital Signal Processor.
A method of simultaneously simulating both the converter hardware and software is developed using VHDL. This method greatly reduced the development effort of the converter. The operation of the proposed converter is verified through this method of simulation.
A prototype converter is constructed and successfully tested, thereby proving the viability of the proposed converter topology and control methodology.
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Investigation into possible mechanisms of light pollution flashover of 275kv transmission lines as a cause of unknown outagesKleinhans, Kevin 03 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / The cause of the largest number of faults on the Eskom main transmission system is unknown. It is believed that a non-uniform pollution layer along an insulator string is the reason for these anomalous flashovers. This non-uniform pollution layer results in the highest electric field strength, and thus the highest voltage, across the cleanest and driest discs. There thus exists a strong possibility that the anomalous flashover phenomenon is caused by a combination of mechanisms involving the pollution and air breakdown flashover mechanisms. This research project attempted to prove that flashover of the insulators is possible in accordance with the above model. Various experiments were set up in the high voltage laboratory and at a natural test site with a low source impedance supply attempting to simulate the conditions that lead to flashover in accordance with the hypothesis. All the tests done have not proven the non-uniform light pollution flashover mechanism successfully. However, future research has proposed an air breakdown flashover mechanism in light pollution conditions where the polluted part of the insulator string has a specific non-uniform distribution. Full scale testing in conditions similar to the normal operating conditions is proposed to prove the validity of this new hypothesis.
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Efficient finite element electromagnetic analysis of antennas and microwave devices : the FE-BI-FMM formulation and a posteriori error estimation for p adaptive analysisBotha, Matthys Michiel 09 1900 (has links)
Dissertation (PhD)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: This document presents a Galerkin FE formulation for the full-wave, frequency domain,
electromagnetic analysis of three dimensional structures relevant to microwave engineering,
together with the investigation of two techniques to enhance the formulation's computational
efficiency. The first technique considered is the fast multi pole method (FMM) and the second
technique is adaptive refinement of the discretization, based on a posteriori error estimation.
Thus, the motivation for the work presented in this document is to increase the computational
efficiency of the FE formulation considered.
The FE formulation considered is widely used within the microwave engineering, finite element
community. Tetrahedral, rectilinear, curl-conforming, mixed- and full order, hierarchical
vector elements are used. The formulation is extended to incorporate a cavity backed
aperture employing the appropriate half-space Green function within a BI boundary condition,
which represents a specific member of a large class of hybrid FE-BI formulations. The
formulation is also extended to model coaxial ports via a Neumann boundary condition, using
a priori knowledge of the dominant modal fields. Results are presented in support of the
formulation and its extensions, including novel results on the coupling between microstrip
patch antennas on a perforated substrate.
The FMM is investigated first, with the purpose of optimizing the non-local BI component
of the cavity FE-BI formulation, in light of its coupling with the differential equation based,
sparse FEM. The FMM results in a partly sparse factorization of the BI contribution to
the system matrix. Error control schemes for the FMM are thoroughly reviewed and an
additional, novel scheme is empirically devised.
The second technique investigated, which is more directly related to the FEM and larger in
scope, is the use of a posteriori error estimation, in order to optimize the FE discretization
through adaptive refinement. A overview of available a posteriori error estimation techniques
in the general FE literature is given as well as a survey of available techniques that are
specifically tailored to Maxwell's equations. Two known approaches within the applied
mathematics literature are adapted to the FE formulation at hand, resulting in two novel,
residual based error estimation procedures for this FE formulation - one explicit in nature
and the other implicit. The two error estimators are then used to drive a single p adaptive
analysis cycle of the FE formulation, experimentally demonstrating their effectiveness. A
quasi-static condition is introduced and successfully used to enhance the adaptive algorithm's
effectiveness, independently of the error estimation procedure employed. The novel error
estimation schemes and adaptive results represent the main research contributions of this
study. / AFRIKAANSE OPSOMMING: Hierdie dokument beskryf 'n Galerkin eindige element (EE) formulering vir die volgolf, frekwensiegebied,
elektromagnetiese analise van driedimensionele strukture relevant vir mikrogolfingenieurwese,
saam met die ondersoek van twee tegnieke om die numeriese effektiwiteit
van die formulering te verbeter. Die eerste tegniek wat ondersoek word, is die vinnige multipooi
metode (VMM) en die tweede is die aanpasbare verfyning van die EE diskretisering,
gebaseer op a posteriori foutberaming. Dus, die motivering vir hierdie werk is om die numeriese
effektiwiteit van die genoemde EE formulering te verbeter.
Die bogenoemde EE formulering word algemeen gebruik deur die mikrogolfingenieurswese,
eindige element-gemeenskap. Tetrahedriese, reglynige, curl-ondersteunende, hierargiese vektorelemente
van gemengde- en volledige ordes word gebruik. Die formulering word uitgebrei
om holtes in 'n oneindige grondvlak te kan hanteer, deur gebruik te maak van die toepaslike
Green funksie binne 'n grensintegraal (GI) grensvoorwaarde, wat 'n spesifieke lid is van 'n
groot klas, hibriede, EE-GI formulerings. Die formulering word ook uitgebrei om koaksiale
poorte to modelleer via 'n Neumann grensvoorwaarde, deur die gebruik van a priori kennis
van die koaksiale, dominante modus-velde. Resultate word gelewer om die formulering, saam
met die uitbreidings daarvan, te ondersteun, insluitende oorspronklike resultate in verband
met die koppeling tussen mikrostrook plakantennes op 'n geperforeerde substraat.
Die VMM word eerste ondersoek, met die doelom die nie-lokale, GI komponent van die EEGI
formulering vir holtes te optimeer, weens die koppeling daarvan met die yl, differensiaalvergelyking-
gebaseerde, eindige element-metode. Die VMM lei tot 'n gedeeltelik-yl faktorisering
van die GI bydrae tot die algehele matriksvergelyking. Skemas om die VMM fout
te beheer word deeglik ondersoek en 'n addisionele, oorspronklike skema word empiries ontwikkel.
Die tweede tegniek wat ondersoek word, wat meer direk verband hou met die eindige elementmetode,
en van groter omvang is, is die gebruik van a posteriori foutberaming om die EE
diskretisasie te optimeer deur middel van aanpasbare verfyning. 'n Oorsig van beskikbare, a
posteriori foutberamingstegnieke in die algemene EE literatuur word gegee, asook 'n opname
van beskikbare tegnieke wat spesifiek gerig is op Maxwell se vergelykings. Twee bekende
benaderings binne die toegepaste wiskunde-literatuur word aangepas by die bogenoemde
EE formulering, wat lei tot twee oorspronklike residu-gebaseerde foutberamingstegnieke vir
hierdie formulering - een van 'n eksplisiete aard en die ander implisiet. Die twee foutberamingstegnieke
word gebruik om 'n enkel, p-aanpasbare analisesiklus aan te dryf, wat die
effektiwiteit van die foutberamingstegnieke eksperimenteel demonstreer. 'n Kwasi-statiese
vereiste word beskryf en suksesvol gebruik om die aanpasbare algoritme se effektiwiteit te
verhoog, onafhanklik van die foutberamingstegniek wat gebruik word. Die oorspronklike
foutberamingstegnieke en aanpasbare algoritme-resultate verteenwoordig die hoof navorsingsbydraes
van hierdie studie.
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Investigating cost-effective EMC methodsWiid, P. Gideon 12 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / Due to the expensive nature of high frequency measurements in the EMC (Electromagnetic
Compatability) field, more affordable methods of measurement instrumentation and
environments are investigated. Different calibration methods for an Automatic Network
Analyser (ANA) are evaluated against each other to determine the most cost-effective
method of calibration. The mathematics for all the calibration methods are used in
MATLAB programs which perform the error-calculation and correction which is usually
done by the ANA software. These programs can be used to develop a simplified homebuilt
ANA at reduced cost. The MATLAB program calibrations are compared to actual
ANA calibrations to determine accuracy. Different measurement environments are considered
as well to decide on a best compromise between cost and accuracy. To achieve this
a reverberation chamber was built in which measurements were done and compared to
measurements done on an Open Area Test Site. The Device Under Test was a standard
radiator constructed specifically for such measurements. The development of both the
radiator and the reverberation chamber are discussed and all the measurement results are
considered in this thesis.
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A comparison of control systems for the flight transition of VTOL unmanned aerial vehiclesKriel, Steven Cornelius 03 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / This thesis details the development of linear control systems that allow a
vertical take-off and landing unmanned aerial vehicle to perform transitions
between vertical and horizontal flight. Two mathematical models are derived
for the control system design. A large non-linear model, describing all
the dynamics of the aircraft, is linearised in order to perform optimal control
using linear quadratic regulator theory. Another model is decoupled using
time scale separation to form separate rigid body and point mass dynamics.
The decoupled model is controlled using classical control techniques. Simulation
results are used to judge the relative performance of the two control
schemes in several fields including: Trajectory tracking, sensitivity to parameters,
computational complexity and ease of use.
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Optimisation of UHF radio SCADA systems for electrical distribution networksDelport, Pierre 03 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis presents the results of an investigation to establish methods to improve the performance of area radio based communication systems for Supervisory Control and Data Acquisition (SCADA) systems. The considerable scale on which an area radio network is used as a telecommunication network is quite unique to South Africa due to a lack of high bandwidth telecommunication systems in rural areas.
Research was done to establish Key Performance Indicators (KPIs) to measure the performance of the area radio telecommunication systems. Two KPIs were chosen, namely Remote Terminal Unit (RTU) availability and Control Success Rate (CSR). These KPIs were trended over a three-year period to measure the effectiveness of measures taken to improve the systems. Two measures were implemented, namely to change the RTU radios, antennas and coaxial feeder cables and to redesign the communication systems in such a way to restrict multiple RTU protocols on the same area radio telecommunication network.
The installation of new digital radios together with low loss coaxial cable and specific antennas improved the RTU availability from 96.87 % for the year 2002 to 99.17 % for the year 2004, which realised in an increase of 2.83 % for 432 installed RTUs. This measure, however, did not influence the CSR. The implementation of the newly designed communication networks had a significant influence on the control success rate of the SCADA systems and the KPI increased from 77.65 % for the year 2002 to 78.76 % for the year 2003 and 80.88 % for the year 2004. A drastic increase in performance was observed after the restriction of multiple RTU protocols on the same network during May 2004, where the value for twelve months prior to September 2005 was measured at 84.38 %.
The utilisation of area radio telecommunication networks was measured for two operational networks and it was found to be well within the specifications of international accepted standards. One of these networks comprised of three repeaters and 84 installed RTUs and an average utilisation of 17 % and a peak utilisation of 25 % was measured. The other network had an installed base of 4 repeaters and 15 installed RTUs’s and an average utilisation of 3 % and a peak utilisation of 8 % was measured. This compares favourable with the IEEE standard 999-1992 [20], which recommends a channel utilisation of between 40 % and 60 %.
Keywords : Key performance indicators, Radio telecommunication networks, Remote terminal unit
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Metrology and modelling of high frequency probesBadenhorst, J. 03 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2008. / This study investigates high frequency probes through good metrology and
computation software such as CST. A factor that can strongly influence the accuracy
of measurements, is common mode (CM) current. Therefore, the main focus of this
project was the CM current on the outside of an SMA, flanged, probe used for
measuring material properties.
In the course of the investigation, a clamp-on CM current probe (CP) was
calibrated using a CST model and good measurements. This calibration data
indicated that the CP was invasive on the measurement setup and could not deliver
the accuracy required for the CM current measurement.
In light of this, a second method was implemented where the material probe
was placed within a cylindrical shield. A cavity was formed between the probe and
the walls of the shield in which the electric fields could be simulated and measured.
These field measurements allowed measurements to be conducted in both the time-
(TD) and frequency-domain (FD).
For the TD measurements, a sampling oscilloscope was used. As the basic
principle of a sampling oscilloscope differs from its real-time counterpart, this
principle, as well as the systematic errors associated with these devices, was
explored.
The results of the final measurements indicated that the TD results were within
an acceptable range of both the FD results, measured on the VNA, and the results
predicted by CST. This study shows that CST can be used to simulate complex
measurement setups and deliver reliable results in cases where an accurate
measurement cannot be guaranteed.
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Investigation of the scanning performance and enhancement of an electrically large arrayCavanagh, Martin N. 03 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2008. / An existing antenna used in satellite communications is investigated with the
goal of improving low angle radiation performance and reducing manufacturing
costs. To understand this antenna it is modelled, simulated and verified
against existing measurements. This verified model is the basis for further
investigation. The antenna is separated into two parts with are individually
examined and enhancements are proposed for each. Simulations of the resulting
final models show that little by way of improvement in the performance can
be obtained but bring several key issues to light. The scanning impedance is
of particular importance in such an investigation and a method of determining
this impedance is recommended.
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Integrated, FPGA Based NMR Teslameter and power supply for accelerator magnetsTaylor, John-Philip 03 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2007. / Particle accelerators today have numerous magnets that require controlling. These include
magnets for analysing, beam-path selection, focusing, etc. Also, design specifications are
becoming tighter. A typical modern magnet power supply is expected to have a resolution
of 16-bit and a stability of 10 ppm.
This thesis addresses two research areas. First, various aspects of high-performance
accelerator magnet power supplies are investigated. An isolated dual-stage 3.5 kW converter
is designed. The concept is verified through practical measurements. The control system
and high-resolution pulse-width modulation are implemented within a field-programmable
gate array.
Second, a nuclear-magnetic resonance probe is designed and simulated. It is intended to
provide a measurement of field-strength for feed-back purposes. Some adjustments are made
with existing technology in order to decrease the time between successive measurements to
the order of 10 μs.
Also, the support systems (central processing unit, hardware drivers, etc.) are designed,
implemented in the field-programmable gate array and tested successfully.
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Modelling the EM properties of dipole reflections with application to uniform chaff cloudsKruger, Neil 12 1900 (has links)
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: The origin of chaff dates as far back as WWII, acting as a passive EM countermeasure it was used to confuse enemy radar systems and is still in use today. The purpose of this study is, firstly, to build up a knowledge base for determining chaff parameters and secondly, to calculate the theoretical Radar Cross Section (RCS) of a chaff cloud.
Initially dipole resonant properties are investigated relative to dipole physical dimensions. This is extended to the wideband spatial average RCS of a dipole with application to chaff clouds. A model is developed for calculating the theoretical RCS of a cloud typically produced by a single, multiband chaff cartridge.
This model is developed on the principles of sparse clouds with negligible coupling; the dipole density for which the model is valid is determined through the statistical simulation of chaff clouds.
To determine the effectiveness of chaff clouds, the E-field behaviour through a chaff cloud is investigated numerically. From simulation results a model is developed for estimating the position and drop in E-field strength. It is concluded that though it would be possible to hide a target behind a chaff cloud given ideal circumstances, it is not practical in reality. Given the presented results, recommendations are made for future work. / AFRIKAANSE OPSOMMING: Die oorsprong van kaf dateer so ver terug as WOII. Dit was gebruik as ‘n passiewe EM teenmaatreël teen vyandelike radar stelsels en is steeds vandag in gebruik. Die doel van hierdie studie is eerstens, om ‘n kennisbasis op te bou vir die bepaling van kaf parameters en tweedens, om die teoretiese RDS van kafwolke te bereken.
Aanvanklik word die dipool resonante eienskappe ondersoek relatief tot die dipool dimensies. Die studie word uitgebrei tot die wyeband ruimte gemiddelde RDS van ‘n dipool met toepassing op kafwolke. ‘n Model word ontwikkel om die teoretiese RDS te bereken vir ‘n tipiese kafwolk geproduseer deur ‘n enkele, multi-band kafpakkie.
Die model is gegrond op die beginsels van lae digte kafwolke met weglaatbare koppeling; die dipool digtheid waarvoor die model geldig is, is bepaal deur statistiese simulasie van kafwolke.
Om die effektiwiteit van kafwolke te bepaal, word die E-veld gedrag deur kafwolke numeries ondersoek. Vanaf simulasie resultate word ‘n model ontwikkel om die ligging van, en daling in E-veld sterkte af te skat. Daar word tot die gevolgtrekking gekom dat, alhoewel dit moontlik is om ‘n teiken agter ‘n kafwolk te versteek in ideale omstandighede, dit nie prakties is nie. Na aanleiding van die resultate verkry, is aanbevelings vir verdere werk gedoen.
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