• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 92
  • 91
  • Tagged with
  • 184
  • 184
  • 184
  • 104
  • 81
  • 75
  • 39
  • 29
  • 28
  • 27
  • 21
  • 21
  • 20
  • 18
  • 17
  • 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.
31

A DSP-based digital controller for a thyristor controlled series capacitor

Pillay, Anand. January 2007 (has links)
The power transfer capability of long high voltage transmission lines is often limited by the inductive reactance of the transmission line. Series compensation is in some instances employed to lower the inductive reactance of the transmission line which increases the transmission line power transfer capability. Numerous methods have been employed to provide series compensation of a transmission line. One such method is to use a thyristor controlled series capacitor (TCSC). A thyristor controlled senes capacitor (TCSC) belongs to the flexible altemating CUlTent transmission systems (FACTS) family of devices. It is a variable capacitive and inductive reactance device that can be used to provide series compensation in high voltage transmission lines. One of the significant advantages that a TCSC has over other series compensation devices is that the TCSC's reactance is instantaneously and continuously variable. This means that the TCSC can be used not only to provide series compensation but can also be used to enhance the stability of the power system. However accurate control of the TCSC is challenging due to its highly non-linear variable reactance characteristic. The TCSC consists of back to back thyristors that control the reactance of the TCSC. By changing the trigger angle of these back to back thyristors it is possible to vary the reactance of the TCSC. The reactance characteristic becomes highly non linear at higher levels of compensation; at such operating points the trigger angle of the thyristors needs to be accurately controlled to avoid small variations in the thyristor trigger angle causing significant variation in the reactance of the TCSC. Literature has shown that there is an acceptable limit to the resolution of the thyristor trigger angle based on the parameters of the components used in the TCSC. If a controller is developed to meet this acceptable level of thyristor trigger angle resolution, then the operation of the TCSC will also be acceptable and its operation will not result in unwanted fluctuations in the transmission line variables. This thesis details the development of such a controller for use in a laboratory-scale TCSC. The thesis then goes on to present the practical results obtained from laboratory experiments on the laboratory-scale TCSC with the TCSC triggering controller being used to control the operation of the laboratory-scale TCSC. For purposes of comparison and benchmarking, a detailed simulation model of the laboratory-scale TCSC is developed to take into account the non-ideal properties of the components used in make-up of the laboratory-scale TCSC since the theoretical model is derived assuming ideal conditions. The detailed simulation model is also used to aid in the redesign the power circuit of the laboratory-scale TCSC in an attempt to improve the perfonnance of the laboratory-scale TCSC by obtaining better agreement between the theoretical and practical results. The redesigned laboratory-scale TCSC is used to obtain practical results to COnfill11 the findings of the simulation studies. Finally, the TCSC triggering controller is tested using a real time digital simulator (RTDS). The simulation model developed on the RTDS consisted of a two area, four generator power, with the TCSC connected between the two areas. The RTDS simulation model is used to study the ability of the TCSC to damp inter-area mode oscillations and hence the RTDS simulation model incorporated a power oscillation controller. The input of TCSC triggering controller was "connected" to the power oscillation damping controller and the output of the TCSC triggering controller was "connected" to the thyristors of the TCSC. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.
32

Effect of ADLash opitical fibre cable on corona onset gradient and electric fields around the shield wire of the Apollo-Cahora Bassa HVDC line.

Bussy, Eamon Riccardo. January 2005 (has links)
ADLash® is the trade name for an optical fibre cable attached to a transmission line shield wire using aramid-reinforced bands. The installation of this type of optic fibre cable has been considered for the HVDC line from the Songo hydroelectric scheme at Cahora Bassa in Mozambique, to Apollo substation in South Africa. The impact that installation of this cable will have on the onset of corona is examined. The shield wire with and without ADLash attached is modelled for both the actual line configuration and for corona cage studies. The electric field is calculated using the Method of Images and the Boundary Element Method to predict the electric field enhancements and to estimate the corona onset gradient. Corona onset gradients and phenomena for smooth and stranded conductors are researched to aid the prediction of voltage onset magnitudes for the corona cage. Estimated values are compared with observed values for both AC and DC applied voltages. Different patterns of corona and different corona onset voltages are observed for AC and negative DC applied voltages. The relative permittivity of the ADLash is higher than the surrounding air and this is shown to lead to greatly enhanced electric fields at the air/ dielectric/ shield wire interfaces that are significant enough to cause corona at the rated operating voltage. The corona will bombard the ADLash and probably damage it. Some alternative methods of installing fibre optic cables are reviewed for further research because the use of ADLash cannot be recommended. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2005.
33

Protection of ultra long HVDC transmission lines.

Naidoo, Divoloshanan. January 2005 (has links)
HVDC transmission is today widely used in modem Power Systems as an alternative to HVAC. Current trends indicate that many future conventional HVDC systems will be systems of increasing power ratings, delivered over larger distances as well as multi-terminal systems. In order to ensure the security and dependability of such systems, the current protection schemes need to be evaluated to assess their ability to provide adequate protection for the envisaged HVDC systems. This research work firstly reviews the present HVDC transmission line protection systems, and highlights their advantages and disadvantages, including factors that adversely influence their performance. The author critically evaluates the current protection schemes and reveals the drawbacks and other factors that render them unsuitable for the protection oflong dc transmission lines. The author then goes on to propose and develop an HVDC line protection system that will be able to provide adequate protection for proposed long HVDC transmission lines. The proposed protection system is able to make decisions based solely on local detection increasing its overall reliability. The author then recommends that the proposed protection system be used in conjunction with the existing main protection system in order to optimise the protection response times for both close in and distant faults. The author also proposes and develops a method of further enhancing the reliability of the protection system by the use of the telecommunication infrastructure when available. Finally the performance and feasibility of the proposed protection system is evaluated using the results obtained from the extensive fault simulations performed in EMTDC and Matlab. The simulations are performed using a bipole model of an HVDC System on which the required line and protection systems are modelled. The simulation results obtained are very favourable and promote the use of this proposed protection system, for the protection of long HVDC transmission lines. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.
34

A theoretical and experimental investigation into fire induced flashover of high voltage transmission lines.

January 2005 (has links)
This thesis documents a research study of High Voltage transmission line faults induced by fire. Conductor to conductor and conductor to ground flashovers have been experienced by electricity utilities around the world under conditions of veld and sugar cane fires. These types of faults are unpredictable and negatively impact line reliability and quality of supply. This is a crucial problem when the revenue of the industry is sensitive to voltage dips. Electricity utilities have taken a preventative approach, like clearing vegetation from the line servitude in order to decrease the frequency of line faults. There has also been a drive to collaborate with sugarcane farmers in order to have harvesting fires planned with utilities. Some success has been achieved with these initiatives however there still remains a large number of faults. The focus of this study is on the mechanism of fire-induced flashover. Previou s work has displayed the existence of two theories. The first theory suggests that flashover is due to the reduction in air insulation strength caused by a reduced air density that results from the thermal effect of the fire. The second theory suggests that small particles present in the fire cause electric field distortions that induce flashover. This study is focused on a theory , which indicates that flashover is induced due to an enhanced electric field which is a result of the conductive properties of the flames present in the air gap (the flame conductivity theory). The effects of particles and a reduced air density is said to support this mechanism that is the primary reason for flashover. This thesis present s a summary of the literature where firstly an understanding of air insulation behavior is displayed. Thereafter specific interest is given to the effect of fire and flames wherein the physics of flames are discussed. This then leads to the description of the flame conductivity theory. Chapter 4 deals with a simulative investigation into the effect a conducting flame has on the electric field distribution. This is looked at with a varying flame conductivity and gap length in mind. The simulations specifically cover the 275 kV and 400 kV line configurations. The simulative investigation results in a mapping of electric field enhancement against conductivity values and gap sizes. Thus a flashover probability is assessed by using the two factor flashover criteria when analyzing the electric field stresses. The objective of the experimental work in this study is to obtain insight on how the flame geometry and orientation affects flashover and the dependence of flashover on gap size. Tests involving a fire beneath a conductor were carried out for different gap sizes . Experimentation with particles above a flame was also conducted. It was concluded that flame structure does have an impact on flashover since a flame with sharp edges is more likely to cause flashover. Particles have a reducing effect on air insulation strength. This is mainly due to the fact that the particle reduces the effective air-gap size. No significant effect over and above this is noticed . For gaps spanned by clean Liquid Petroleum Gas (LPG) flames flashover voltage increases as gap-length increases with some degree of nonlinearity. Flame resistances and conductivity were approximated from measured currents and voltages. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.
35

The development of a guideline to assist with compiling asset management plans for transmission lines.

Mansingh, Sharan. January 2010 (has links)
The overhead transmission line is a fundamental component in the power supply system as it links electricity supply to the various points on the electrical network. Failure of the transmission overhead line will result in interruption of supply and depending on the network configuration may result in long term outages. It is therefore essential that the overhead transmission line asset is inspected and maintained regularly to prevent premature failure. Newer approaches to maintenance management are required to improve the overhead transmission lines performance and reduce the cost and risk associated with the asset. Asset management is seen as the process that can be adopted to enhance overall management of the overhead transmission line. The review of maintenance practices of various Utility’s and that of a pilot site made up of selected lines within Eskom’s North East Transmission Grid revealed numerous shortcomings in the current practices largely due to the application of traditional (non-holistic) methods. This situation supports the development of asset management plans which will cater for improvement in performance, reduction in the risk and cost and achieving service level targets. This research has used asset management principles to design a guideline in the form of a flowchart for effective maintenance management for overhead transmission lines. The key benefits/advantages of the maintenance management guideline are as follows: It is closed loop and process driven. Decision making is more scientific because it requires the use of historical performance data, detailed asset condition information and encourages quantitative analysis. Promotes defect and condition assessment tracking via the condition database. Rather than focusing mainly on defect management, the asset manager will be directed towards the performance specifications and the condition database to establish individual action plans which can be prioritized against short, medium and long term improvement plans per specific asset. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2010.
36

Evaluation of overcurrent protection performance and application on the Eskom shunt capacitors during system disturbances.

Boodhraj, Revana. January 2009 (has links)
This dissertation report began as an investigation into an overcurrent relay protection operation on a shunt capacitor bank (SCB) at ESKOM’s Westgate substation. Westgate substation has two SCBs, both of which were in service at the time of the 2007 incident. However, only the overcurrent protection scheme applied on SCB No.2 operated due to an external feeder fault on the Eltro feeder at Westgate substation. In 2004, SCB No.2 had tripped also on an overcurrent relay protection operation for an external fault. The difference identified in the otherwise identical SCBs was the relay technology employed by the overcurrent protection schemes i.e. electromechanical and electronic overcurrent relays were utilised. Therefore an investigation was initiated to determine any difference in the performance and reliability of overcurrent relay technologies in the SCB environment. The purpose of this work is to present the performance of the different technologies of overcurrent relays (electromechanical, electronic and digital) as applied to an ESKOM SCB during system disturbances and to compare their operation and behaviour. MatLAB and DigSILENT simulation packages were used to conduct preliminary fault studies to determine overcurrent relay performance, for a definite time overcurrent setting. These simulation results indicated that the simple electromechanical and electronic overcurrent relay could operate incorrectly in the SCB environment, during system disturbances. Practical laboratory tests were also conducted. This comprised of injecting DigSILENT simulations, comprising of system switching events and external faults, into three technologies of overcurrent relays. These Omicron injection tests found that the Westgate electronic relay would operate incorrectly for certain fault events in the SCB environment. Due to the results observed, further frequency response tests were conducted. These results suggested that the electronic and electromechanical overcurrent relays were susceptible to harmonics i.e. harmonics impact both the pick-up current setting and operating time of electronic and electromechanical overcurrent relays. The digital relay did not exhibit this vulnerability. Finally, recommendations were made to address the incorrect operation of the Westgate electronic relay in its SCB application. These recommendations could be applied in other ESKOM SCB overcurrent protection schemes, to prevent incorrect operation for system disturbances. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2009.
37

Development of a scaled down laboratory test bed system for use in the optimisation of centrifugal fan driven air ventilation systems.

Harcharan, Ashvir. 12 October 2011 (has links)
This thesis describes work that has been carried out to develop a scaled down laboratory test bed for use in the optimisation of fan driven air ventilation systems found at the various Anglo Coal South African mines. The present system involved in the movement of air underground comprises a fixed speed centrifugal fan driven damper controlled system. National Power Contractors (NPC) together with the University of KwaZulu-Natal (UKZN) proposed a variable speed automated controlled system, but the costs of installing Variable Speed Drives (VSD) and their impact on the energy consumption of a system prior to being installed are important. In addition deliverables required by the industrial partner NPC was to develop a calibrated simulation model where any fan system could be simulated showing potential energy savings. A test bed was therefore constructed to evaluate the power usage of a VSD while driving a simulated fan. The test bed comprised of two Field Oriented Controlled induction machines. A ventilation system at Anglo Coal’s Vlaklaagte colliery was proposed to be simulated by the test bed to develop the Measurement and Verification (M&V) methodology required to represent a business case, but since data from this fan system was not available two other fan systems were studied. One fan system was built at the UKZN whilst the other industrial fan system was at Anglo Coal’s Greenside colliery. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2010.
38

Development of a model of the alpha/beta 765kV line for the evaluation of auto-reclosing.

Van Heerden, Peter Robert. January 2009 (has links)
The intention of this dissertation is to determine, on the Eskom system, if the pre-insertion closing resistors installed on the Alpha-Beta 765 kV line breakers are preventing overvoltages from being caused during auto-reclosing. Other possible solutions of reclosing protection are investigated. It has been shown on two occasions, from actual field data that overvoltages have occurred on the lines after reclosing. High overvoltages on this network could be the cause of the many reactor failures that have occurred. A mathematical model of the Alpha-Beta 765kV system was produced on Matlab/Simulink to simulate the resonance of the line during opening and then the effect on the voltage when reclosing takes place. The effects of installing pre-insertion resistors to reduce overvoltages on reclosing were analysed, as well as looking at controlled reclosing at the optimal voltage across the line breakers. It was shown from the studies, that pre-insertion resistors do limit the overvoltages to within the surge capabilities of the line (10% overvoltage) and that the cause of the previous overvoltages were actually due to insertion resistor operations failure. It was also shown that the method of controlled closing at optimal voltage across the breakers is also a successful method of preventing overvoltages. This dissertation also evaluates a design specification for a switching relay for controlled re-closing of the line. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2009.
39

Cost effective DSL solutions for the developing countries.

January 2004 (has links)
Developing countries in Africa present a graphic picture of the digital divide. High costs associated with serving rural customers are the major cause of uneven distribution of services. Rural areas are characterised by a high rate of unemployment and a poor level of education. This results in a scenario where most of the residents are unable to utilize IT resources. Some people in these areas are not informed about the availability and importance of these technologies in the market. Those who are academically fit for accessing these technologies often cannot afford them. Some of the areas still have no existing telecommunications infrastructure. High deployment costs associated with broadband services makes it even more challenging to deploy such services in this environment. In Africa approximately 80% of the population is living in rural areas, which alone creates a demand for the coverage of rural regions. Leaving such a large number of residents not connected, means poor medical care, students cannot participate in distance learning programs which means poor quality of education, poor performance in businesses, poor farming and crippling delivery of government services. DSL technologies were originally designed to suit suburban to urban conditions. In this research it is shown that broadband services can be delivered to rural people by applying DSL technologies, using the existing telecommunications infrastructure. This will mean significant savings, as it does not need core network investments. DSL increases network capacity to a network, which is no longer limited to voice. With this technology a number different high bandwidth applications are delivered to the homes, schools, hospitals, telecentres and small businesses. The cost effectiveness of these technologies for several reach and rural traffic environment is investigated. This is done by investigating several promising DSL solutions in terms of diverse geography, demographics and other cost dictating parameters. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Westville, 2004.
40

An application specific low bit-rate video compression system geared towards vehicle tracking.

Spicer, Ryan David. January 2003 (has links)
The ability to communicate over a low bit-rate transmission channel has become the order of the day. In the past, transmitted data over a low bit-rate transmission channel, such as a wireless link, has typically been reserved for speech and data. However, there is currently a great deal of interest being shown in the ability to transmit streaming video over such a link. These transmission channels are generally bandwidth limited hence bit-rates need to be low. Video on the other hand requires large amounts of bandwidth for real-time streaming applications. Existing Video Compression standards such as MPEG-l/2 have succeeded in reducing the bandwidth required for transmission by exploiting redundant video information in both the spatial and temporal domains. However such compression systems are geared towards general applications hence they tend not to be suitable for low bit-rate applications. The objective of this work is to implement such a system. Following an investigation in the field of video compression, existing techniques have been adapted and integrated into an application specific low bit-rate video compression system. The implemented system is application specific as it has been designed to track vehicles of reasonable size within an otherwise static scene. Low bit-rate video is achieved by separating a video scene into two areas of interest, namely the background scene and objects that move with reference to this background. Once the background has been compressed and transmitted to the decoder, the only data that is subsequently transmitted is that that has resulted from the segmentation and tracking of vehicles within the scene. This data is normally small in comparison with that of the background scene and therefore by only updating the background periodically, the resulting average output bit-rate is low. The implemented system is divided into two parts, namely a still image encoder and decoder based on a Variable Block-Size Discrete Cosine Transform, and a context-specific encoder and decoder that tracks vehicles in motion within a video scene. The encoder system has been implemented on the Philips TriMedia TM-1300 digital signal processor (DSP). The encoder is able to capture streaming video, compress individual video frames as well as track objects in motion within a video scene. The decoder on the other hand has been implemented on the host PC in which the TriMedia DSP is plugged. A graphic user interface allows a system operator to control the compression system by configuring various compression variables. For demonstration purposes, the host PC displays the decoded video stream as well as calculated rate metrics such as peak signal to noise ratio and resultant bit-rate. The implementation of the compression system is described whilst incorporating application examples and results. Conclusions are drawn and suggestions for further improvement are offered. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2003.

Page generated in 0.1008 seconds