1,721,075 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Network studies and mitigation of high 132 kV fault currents in eThekwini electricity.
Get PDFMaster of Science in Electrical Engineering. University of KwaZulu-Natal, Durban 2016.The growth of the world population has led to an increase in the demand for electricity. This has resulted in the expansion of electric power networks and this evolution brought with it many challenges. One of which is that power networks are experiencing increased fault current levels. This is as a result of growth in demand which has led to interconnected networks and increases in generation capacity. Fault current levels have been increasing steadily and are at a point where mitigation measures have to be evaluated to ensure that equipment operate within designed limits. Alternatively, equipment would have to be replaced with adequately rated equipment. In some cases, replacement would have to take place prematurely, since equipment would not have reached their “end of life”. This study investigates the problem at a 132 kV sub-transmission voltage, and the various factors involved with increasing fault levels and mitigation methods being used. Essentially, mitigation measures increase the impedance in the network, thereby reducing fault currents. Mitigation measures are classified as passive or active, and have varied degrees of effectiveness, usage and network losses. Active measures do not have any effect on the network under normal operating conditions, and only operate during a fault. An example is the superconducting fault current limiter. Passive measures operate under normal and abnormal conditions and affect network parameters. These are usually topological changes which increase the system impedance. Passive measures were chosen for the network studies since active measures are in the developmental stage at the 132 kV voltage level. In this research investigation, the measures tested include: network splitting by creating sub-grids, network reduction, high impedance transformers, introducing a higher voltage network and current limiting reactors. Reducing the 132 kV interconnectivity by creating a northern, southern and central grid reduced the fault levels significantly and does not require any capital investment. However, under abnormal conditions the grids are reconnected to ensure that there is no loss of supply. A solution is to construct a network at a higher voltage level that will support the 132 kV sub grids. A reduction in 275/132 kV transformation lowers the fault levels, while a reduction in generation had little effect on the network. High impedance transformers and current limiting reactors increase the losses in the network, but can be used to limit fault currents to pre-determined values. Electric utilities have to investigate the various measures in order to ascertain the most beneficial to that particular network, given the high cost of infrastructure, the ability to experience outages, space constraints in substations, and the electrical losses that might be incurred. The results obtained from this study carried out on the 132 kV eThekwini Network is presented and discussed
A comparative study and analysis of PHES and UGPHES systems.
Get PDFMaster of Science in Power and Energy Systems. University of KwaZulu-Natal, Durban, 2015.Underground Pumped Hydroelectric Energy Storage (UGPHES) is a similar energy
storage concept to the conventional Pumped Hydroelectric Energy Storage (PHES) with
the major difference being that the lower reservoir is in an underground cavern system.
Electricity is stored in the form of gravitational potential energy between a surface reservoir
and an underlying subterranean reservoir. In this study, various existing energy storage
systems are examined with the UGPHES introduced as an alternative technology for bulk
energy storage in South Africa to contribute to the constrained electricity network with
environmental and economic benefits. The use of existing infrastructure for the
implementation of UGPHES systems is explored, which includes the use of aquifers and
abandoned mines. South Africa has large amounts of groundwater as well as
transboundary aquifers which may be used for UGPHES systems. A mathematical model
is presented which highlights the considerations for the implementation of an aquifer
UGPHES system including head and aquifer transmissivity. The use of abandoned mines
in South Africa is also explored as it presents an existing underground cavern as well as
large amounts of groundwater. Finally, a mathematical model is presented to provide an
analysis of the water hammer phenomenon as well as an economic analysis for the use of
abandoned mines for UGPHES systems
Stability analysis of the Namibian power grid with integration of large offshore wind farms using a VSC-HVDC scheme
No full textA thesis submitted in fulfillment of the requirements of the Degree of Master of Engineering in Electrical Power, Durban University of Technology, Durban, South Africa, 2021.The use of the wind for electrical power production has seen a meteoric increase due to the
wind being a free and abundantly available resource, especially when the site is offshore. The
wind resource along the Namibian coastline could therefore be implemented to develop
offshore wind farms that would enable Namibia to meet its steadily increasing power demand.
The efficient transmission of bulk power from offshore sites to the onshore AC grid is widely
achieved through voltage source converter-based high voltage direct current (VSC-HVDC)
schemes.
This study aims to investigate the power system stability response of the Namibian network,
particularly in terms of rotor angle stability, to the integration of large offshore wind farms. A
single machine infinite bus (SMIB) model developed in DIgSILENT PowerFactory was used
as a test bed for the study. Transient and small-signal stability analysis in relation to different
fault scenarios on the main transmission lines were then carried out after doubly-fed induction
generators (DFIGs) representing offshore wind farms were integrated into the SMIB model.
The same methodology was applied on a reduced model of the NamPower network.
DigSILENT PowerFactory’s VSC-HVDC offshore wind farm template model was integrated
to a reduced model of the NamPower network. The entire network was then subjected to
different fault scenarios along backbone transmission lines, major busbars and the HVDC link
at different penetration levels of offshore wind power.
The study established that the integration of large offshore wind farms using a VSC-HVDC
scheme to the reduced NamPower network negatively affected the network's transient and
small-signal stability. However, there was a positive impact on the voltage levels of the
network due to the reactive power compensation supplied by the VSC-HVDC link. The VSC-HVDC link also maintained low-voltage ride-through of the offshore wind farms during faults
that comply with the Namibian transmission grid code.
Photovoltaic system with multilevel converter coupled to a compressed air energy storage system for grid integration.
Get PDFM. Sc. Eng. University of KwaZulu-Natal, Durban 2014.Electricity demand is continuously increasing and nations including South Africa, are looking to exploit renewable energy sources to augment conventional electricity generation. An analysis of electricity demand and supply, the electricity infrastructure and the status of renewable electricity including the progress and plans made thereof in South Africa were carried out. A review of the challenges affecting the bulk exploitation of renewable energy (RE) resources and its future prospects are discussed to determine the sustainability of these efforts.
This research investigation focuses on a simulation model designed to harness the energy from the sun through a photovoltaic system. Based on empirical data of environmental conditions, a photovoltaic (PV) system model to generate 30 MW of electricity at Witkop substation, Polokwane (South Africa) was developed and to be fed into the grid. A maximum power point tracker (MPPT) control scheme is utilised to ensure that maximum power can be derived from the PV plant. A modular multi-level converter (MMLC) is utilised to convert the electricity generated by the PV system from direct current (DC) to alternative current (AC). The MMLC coupled to compressed air energy storage (CAES) stores the electricity generated during the day and injects it into the grid during peak periods of electricity demand.
Mathematical models of the PV system, the MMLC, the CAES and the grid integration were developed, modelled and simulated to describe the electrical behaviour and to establish the ideal operational parameters of the various systems components. Furthermore, validation of the performance of the system components in the simulation model were carried out against manufacturers’ data sheets for similar studies and prototypes. The simulation model were used to combine all the system components effectively into the grid based on the electricity generation system configurations, electricity demand and the environmental conditions of the selected site.
More importantly, this investigation seeks to increase the effort of development of PV generation models in the field of renewables when compared to other alternative energy sources such as wind energy generation
Dynamic analysis of the Southern Africa power pool (SAPP) network.
Get PDFMaster of Science in Electrical Engineering. University of KwaZulu-Natal, Durban 2016.Synchronous generators have been connected through overhead transmission lines and interconnected to a regional power system network to improve reliability, enhance the security of supply, trade electricity and share the available natural resources for energy fuel supply. The interconnected power system experiences disturbances during normal operation such as load variations and faults, causes stress on the generators to control and remain in synchronism. This dissertation analyses the natural damping oscillations that will emanate during a disturbance in the interconnected power system and provide understanding to the system operator to monitor and operate effectively the Southern African Power Pool (SAPP). It is required to carry out the performance analysis and characteristics of the generators during the disturbances in order to identify the synchronizing and damping torque in respect of rotor angle and speed deviations. In power system control, the frequency is monitored continually since the speed of the generators is synchronized into the transmission lines. Any small variation on the interconnected system will affect the others machines. The effect can either be the system maintaining its stability or loss of synchronism. The latter can be avoided by identifying the nature and behaviour of oscillations and determining effective means to minimize them in interconnected power pool. It can also notify the system operator of an impending power outage. In this research investigation, a simplified model of the Southern African Power Pool is modeled using DIgSILENT Powerfactory power systems analysis software tool, using input data of the primary plant (synchronous generators) and associated interconnected power system network. Nodal and modal analysis tools were used to determine the dynamic status of the interconnected power network. A dynamic analysis will enable participating members of the power pool understand the nature of oscillations when affected by different types of events with continuous monitoring of the modes and eventually assist in re-tuning of secondary control equipment to improve the service delivery of electricity of a pool such as of the Southern African region.
The study has identified the focal points of power oscillations in the modelled SAPP grid through simulations that affects the behaviour of system voltages during a disturbance and require to control damping oscillations on the synchronous generators
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Utilisation of line surge arrestors to improve overhead HVAC and EHVDC line performance under lightning conditions.
Get PDFDoctoral Degree. University of KwaZulu-Natal, Durban.In high lightning areas, lightning strokes play an important role in the performance of overhead EHV AC and DC lines. A single lightning stroke, that terminate on the earth wire and/or tower can lead to back flashovers. This flashover depends on factors such as conductor type, tower, soil resistivity and magnitude of the stroke. The flashover across the insulator and the resultant fault current surge will propagate along the line, until it is extinguished or the breaker operates. This movement of the surge currents tend to damage and reduce the life span of associated equipment such and circuits breakers, insulators, transformers and impact network performance adversely. Furthermore, this operation of the protective devices leads to power interruption to consumers on that network, and loss of production, thus negatively impacting the economy.
This thesis investigates the incidences of network failure due to lightining strokes occuring on Eskom HVAC network as well as HVDC networks, considering soil resistivity, tower footing resistance and factors that influence the earthing resistances. Tower footing resistance needs to be kept uniform and as low as possible to extinguish the surge across the tower and hence reducing the back flashovers across the insulator under lightning conditions. Theoretical simulations were conducted on the different methods that are available to improve the tower footing resistance values. A case study was undertaken to ascertain the tower footing resistance of an 88kV Eskom line. The crows earthing configuration was then utilized to reduce the footing resistance to a value less than 30 ohms, using line surge arrestors (LSA) which are devices that can drain power surges to ground, if placed adequately and in sufficient numbers.
Furthermore the thesis determines the relationship between the magnitude of the lightning stroke, the tower top voltage, tower footing resistance and hence the back flashover voltage that would appear on the line, which would lead to power interruptions. Surge arrestors were modelled using MATLAB software. The required number of surge arrestors per phase is thus determined that is required to drain the surge current down to earth., thus preventing power interruptions. EHV AC and DC cases studies are simulated and results are presented snd discussed.Publications listed on page iii
The safety risk assessment and mitigation measures of the LV networks with embedded generators.
Get PDFMaster of Science in Electrical Engineering.Electricity industry liberalization across the world has seen a significant growth in the utilization of autonomous- and distributed power sources deployed at sub-transmission (132 - 33 kV) and reticulation levels (<33kV) in stand-alone or grid connection notations. With the electricity industry reform, an open access regime is a standard policy governing the transmission grid, and this provides for full competition at generation and distribution end of the delivery value chain. The National Electricity Regulator of South Africa (NERSA) is currently examining a roll out plan for a nation-wide rooftop photovoltaic (PV) system. Most of these roof top PV systems are expected to be connected on the low voltage (LV) networks (<1kV). The widespread deployment of such PV installations have associated risks to personnel and could pose challenges to system operations. Most utility field service engineers are not aware of the dangers posed by such installations. Dangers may include but are not limited to reverse power flow from installed PV systems should the anti-islanding protection fail after the loss of utility supply. This research investigation presents results from the analyses of the impact of statutory requirement, load demand and load type on the embedded generator (EG) grid–tied inverter anti-islanding protection settings and anti-islanding non-detection zone to minimize or reduce LV network operating safety risk upon the loss of utility supply
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