Electrical Engineering

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Design of a direct PI controller without the feedforward terms for a VSC-based PMSG wind turbine.
(2018) Sukati, Khethizwe Sihlangu.; Dorrell, David George.; Agee, John Terhile.
Renewable energy (RE) sources – wind and solar, have received attention in South Africa and across the world from investors. The development of the Voltage Source Converter (VSC) has enabled large amount of RE to be integrated to the grid. Direct-drive permanent magnet synchronous generators (PMSGs) are now widely available. A PMSG wind turbine employs a full back-to-back VSC arrangement which is controlled via a pulse width modulated system of insulated gate bipolar transistors (IGBTs). Conventional PI vector controllers have been widely used to control VSC based PMSGs. This thesis proposes a new direct PI controller which excludes the decoupled terms in the configurations. A 690 V, 2 MW direct-drive PMSG wind turbine is modelled in PowerSim (PSIM) to investigate the proposed vector control configuration performance under varying wind speeds. A detailed mathematical model of a wind turbine and a PMSG is developed. Then a VSC-HVDC detailed modelled is presented. The proposed control configuration is introduced. An optimal torque control Maximum Power Point Tracking (MPPT) algorithm and vector control method are applied to implement the system. Finally, simulation studies are carried out to investigate the performance of the proposed controller configuration. A deduction from the results obtained, this thesis successfully implemented the new proposed controller configuration. It can be concluded, therefore, that VSC-based PMSG can be implemented in practice using a direct PI controller to reduce system complexity and improve controller performance.
Performance analysis and mitigating the effects of stray currents on underground metal pipelines in South Africa.
(2016) De Lange, Gerald Benjamin.; Davidson, Innocent Ewean.; Agee, John Terhile.
The transport, distribution and utilization of electrical energy can often negatively influence other services in the vicinity due to leakage of stray currents or the influence of varying magnetic fields causing induction in nearby pipelines. When pipeline operators are informed of electrical abnormalities in the vicinity of their facilities, it raises considerable concern and triggers priority remedial reaction. It is well known that in the vicinity of DC stray current the risk of electrolytic corrosion on buried pipelines is very high. In the case of a good external pipe coating with a small defect in the coating, electrolytic corrosion current density at the coating defect will be high and penetration of the pipe wall can result in a short period of time. Invariably the license agreement to operate a pipeline is granted on condition that all necessary steps are taken to prevent spillages and that all actions taken to ensure the integrity of the pipeline are accurately recorded and be available at all times for inspection to the applicable investigating authorities. The primary protection of pipelines against electrolytic corrosion is the external coating of the pipe. As no coating is deemed to be perfect and all coatings are subject to deterioration from aging and subject to damage during installation, a secondary or back-up means of corrosion protection is required. The most commonly used means of secondary corrosion protection for pipelines is cathodic protection. Cathodic protection works on the principle of polarizing all areas on the pipe surface to the same potential so that no corrosion cells can exist on the pipe surface. In addition the pipe surface is maintained at a potential more negative than its immediate surrounding so that it becomes the cathode with respect to the anode of the cathodic protection circuit. A cathodic protection system may consist of a combination of galvanic and impressed current technologies depending on varies factors such as, availability of power sources, soil resistivity, land availability for installation of anode beds and length of pipe to be protected. In South Africa metal pipelines are frequently subjected to the influence of stray currents as a result of the DC rail traction network than spans across the country. Stray current is known to cause the most severe form of corrosion to underground metallic structures that could result in a pipeline leak in a very short period of time with devastating damage to the environment. Stray currents may also interfere with existing cathodic protection systems rendering them inadequate of providing the necessary protection to the pipeline. These currents also interfere in such a manner that maintenance and operation of the existing cathodic protection becomes difficult and unpredictable. Knowledge of the presence of stray current is also critical when designing new pipelines and cathodic protection systems so that the necessary mitigating factors can be implemented in new designs. In South Africa cathodic protection has been proven to be an effective means of protecting both new and old pipelines. This study analyses the challenges associated with the mitigation of stray current and the existing methodology used by a major South African pipeline operator to monitor the presence of stray current. Methodologies for detection and mitigation are briefly discussed in this dissertation. Based on the information studied proposals are suggested to enhance the current methodology and create greater awareness about the damaging effects of stray currents.
Power demand and supply allocation using inherent structural theory of network systems and voltage stability index based on multi-bus reactive power loading.
(2021) Mutuku, Peter Munyao.; Agee, John Terhile.; Tiako, Remy.
Power availability is a crucial factor in determining new load centers. There is a need for adequate power reserve and maximum load capacity allocation to ensure continued power demand and supply electrical systems. In modern interconnected power systems, a high peak load power demand is met by the contribution of the available generator units. There is an urgency to solve the challenges arising from interconnected network configurations such as the loss of generation, inadequate supply capacity to meet load demand during peak time, transmission losses and significant voltage drop at the heavily loaded buses. This dissertation investigates the influence of inter-connected load buses on the system’s voltage profile and the electrical proximity from generation sites to load centers as captured by the Y-admittance matrix. The inherent structural theory of networks was used in determining the required power reserve and load capacity allocation using the ideal generator contribution index. PowerWorld simulator, Dig SILENT Power Factory and MATLAB were used as simulation and presentation tools for the modified IEEE 14 bus system and the Southern Indian 10 bus system. From the analysis of the results, much of the load capacity needed for electrical load growth is feasible for the bus that is most electrically proximal to a high-rated power source. The use of the ideal generator contribution index exploits the structural properties of the network. That being the case, its advantages include minimum expansion of existing structures and minimal transmission active power losses. Also, in this dissertation, a V-Q curve characteristic approach was used to identify the weak load buses in an interconnected power system. This was done by simulating uniformly distributed multi-bus loading conditions and the conventional analysis of the sole bus loading method in a power network system up to the minimum acceptable per unit voltage point. This lead to the formulation of a novel V-Q curve-based index. The voltage critical multi-bus index is a variable state-based index. This index was compared with the self-sensitivity index of the reduced Jacobian matrix and the ‘load structural electrical attraction region’ index of the inherent structural theory of power networks, giving a deeper insight into the system characteristics under light and heavy loading states.
Security and privacy in smart grid advanced metering infrastructure network.
(2018) Diovu, Remigius Chidiebere.; Agee, John Terhile.
a group of consumers are sufficient for data recipients to perform their duties. Unfortunately, this approach usually relies on cryptographic algorithms which increases the overhead on computations and communication for the network. This often drives the network to a congestive state, thereby resulting in delays which can minimize the task of a cyber-criminal targeting the availability of data in the network. The congestion problem was tackled in this work by designing a robust state-of-the-art communication architecture herein referred to in this thesis as Ring Triangulation Communication Architecture (RTCA). Congestive scenarios in Wireless Fidelity (Wi-Fi) and ZigBee wireless communication technology standards were modeled. The designed architecture was then applied to those networks and its performance was analyzed through extensive simulations. The results of the simulations show that notwithstanding the congestive effects of data aggregation, the designed architecture provides a good QoS guarantee for the two considered networks.

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