The research focus of this thesis is on modelling techniques for river kinetic turbines, to develop predictive numerical tools to further the design of this emerging hydro technology. The performance benefits of enclosing the turbine in a shroud are quantified numerically and an optimized shroud design is developed. The optimum performing model is then used to study river kinetic turbines, including different anchoring systems to enhance performance. Two different turbine numerical models are studied to simulate the rotor. Four different computational fluid dynamics (CFD) turbulence models are compared against a series of particle image velocimetry (PIV) experiments involving highly-separated diffuser-flow and nozzle-flow conditions. The risk of cavitation is briefly discussed as well as riverbed boundary layer losses. This study is part of an effort to develop this emerging technology for distributed power generation in provinces like Manitoba that have a river system well adapted for this technology.

The research focus of this thesis is on modelling techniques for river kinetic turbines, to develop predictive numerical tools to further the design of this emerging hydro technology. The performance benefits of enclosing the turbine in a shroud are quantified numerically and an optimized shroud design is developed. The optimum performing model is then used to study river kinetic turbines, including different anchoring systems to enhance performance. Two different turbine numerical models are studied to simulate the rotor. Four different computational fluid dynamics (CFD) turbulence models are compared against a series of particle image velocimetry (PIV) experiments involving highly-separated diffuser-flow and nozzle-flow conditions. The risk of cavitation is briefly discussed as well as riverbed boundary layer losses. This study is part of an effort to develop this emerging technology for distributed power generation in provinces like Manitoba that have a river system well adapted for this technology.

We discuss a model of a virtual power plant (VPP) that provides market access to privately-owned distributed generations (DGs). The VPP serves passive loads, processes bids from generators, and trades in the wholesale market. The generators can be renewable or thermal, and they act strategically to maximize their own profit. The VPP establishes the rules of the internal market to minimize the cost of energy and the cost of balancing while ensuring generator participation and load balancing. We derive a heuristic mechanism for internal market and propose a dynamic programming approach for minimizing the VPP cost. We present illustrative simulations for both single and multistage market bidding and then compare the resulting performance to the centralized VPP model, where the DGs are assumed to be owned by the VPP. We show that the proposed design incentivizes the DG agents to behave the same as in the centralized case, but the optimal cost paid by VPP is higher due to the payments to the DG owners.

In recent years, the world has seen a significant growth in energy requirements. To meet this requirement and also driven by environmental issues with conventional power plants, engineers and consumers have started a growing trend in the deployment of distributed renewable power plants such as photovoltaic (PV) power plants and wind turbines. The introduction of distributed generation pose some serious issues for power system protection and control engineers. One of the major challenges are power system protection. Conventional distribution power systems take on a radial topology, with current flowing from the substation to the loads, yielded unidirectional power flow. With the addition of distributed generation, power flow and fault current are becoming bi-directional. This causes loss of coordination between conventional overcurrent protection devices. Adding power sources downstream of protection devices might also cause the upstream protection device to be blinded from faults. Conventional overcurrent protection is mainly based on the fault levels at specific points along the network. By adding renewable sources, the fault levels increase and become dynamic, based on weather conditions. In this dissertation, power system faults are modelled with sequence components and simulated with Digsilent PowerFactory power system software. The modeling of several faults under varying power system parameters are combined with different photovoltaic penetration levels to establish a framework under which protection challenges can be better defined and understood. Understanding the effects of distributed generation on three phase power systems are simplified by modeling power systems with sequence networks. The models for asymmetrical faults shows the limited affect which distributed generation has on power system protection. The ability of inverter based distributed generators to provide active control of phase current, irrespective of unbalanced voltage occurring in the network limits their influence during asymmetrical faults. Based on this unique ability of inverter based distributed generators (of which PV energy sources are the main type), solutions are proposed to mitigate or prevent the occurrence of loss of protection under increasing penetration levels of distributed generation. The solutions include using zero and negative sequence overcurrent protection, and adapting the undervoltage disconnection time of distributed generators based on the unique network parameters where it is used. Repeating the simulations after integrating the proposed solutions show improved results and better protection coordination under high penetration levels of PV based distributed generation. / Dissertation (MEng)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

The basic Power Electronics Building Block (PEBB) configurations are identified and conceptual PEBB modules are constructed and tested. Using the INCA (Inductance Calculator) parasitic extraction and the Saber circuit simulation software, the microscopic relationships between the parasitics of the packaging layout and their circuit electrical effects are cross-examined. The PEBB module with advanced packaging techniques is characterized in comparison with the wire-bond module. The soft-switching techniques are evaluated for PEBB applications. The Zero-Current-Transition (ZCT) is proved better because the parasitics in the power current flow path are absorbed into the resonant soft-switching operation. This makes the PEBBs insensitive to system integration. Based on the building block concept, the discrete and large signal average models are developed for simulation, design, and analysis of large-scale PEBB-based systems. New average models are developed for half-bridge PEBB module and Space Vector Modulation (SVM). These models keep the exact information of the discontinuous SVM and the common mode component of the three-phase system. They can be used to construct the computer models of a power electronics system the same as the modularized hardware and perform time domain simulations with very fast speed. Further more, even though the system is modeled based on modularized concept on the ABC coordinates, it can be used to perform small signal analysis on the DQ coordinates as well. Based on the developed models, the system-level interactions in integrated systems are investigated. Three interaction scenarios are presented: (1) the zero-sequence circulation current in paralleled three-phase rectifiers caused by the interleaved discontinuous SVM, (2) the load and source interactions caused by unbalanced load and small signal impedance overlap, and (3) the combined common mode noise caused by both front-end PWM rectifiers and load inverters. The interaction phenomena and mitigation methods are demonstrated through hardware testbed system. The concept of dc bus conditioning is proposed. The bus conditioner is a bi-directional dc/dc converter programmed as a current controlled current source, which shunts the large signal ac current, which otherwise goes to the dc bus, into an isolated energy storage component. In addition to alleviate the source and load interactions, it increases the load impedance/decreases the bus impedance and provides more stability margins to the distribution system. The dc bus conditioner concept and its functions are demonstrated through system simulation and preliminary hardware experiment. / Ph. D.

With the fast advance in VLSI technology, smaller, more powerful digital system is available. It requires power supply with higher power density, lower profile and higher efficiency. PWM topologies have been widely used for this application. Unfortunately, hold up time requirement put huge penalties on the performance of these topologies. Also, high switching loss limited the power density achievable for these topologies. Two techniques to deal with hold up time issue are discussed in this dissertation: range winding solution and asymmetric winding solution, the efficiency at normal operation point could be improved with these methods. To reduce secondary rectifier conduction loss, QSW synchronous rectifier is developed, which also helps to achieve ZVS for symmetrical half bridge converter. Although with these methods, the efficiency of front end DC/DC converter could be improved, the excessive switching loss prohibited higher switching frequency. To achieve the targets, topologies with high switching frequency and high efficiency must be developed. Three resonant topologies: SRC, PRC and SPRC, are been investigated for this application because of their fame of low switching loss. Unfortunately, to design with hold up requirement, none of them could provide significant improvements over PWM converter. Although the negative outcome, the desired characteristic for front end application could be derived. Base on the desired characteristic, a thorough search is performed for three elements resonant tanks. LLC resonant topology is found to posses the desired characteristic. From comparison, LLC resonant converter could reduce the total loss by 40% at same switching frequency. With doubled switching frequency, efficiency of LLC resonant converter is still far better than PWM converters. To design the power stage of LLC resonant converter, DC analysis is performed with two methods: simulation and fundamental component simplification. Magnetic design is also discussed. The proposed integrated magnetic structure could achieve smaller volume, higher efficiency and easy manufacture. To make practical use of the topology, over load protection is a critical issue. Three methods to limit the stress under over load situation are discussed. With these methods, the converter could not only survive the over load condition, but also operate for long time under over load condition. Next small signal characteristic of the converter is investigated in order to design the feedback control. For resonant converter, state space average method is no longer valid. Two methods are used to investigate the small signal characteristic of LLC resonant converter: simulation and extended describing function method. Compare with test results, both methods could provide satisfactory results. To achieve both breadth and depth, two methods are both used to reveal the myth. With this information, compensator for feedback control could be designed. Test circuit of LLC resonant converter was developed for front end DC/DC application. With LLC topology, power density of 48W/in3 could be achieved compare with 13W/in3 for PWM converter. / Ph. D.

Front-end converters with power factor correction (PFC) capability are widely used in distributed power systems (DPSs). Most of the front-end converters are implemented using a two-stage approach, which consists of a PFC stage followed by a DC/DC converter. The purpose of the front-end converter is to regulate the DC output voltage, supply all the load converters connected to the distributed bus, guarantee current sharing, and charge a bank of batteries to provide backup energy when the power grid breaks down. One of the main concerns of the power supply industry is to obtain a front-end converter with a low-cost PFC stage, while still complying with required harmonic standards, especially for high-power three-phase applications. Having this statement in mind, the main objective of this dissertation is to study front-end converters for DPS applications with PFC to meet harmonic standards, while still maintaining low cost and performance indices. To realize the many aforementioned objectives, this dissertation is divided into two main parts: (1) two-stage front-end converters suitable for telecom applications, and (2) single-stage low-cost AC/DC converters suitable for mainframe computers and server applications. The use of discontinuous conduction mode (DCM) boost rectifiers is extensively explored to achieve simplicity, while reducing the cost for DPS applications. Interleaving of DCM boost rectifiers is also explored as an alternative approach to further reduce the system cost by reducing the filtering requirements. All the solutions discussed are implemented for 3kW applications, while 6kW is obtained by interleaving two converters. / Ph. D.

Small-signal stability is of great concern for distributed power systems with a large number of regulated power converters. These converters are constant-power loads (CPLs) exhibit a negative incremental input resistance within the output voltage regulation bandwidth. In the case of dc systems, design requirements for impedances that guarantee stability have been previously developed and are used in the design and specification of these systems. In terms of three-phase ac systems, a mathematical framework based on the generalized Nyquist stability criterion (GNC), reference frame theory, and multivariable control is set forth for stability assessment. However, this approach relies on the actual measurement of these impedances, which up to now has severely hindered its applicability. Addressing this shortcoming, this research investigates the small-signal stability of three-phase ac systems using measured d-q frame impedances. Prior to this research, negative incremental resistance is only found in CPLs as a results of output voltage regulation. In this research, negative incremental resistance is discovered in grid-tied inverters as a consequence of grid synchronization and current injection, where the bandwidth of the phase-locked loop determines the frequency range of the negative incremental resistance behavior, and the power rating of inverter determines the magnitude of the resistance. Prior to this research, grid synchronization stability issue and sub-synchronous oscillations between grid-tied inverter and its nearby rectifier under weak grid condition are reported and analyzed using characteristic equation of the system. This research proposes a more design oriented analysis approach based on the negative incremental resistance concept of grid-tied inverters. Grid synchronization stability issues are well explained under the framework of GNC. Although stability and its margin of ac system can be addressed using source and load impedances in d-q frame, method to specify the shape of load impedances to assure system stability is not reported. This research finds out that under unity power factor condition, three-phase ac system is decoupled. It can be simplified to two dc systems. Load impedances can be then specified to guarantee system stability and less conservative design. / Ph. D.

Les Systèmes électriques à Puissance Distribuée (SPD) sont utilisés dans de nombreux secteurs industriels. La sûreté de fonctionnement (SDF) et la continuité de service de ces SPDs sont aujourd'hui des préoccupations majeures. Une stratégie de gestion globale de l'énergie adaptée ainsi que leur stabilité sont des exigences fondamentales pour que ces systèmes puissent fonctionner correctement. La présence de charges déséquilibrées ainsi que les interactions entre convertisseurs dans ces systèmes peuvent conduire à l'instabilité du bus DC commun. Un des cas les plus connus en terme de cause d'instabilité est celui d'une charge "à puissance constante" (CPL). Par ailleurs, toute défaillance au niveau de l'interrupteur commandable du convertisseur peut provoquer de graves dysfonctionnements du système. Tout défaut non détecté et non compensé en temps réel peut rapidement mettre en danger l'ensemble du système de puissance. Par conséquent, la mise en oeuvre de méthodes efficaces et rapides de détection et de compensation de défaut est impérative. Afin d'assurer la continuité de service de ces systèmes. Dans ce mémoire, nous étudions la gestion de l'énergie, la stabilité et la continuité de service d'un DC-SPD. Après l'étude de la gestion de l'énergie et la stabilité du système, une méthode de stabilisation active décentralisée est proposée afin d'augmenter le domaine de stabilité du SPD et afin deéviter l'instabilité en présence de charges déséquilibrées. Par ailleurs, des méthodes de détection de défaut au niveau d'un interrupteur commandable, efficaces et très rapides, sont également proposées. Nous présentons également une topologie de convertisseur DC-DC à tolérance de pannes, intégrant un interrupteur redondant ; dans tous les cas de défaut (court-circuit ou circuit-ouvert), cette topologie doit permettre deassurer la continuité de service du système de puissance en mode normal. Les études théoriques ont été validées par la simulation et par des tests expérimentaux / Electric Distributed Power Systems (DPS) are used in many industrial sectors. Continuity of service of these systems as well as their reliability are now of the major concerned. Energy management and stability are fundamental requirements for these systems to operate normally. In these systems, load converters exhibit Constant Power Load (CPL) behavior tend to destabilize the system. The system stability also can be threatened by unbalanced loads connected to the common bus. Furthermore, the failure of the controllable switch of the converter can cause serious malfunctions of the system. Undetected and uncompensated fault in real time can quickly endanger the entire power system. Therefore, the implementation of effective and rapid fault detection methods and compensation is mandatory. In this thesis, we study the energy management, stability and continuity of service of a DC-DPS. After the study of energy management and system stability, an active decentralized stabilization method is proposed to increase the stability domain of the system and to avoid instability in the case of unbalanced loads. Furthermore, efficient and very rapid methods of fault detection for a controllable switch are also proposed. We have presented a fault tolerant topology with redundancy for a DC-DC converter, which must ensure continuity of service of the DPS in any fault conditions (open or short circuit). Theoretical studies have been validated by simulation and experimental tests

Η παρούσα διπλωματική εργασία έχει ως σκοπό την παράθεση και ανάλυση των μεθόδων που έχουν παρουσιαστεί αυτή τη στιγμή στη διεθνή βιβλιογραφία και αφορούν τον έλεγχο και συγχρονισμό σε δίκτυο των Διασπαρμένων Συστημάτων Παραγωγής Ισχύος (Δ.Σ.Π.Ι.). Στο πρώτο κεφάλαιο γίνεται μια εισαγωγή στη Διασπαρμένη Παραγωγή (Δ.Π.) και στα Δ.Σ.Π.Ι. Στο δεύτερο κεφάλαιο παρουσιάζονται και αναλύονται τα κύρια χαρακτηριστικά των στρατηγικών ελέγχου που σχετίζονται με τον έλεγχο των Δ.Σ.Π.Ι. Στο τρίτο κεφάλαιο γίνεται παρουσίαση και ανάλυση των μεθόδων που χρησιμοποιούνται για το συγχρονισμό των Δ.Σ.Π.Ι. με το δίκτυο. Στο τέταρτο κεφάλαιο, όμοια με προηγούμενα, έγινε προσπάθεια για παρουσίαση των στρατηγικών ελέγχου των Δ.Σ.Π.Ι. σε περίπτωση που έχουμε εσφαλμένο δίκτυο. / The aim of this master thesis , is the presentation and analysis of the control strategies which are implemented on the distributed power generation systems(grid connection. Moreover, grid synchronization methods of DPGS are presented and evaluated. Control strategies when running on grid faults are also presented.