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The quest for energy is leading the industry into drilling deeper wells. Typically, a temperature gradient of 1°C/150 ft can be expected, with bottom hole temperatures reaching beyond 200°C in many areas of the world. Moreover, the increased recovery benefits and cost reductions possible with the use of horizontal and multilateral wells has triggered a need for higher power energy conversion systems in bottom hole assemblies, such as rotary steerable tools and downhole tractors. The concepts developed throughout this work address some of these new needs. This research investigated improvements, novel solutions and considerations that will lead to significant advantages in terms of reliability, extended temperature operation, increased power capability and reduced size and cost of compact harsh environment energy conversion systems. Improvements to both the electromechanical subsystem and the power electronic subsystem are introduced. Air gap viscous losses were shown to a have a significant effect on the optimal design of submersible PM (permanent magnet) machines, and a design procedure to account for this loss component in the design was developed. The application of a dual winding exterior rotor PM machine in a downhole environment enabled a significant increase in the application’s torque capability, provided protection against generator winding over voltage, and reduced parts count. Comprehensive switching device qualification, testing, and simulation lead to a simple failure mitigation technique for the operation of the most suitable devices at elevated temperature. A flying capacitor multilevel inverter was then successfully constructed and temperature tested. A novel motor drive concept suited for elevated temperature oil filled environment applications concluded the research.

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published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Power Electronic Building Block (PEBB) concept is to provide generic building blocks for power conversion, regulation and distribution with control intelligence and autonomy. A comprehensive modeling and analysis of a PEBB based DC distributed power system (DPS), comprising of a front end power factor correction (PFC) boost rectifier, a DC-DC converter and a three phase four leg inverter is performed. All the sub-systems of the DC DPS are modeled and analyzed for stability and good transient performance. A comprehensive stability analysis of a PEBB based DC DPS is performed. The effect of impedance overlap on the system and individual sub-systems is examined. Ability of a PEBB based converter to stabilize the integrated system by actively changing the system bandwidth is presented. The fault tolerance capability in a PEBB based rectifier is established by ensuring stable system operation, with one leg of the rectifier failed open-circuited. / Master of Science

The kinetic energy in river streams, tidal currents, or other artificial water channels has been used as a feasible source of renewable power through different conversion systems. Thus, hydrokinetic energy conversion systems are attracting worldwide interest as another form of distributed alternative energy. Because these systems are still in early stages of development, the basic approaches need significant research. The main challenges are not only to have efficient systems, but also to convert energy more economically so that the cost-benefit analysis drives the growth of this alternative energy form. One way to view this analysis is in terms of the energy conversion efficiency per unit cost. This study presents a detailed assessment of a prototype hydrokinetic energy system along with power output costs. This experimental study was performed using commercial low-cost blades of 20 in diameter inside a tank with water flow speed up to 1.3 m/s. The work was divided into two stages: (a) a fixed-pitch blade configuration, using a radial permanent magnet generator (PMG), and (b) the same hydrokinetic turbine, with a variable-pitch blade and an axial-flux PMG. The results indicate that even though the efficiency of a simple blade configuration is not high, the power coefficient is in the range of other, more complicated designs/prototypes. Additionally, the low manufacturing and operation costs of this system offer an option for low-cost distributed power applications.

This thesis reports an investigation into the design of converters for induction heating systems based upon resonant switch mode power converter techniques. The proposed three phase unity power factor induction heating system consists of two stages of power conversions. The important requirements for each stage of the power conversion of a typical induction heating system working from a three-phase supply are identified. A wide range of power converters which fulfil these requirements are compared and evaluated. From the evaluation, the most applicable converter topologies are selected. Each selected converter class is investigated in great detail to outline their advantages and disadvantages. The first stage consists of a push-pull buck converter connected to a unity power factor rectifier stage. This stage converts the three phase AC mains supply to a required DC value. The second stage, which converters the DC into AC is a single ended resonant inverter system. Analysis of the converters has been made and the design procedure has been formulated. The design procedure allows a strenuous design of each resonant converter for particular converter applications. The final converter design has been simulated using the circuit simulation software packages Design Architect and Accusim to verify the results of analysis. The most important design and construction achievements can be summarised as follows: I A novel push-pull buck quasi-resonant converter with a three-phase rectifier stage has been built and tested. At its maximum operating frequency of 40kHz, the prototype converter delivers an output power of 500W. The converter draws nearly sinusoidal currents from the three-phase mains supply and has an input power factor approaching unity. A secondary stage resonant converter provides AC for the induction heater coil. This AC current flowing in the induction coil creates an alternating electromagnetic field for the workpiece. An induction heating coil has been designed and built by using electrical equivalent coil design method. A novel control strategy was developed to provide output power control. Both converter and inverter stage of the system are operated in the zero-current switching condition. The use of this technique allows higher switching frequencies and provides low switching losses. The full design details are presented along with simulation and practical results. The simulation and practical performance results presented show good correlation with theoretical predictions.

With the use of the simplistic equivalent circuits, loss mechanism in conventional power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) gate drive circuits is analyzed. Resonant gate drive techniques are investigated and a new resonant gate drive circuit is presented. The presented circuit adds minor complexity to conventional gate drivers but reduces the MOSFET gate drive loss very effectively. To further expand its use in driving Half-Bridge MOSFETs, another circuit is proposed in this thesis. The later circuit simplifies the isolation circuitry for the top MOSFET and meanwhile consumes much lower power than conventional gate drivers. / Master of Science

The electrical efficiency of telecommunication power supplies is increasing to meet customer demands for lower total cost of ownership. Increased capital cost can now be justified if it enables sufficiently large energy savings, allowing the use of topologies and devices previously considered unnecessarily complex or expensive. Silicon carbide Schottky diodes have already been incorporated into commercial power supplies as expensive, but energy saving components. This thesis pursues the next step of considering silicon carbide transistors for use in telecommunications power converters. A range of silicon carbide transistors was considered with a primary focus on recently developed, normally-off, junction field effect transistors. Tests were devised and performed to uncover a number of previously unpublished characteristics of normally-off silicon carbide JFETs. Specifically, unique reverse conduction and associated gate current draw relationships were measured as well as the ability to block small reverse voltages when a negative gate-source voltage is applied. Reverse recovery-like characteristics were also measured and found to be superior to those of silicon MOSFETs. These characteristics significantly impact the steps that are required to maximize efficiency with normally-off SiC JFETs in circuits where synchronous rectification or bidirectional blocking is performed. A gate drive circuit was proposed that combines a number of recommendations to achieve rapid and efficient switching of normally-off SiC JFETs. Specifically, a low transient output impedance was provided to achieve rapid turn-on and turn-off transitions as well as a high dc output impedance to limit the steady state drive current while sustaining the turned-on state. A prototype circuit was constructed using building blocks that are typically found in single chip MOSFET drivers. The circuit was shown to operate well from a single supply, alleviating the need for a split supply such as that required by many published JFET drive circuits. This demonstrated a proof of concept for a single chip JFET driver solution. An active power factor correction circuit topology was extensively modelled and a prototype designed and tested to verify the model. The circuit was able to operate at switching frequencies in excess of 100kHz when using SiC JFETs, whereas silicon MOSFETs could only achieve switching frequencies of several kHz before switching losses became excessive. The circuit was designed as the dc equivalent for a 2kW, 230V AC input power converter with a split +/-400V dc output. A commercial single phase telecommunications power converter was modified to utilise normally-off SiC JFETs in its power factor correction circuit. The converter was tested and found to achieve similar electrical efficiency with 1200V SiC JFETs to that achieved with 600V silicon MOSFETs. The performance of the 1200V SiC JFETs in this application was also compared to that of 900V silicon MOSFETs and found to be superior. Finally, a prototype three-phase cyclo-converter was modified to use 1200V normallyoff SiC JFETs in place of 600V silicon MOSFETs and found to achieve similar electrical efficiency to the silicon MOSFETs in a 208V three phase system. These results strongly indicate that the 1200V SiC JFETs would provide better performance than 900V silicon MOSFETs in a 400V three phase system (that had been considered for commercial development).