Licentiate theses at Electricity

Abstract

In order to achieve net zero emissions from the electricity sector, the proportion of renewable energy sources connected to the electrical grid needs to be increased significantly in the coming years. Established renewable energy sources, such as wind power and solar power, will certainly be crucial in achieving this. However, marine energy sources, like marine current power and wave power, have the potential to significantly contribute to the increase of electricity from renewable energy. An important area of study to enable the use of marine energy sources is how to construct electrical systems for offshore renewable energy. Therefore, this thesis addresses some challenges regarding the grid connection of offshore renewable energy.

Two important questions for offshore renewable energy are how the offshore electrical grid is constructed and how the power is transmitted to the shore. In the thesis, a review of AC and DC collection grid topologies is presented. Furthermore, HVAC and HVDC transmission for offshore applications are compared in a literature review. It is concluded that for transmission distances longer than 50 km to 100 km, the preferred technology appears to be HVDC.

Regardless of how the offshore collection grid is constructed, the energy converters need to be connected to the collection grid and the distribution grid. Uppsala University has deployed a marine current energy converter in the river Dalälven in Söderfors, Sweden. The electrical grid connection system at the test site is based on a B2B converter technology. In the thesis, a simulation model of the grid connection system of the energy converter is presented.

The grid connection system at the Söderfors test site includes an LC-filter connected to a power transformer. A novel transfer function is derived for this system and the transfer function is verified with simulations and experimental investigations. It is shown that the derived transfer function is able to capture the frequency response of the experimental system.  

Abstract

Driven by the need for automation and autonomy as well as the need to reduce resources and emissions, the automotive industry is currently undergoing a major transformation. Technologically, this transformation is addressing a wide range of challenges and opportunities. The optimal control of all components is significant for the sustainable development and eco-friendly operation of vehicles. Additionally, robust control of the actuators forms the basis for the development of driver assistance systems and functions for autonomous driving. The actuators of the steering system are particularly important, as they enable safe and comfortable lateral vehicle control. Therefore, the model-based development and virtual simulation of an innovative highly robust control approach for modern Steer-by-Wire systems were conducted in this thesis. The approaches and algorithms described in this thesis allow the design of robust Steer-by-Wire systems and offer the opportunity to conduct many investigations in a computer-aided virtual environment at an early stage in the development process. This reduces time- and cost-intensive testing on prototypes, avoids unnecessary iterations in the design and significantly increases the efficiency and quality of the development. The desired high degree of robustness of the steering control also ensures that the parameterization of the steering feel generator can be freely selected for the individual application. This enables safe and comfortable vehicle lateral control.In summary, the research results described in this thesis accelerate the development of new, modern Steer-by-Wire systems whose robust design forms the basis for the realization of functions for highly automated and autonomous driving.

Abstract

Wave energy conversion technology has gained popularity due to its potential to be-come one of the most preferred energy sources. Its high energy density and low car-bon footprint have inspired the development of many wave energy converter (WEC) technologies, few of which have made their way to commercialisation, and many are progressing.

The Floating Power Plant (FPP) device is a combined floating wind and wave converter. The company, Floating Power Plant, was established in 2004 and has developed and patented a floating device that consists of a semi-submersible that serves as a foundation for a single wind turbine and hosts four wave energy converters (WECs). Each WEC consists of a partially submerged wave absorber whose pitching motion generates energy from incoming waves. The wave absorbers are connected to an oil hydraulic power take-off system located in a dry “engine room” above the free water surface, where the mechanical energy in the absorber is converted to electricity. When undergoing pitching movements, there are interactions between individual wave absorbers and the surrounding platform. This thesis focuses on developing methods to improve the FPP WEC’s hydrodynamic interactions.

The first part of this thesis optimises the wave absorber (WA) ballast. An ana-lytical model is developed to enable systematic selection of WA ballast combination with significantly less computational effort when compared with the more conven-tional means, such as using CAD software. The study suggests an algorithm with which the absorbed power and resonance frequency can be improved and adjusted by manipulating the ballasts’ mass, the position of its centre of gravity, placement and inclination of the WA. The proposed method is generic and can be applied to other WEC concepts or submerged bodies in general. The results show the feasibility of designing the absorber ballast to offer passive control for increased wave absorption. It demonstrates the effect of ballast on the WA inclination, resonance frequency and response amplitude operator (RAO).

The second part focuses on the optimisation of the FPP platform geometry. The genetic algorithm optimisation technique is implemented to maximise the annual en-ergy produced by the relative pitch motion of the WA to the floating platform. The optimised variables are characteristic lengths of the floating platform, most of which are part of the immediate surrounding walls of the absorber. The objective function is a function of the WA’s annual energy production (AEP) and RAO. Results show the feasibility of improving the hydrodynamic interaction between the floating platform and its integrated wave absorbers for a given wave climate by using a heuristic search technique. The number of iterations to convergence tends towards increased values when considering more optimised variables. It is also observed that the computational time appears to be independent of the number of variables but is significantly impacted by the computational power of the machine used.

Abstract

The interest in renewable energy has significantly increased in the last decades which has led to an increased amount of renewable energy sources in the grid. In the Nordic grid, the major contribution to renewable energy is hydro power and wind power and an increase in the amount of wind power is expected in the future. The increase in wind power and decommissioning of nuclear power is expected to decrease the mechanical inertia in the system which helps to stabilise the electrical grid frequency. The inertia is expected to decrease by a factor of two within 20 years and other solutions for frequency stability must be implemented to assure a stable power system. At Uppsala University several projects are investigating how grid-connected energy storages can increase the frequency stability with a high penetration of intermittent renewable energy sources. In this thesis, a linear synthetic inertia control algorithm is implemented on a national Instruments FPGA for controlling the power flow from a supercapacitor energy storage via a two-level three-phase inverter. The control strategy is evaluated both via simulations and experimental tests in a nano grid. The results of the simulations and experimental work are presented and show that it is possible to calculate the frequency derivative in real time to reduce the frequency ROCOF and nadir. The results of the increased frequency stability are presented.

Abstract

The amount of energy enclosed in ocean waves has been classified as one of the most promising renewable energy sources. Nowadays, different wave energy conversion (WEC) systems are being investigated, but only a few concepts have been operated in a sea environment. One of the largest challenges is to guarantee the offshore survivability of the devices in extreme wave conditions. However, there are large uncertainties related to the prediction of extreme wave loads on WECs.  Highfidelity computational fluid dynamics (CFD) simulations can resolve nonlinear hydrodynamic effects associated with wave-structure interaction (WSI).

This thesis explores the point-absorbing WEC developed by Uppsala University in extreme wave conditions. The dynamic response and the forces on key components (mooring line, buoy, generator's end-stop spring) of the device are studied and compared. The high nonlinear phenomena accompany the steep and high waves, i.e., breaking behavior, slamming loads can be well-captured by the highfidelity CFD simulations. A commonly used methodology for extreme waves selection, recommended by technical specifications and guidelines, is the environmental contour approach. The 100-year contour in Hamboldt Bay site in California and the 50-year contour in the Dowsing site, outside the UK, are utilized to extract the extreme waves examined in the present thesis. Popular methodologies and data from different sources (observational and hindcast data) are examined for the environmental contour generation providing useful insights. Moreover, two popular approaches for the numerical representation of the extreme sea states, either as focused wave or as equivalent regular wave, were examined and compared. A midfidelity model of the WEC is successfully verified, as the utilization of lower fidelity tools in the design stage would reduce the computational cost. Last but not least, in CFD simulations the computational grid is sensitive in large motions, something often occurs during extreme-WSI. The solution of this issue for the open source CFD software OpenFOAM is provided here.

Abstract

Many AC grids suffer from decreased frequency stability due to less system inertia. This has increased the risk of large-scale blackouts. This thesis and its papers address the frequency stability problem from a hydropower perspective.

Grid frequency stability assessments often require accurate system inertia estimates. One approach is to estimate the inertia of all individual power plants and sum up the results. We implemented three inertia estimation methods on a Kaplan unit and compared their results. The generator contributed with 92-96% of the unit inertia, which verified the results from previous studies. However, the three methods estimated slightly different values for the unit inertia, which raises the question of when to use which method.

Hydropower often deliver frequency control, yet we found no studies which validate Kaplan turbine models for large grid frequency disturbances on strong grids. Therefore, we performed frequency control tests on a Kaplan unit, implemented three hydropower models, and compared the simulation results to the measurement data. The models overestimated the change in output power and energy delivered within the first few seconds after a large change in frequency. Thus, it is important to have sufficient stability margin when using these types of hydropower models to assess the grid frequency stability.

The Nordic transmission system operators are updating their frequency control requirements. We used measurement data and simulation models to assess whether improved runner blade angle control could help a Kaplan unit fulfill the coming requirements. The results showed that improved runner control does not improve the performance sufficiently for requirements fulfillment. The requirements are based on an assumption on minimum system inertia and became easier to fulfill if they were implemented with more system inertia. Thus, more inertia could allow more participants to deliver frequency control in the Nordic grid.

Abstract

Wound Field Synchronous Generators provide more than 95% of the electricity need worldwide. Their primacy in electricity production is due to ease of voltage regulation, performed by simply adjusting the direct current intensity in their rotor winding. Nevertheless, the rapid progress of power electronics devices enables new possibilities for alternating current add-ins in a more than a century long DC dominated technology. Damping the rotor oscillations with less energy loss than before, reducing the wear of the bearings by actively compensating for the mechanic unbalance of the rotating parts, speeding up the generator with no need for additional means, these are just few of the new applications which imply partial or total alternated current supplying of the rotor winding.

This thesis explores what happens in a winding traditionally designed for the direct current supply when an alternated current is injected into it by an inverter. The research focuses on wound field salient pole synchronous machines and investigates the changes in the field winding parameters under AC conditions. Particular attention is dedicated to the potentially harmful voltage surges and voltage gradients triggered by voltage-edges with large slew rate. For this study a wide frequency band simplified electromagnetic model of the field winding has been carried out, experimentally determined and validated. Within the specific application of the fast field current control, the research provides some references for the design of the rotor magnetic circuit and of the field winding. Finally the coordination between the power electronics and the field winding properties is addressed, when the current control is done by means of a long cable or busbars, in order to prevent or reduce the ringing.

Abstract

Waves, just like wind and solar radiation, is a widely available renewable energy source. Waves are created when winds move across the sea surface. The estimated potential of wave energy is abundant, but the commercial harvesting technologies are still in their infancy. There are different wave energy converter designs, one of which is developed at Uppsala University and is based on a permanent magnet linear generator. A semi-submerged buoy on the water surface absorbs the energy of the wave and converts the mechanical energy into electricity with a direct drive linear generator.

One of the main goals in wave energy research is to enhance the absorbed power for a single device and multiple devices in wave energy parks. The energy harvest can be increased in different ways: by optimizing the buoy, the generator, and/or by implementing control strategies. To assess the best optimization strategies numerical modeling is an inexpensive tool, aimed to predict the complex behavior of the system. 

This licentiate thesis focuses on the study of wave energy converters in irregular waves for testing sites, such as in Lysekil (Sweden) and Wave Hub (UK). The numerical model is used to analyze the wave energy conversion power performance. The hydrodynamic model involves radiation force approximation for a state-space model. It has been shown that a higher order of approximation can be achieved by vector fitting than by the transfer function fitting in the frequency domain, especially for the interaction of several bodies with the incident wave. Wave energy converter concepts are evaluated in terms of absorbed power for the resistive load connection, representing the passive control of the currents in the generator windings. Additionally, RC-load intends to model a grid-connected generator with active rectification, such that phase angle is compensated. Finally, a power-hardware-in-the-loop study of a grid-connected wave energy converter is presented. The current and voltage profiles of a grid-connected wave energy converter are shown with a suggestion on the implementation of RLC filter for power smoothing.

Abstract

This licentiate thesis presents a new way of understanding Electric Power. The new perspective on Electric Power highlights the similarities between our banking system and our power system. The two different systems share a common abstraction. In the case of the banking system this abstraction is fully accepted. In the case of the power system this abstraction is not yet fully accepted. This thesis aims to clarify this abstraction and show the parallelism between the two systems and the two abstractions.

This thesis examines what the Product Electricity really is. What is it, in reality, we sign a power contract to buy and pay for, and how is this product transferred to us? This new understanding challenges the old, physical understanding of Electric Power. Understanding the similarity between our banking system and our power system becomes important when we examine our Power Markets, and it becomes absolutely vital to understand for those investing in new power generation.

In no way does this new understanding of Electric Power question or challenge the physics behind power generation and power transfer. Maxwell’s equations holds true. The laws of Ohm and Kirchhoff are still the laws by which the electrical and power system engineers must abide. But when it comes to the Product Electricity, the product traded on our Power Markets, the product that we sign power contracts to buy and pay for, there is a major difference. The new understanding challenges the old quite dramatically. It does in fact show that the old, physical perspective on the Product Electricity is flawed and has been a misconception for over a century. My primary goal in this thesis is to thoroughly explain the new perspective and by so doing clarify and dissolve the old misconception of what the Product Electricity actually is.

The scientific theory of the Greenhouse Effect now has over 195 years of published peer-reviewed science.^[[i]] The threat of accelerating Climate Change is a scientifically solid fact. The Paris agreement must be met. My second goal with this thesis is to make credible that an electrification of our society is a possible and viable option. The possibility of a swift energy transition from fossil fuels to renewable electricity is made more likely, and more viable, thanks to the new understanding of what Electric Power really is, because now we have the de facto choice of not consuming any fossil power.

© Per Ribbing, April 2019

[i] Joseph Fourier, Remarques générales sur les températures du globe terrestre et des espaces planétaires”, Annales de Chimie et de Physique, 27, 1824, p. 136-167

Abstract

An increased need for electric energy and share of renewable energy is important for a more sustainable future, to which wave power can contribute. Some concepts for wave power use linear generators, and the studies presented here focus on the power transfer and forces in the air gap of linear generators.

Based on Poynting's theorem, analytical expressions were derived for how much power is transmitted in the air gap of linear electric machines. The equations were derived for flat linear generators as well as for cylindrical linear generators, and for both the radial and the longitudinal power flows. The radial power flow contributes to the electricity that can be taken from the generator, while the longitudinal contributes to unwanted effects such as vibration. The equations show that for flat linear electric machines, vibrations and other unwanted power transmissions can be reduced by increasing the number of poles in the machine, without decreasing the desired power transfer. Otherwise, the same factors that increase the desired power transfer also increase the unwanted.

By studying the generator as a damping force and a lumped circuit, it was derived that the absorbed energy of a linear generator is quadratically dependent on the partial stator-translator overlap if the inductance of the generator is not affected by a partial overlap. Consequently, the forces in the air gap also depend quadratically on the relative overlap between the stator and the translator.

Data were collected during two experiments in order to determine whether the quadratic dependence between partial stator-translator overlap and energy transfer could be seen experimentally. Linear regression analysis of the experimental data shows that the relationship is somewhere between linear and cubic, but did not clearly show a quadratic relationship. The inductance showed no dependence of the partial stator-translator overlap.

Abstract

One way to lower the levelized cost of energy for wave power plants and paving so the way for commercial success, is to increase the power absorption by use of advanced control algorithms. This thesis investigates the influence of the generator inertia, the generator damping and the layout on power absorption and presents a new model free strategy of controlling wave energy converters.

The evaluation of all control strategies was done in a numerical simulation and in experimental 1:10 model scale wave tank tests conducted in the COAST laboratory at the University of Plymouth. The WECs used are inspired by the wave energy concept developed at Uppsala University.

The influence of the generator inertia on the power absorption was tested with an uncontrolled WEC. Compared to a conventional WEC the power output could be significantly increased for small waves and high wave periods.   

As a simple and easy to implement control strategy, a WEC with sea state optimized generator damping was used to create a power matrix. The optimal damping factor depends on both, wave period and wave height. The power absorption increases with the wave height and when the wave period converges towards the oscillation period of the WEC.

A genetic algorithm was used to obtain the optimized layouts for wave energy farms, which suggest that the converter should be placed in rows parallel to the wave front, and the position in the array has nearly no influence on the optimal control parameter.

Then a collaborative learning approach using machine learning is presented, with several identical wave energy converters in a row to parallelise the search of the optimal control parameter. It was implemented to control the generator damping factor and the latching time. With the latter the power could be increased significantly.

Abstract

Renewable energies are believed to play the key role in assuring a future of sustainable energy supply and low carbon emissions. Particularly, this thesis focus on wave energy, which is created by extracting the power stored in the waves of the oceans. In order for wave energy to become a commercialized form of energy, modular deployment of many wave energy converters (WECs) together will be required in the upcoming future. This design will thus allow to benefit, among others, from the modular construction, the shared electrical cables connections and moorings, the reduction in the power fluctuations and reduction of deployment and maintenance costs. When it comes to arrays, the complexity of the design process increase enormously compared with the single WEC, given the mutual influence of most of the design parameters (i.e. hydrodynamic and electrical interactions, dimensions, geometrical layout, wave climate etc.). Uppsala University has developed and tested WECs since 2001, with the first offshore deployment held in 2006. The device is classified as a point absorber and consists in a linear electric generator located on the seabed, driven in the vertical direction by the motion of a floating buoy at the surface. Nowadays, one of the difficulties of the sector is that the cost of electricity is still too high and not competitive, due to high capital and operational costs and low survivability. Therefore, one step to try to reduce these costs is the development of reliable and fast optimization tools for parks of many units. In this thesis, a first attempt of systematic optimization for arrays of the Uppsala University WEC has been proposed. A genetic algorithm (GA) has been used to optimize the geometry of the floater and the damping coefficient of the generator of a single device. Afterwards, the optimal layout of parks up to 14 devices has been studied using two different codes, a continuous and a discrete variables real coded GA. Moreover, the method has been extended to study arrays with devices of different dimensions. A deterministic evaluation of small array layouts in real wave climate has also been carried out. Finally, a physical scale test has been initiated which will allow the validation of the results. A multi--parameter optimization of wave power arrays of the Uppsala University WEC has been shown to be possible and represents a tool that could help to reduce the total cost of electricity, enhance the performance of wave power plants and improve the reliability.

Abstract

The aim of this licentiate thesis is to investigate opportunities to produce fresh-water and electricity using marine renewable energy sources, such as wave power and marine current power, for water scarce areas. It summarizes the ongoing work within the field and suggests some possibilities of future re-search. Specifically, the desalination process reverse osmosis has been dis-cussed in combination with the wave energy concept designed at Uppsala Uni-versity for a site by the Kenyan coast. A review on wave powered desalination systems was presented, and the wave climate of the Kenyan coast was reana-lyzed and discussed with respect to the wave power and desalination applica-tion. Also, the magnetic circuit of the linear generator was investigated, as well as the control of the system, to enhance its sustainability. Moreover, the marine current energy converter designed at Uppsala University was investi-gated for desalination purposes. Only literature studies and simulations were performed, and the research would benefit from experimental work.

Abstract

With the advent of modern power electronics there is reason to explore what can be achieved when it is applied to a mature technology like synchronous machines. In this text several concepts will be presented on how it is possible to control forces and how to get better performance out of synchronous machines by using power electronics. Methodologies to create radial forces by controlling the field current in a standard series connected rotor winding as well as when the winding is split in to several segments is presented. By segmenting the rotor a resulting force vector can be created to cancel forces due to unbalanced magnetic pull.

It is also shown that inverting the field current with respect to the stator field enables line start of synchronous machines without using damper bars, frequency converters, or starting motors.

Some first results from the installation and testing of an electromagnetic thrust bearing installed in unit U9 in the hydropower station in Porjus are presented. The benefits of the system is increased reliability and higher efficiency of the thrust bearing system.

An evaluation of a 2-stage brushless excitation system was done, different rotating power electronics topologies were tested in the stationary frame connected to a six-phase permanent magnet brushless exciter. The rotating control and measurement system for the power electronics is presented. Potential benefits of the system is that there is no need for brushes to transfer the field current to the rotor winding, fast response time due to actively controlled electronics, independence of the station bus voltage, and reduced maintenance.

Abstract

Wind power has become an attractive alternative for clean energy production. Horizontal axis turbines are increasing in power rating and are maturing as products. The cost of maintenance, noise levels and durability are possible concerns of the modern horizontal axis wind turbines. The vertical axis wind turbine project at Uppsala University addresses these issues among others. The concept removes the need for any yaw or pitch mechanism. Even the gearbox is excluded with the use of a directly driven multi-pole permanent magnet generator. Additionally, the generator and power electronics is placed at ground level. The project has resulted in several open site prototypes of which a 12kW and 200 kW turbine are still subject to active research.

One major disadvantage of vertical axis wind turbines are the oscillating forces acting on the turbine. The first part of this work presents a measurement system developed to capture the forces acting on a blade during operation of the 12 kW prototype. The systems performance and data output is presented and thorough error estimations has been performed.

In modern turbines it is common to have the generator power rectified into a DC-voltage, feeding an inverter connected to the grid. The rectifier is usually also an inverter allowing full control of the generator and the DC-link voltage. In the concept used on the 12 kW prototype, the electric power of the generator is instead passively rectified using diodes to produce the DC-voltage. Diodes are considered a cheap and robust solution for producing the DC-voltage. However, one drawback is the high current ripples produced. The current ripples lead to an electric torque ripple. This work studies how this electric torque ripple translates into a mechanical torque ripple on the turbine. A simulation model is developed and the results are compared with the force measurements on the 12 kW prototype.

Abstract

The Norwegian Network Code FIKS from the Norwegian Transmission System Operator (TSO) Statnett, states that synchronous generators ≥ 25 MVA must have a static excitation system. It also includes requirements on the step time response and the available field winding ceiling voltage of the excitation system. An improved brushless excitation system is in operation in some pilot power plants. A rotating thyristor bridge is controlled via Bluetooth. The step time response is as fast as conventional static excitation systems. However, a ceiling voltage factor of 2 requires the thyristor bridge to operate at firing angles about 60 degrees. High torque pulsations, low power factor and low utilization of the exciter is the end result. New power electronic interfaces on the shaft results in a betterutilization of the designed exciter and improves the mechanical performance as well as the controllability of the generator field winding. Permanent magnet rotating exciters increase the field forcing strength of the synchronous generator, yielding improved transient stability (Fault Ride-Through req.). Brushless exciters also reduces regular maintenance of the generator. The thesis includes experiments on a state of the art synchronous generator test setup including constructed PM exciter and different power electronic solutions. Some investigations has been done on industrial power plants as well.

Abstract

Permanent magnet (PM) synchronous generators (SGs) are used in renewable energy production. The preferred PM material is neodymium-iron-boron (NdFeB), which has very high performance and, until recently, low cost. In recent years there has emerged a cost and supply insecurity in NdFeB PMs due to export restrictions imposed by China, where the majority of the raw materials are produced. In this thesis various methods of avoiding the use of PM materials based on NdFeB, or other rare earth metals, are investigated.

One approach is the use of the cheap and abundant ferrite PM. A ferrite PM rotor intended to be interchangeable with an NdFeB PM rotor is designed and built. Some initial investigation of the performance of the new rotor, and how this relates to manufacturing tolerances, is also made.

Another approach is to make parameter studies in anticipation of new PM materials. A study of how three different rotor topologies perform with different PM materials, described by their remanence and recoil permeability, is made. The rotor topologies are: a spoke type PM rotor, a surface mounted PM rotor and a capped PM rotor.

It is concluded that a viable replacement rotor can be designed using ferrite PMs. The new rotor will be heavier and mechanically more complex, and give slightly lower output voltage. Losses in the machine will increase slightly. A study is made on the impact of manufacturing tolerances on the performance of the resulting rotor.

The different rotor topologies work best with different PM material properties. The surface mounted PM and capped PM rotors require higher remanence for good performance; the spoke type PM rotor works well with larger amounts of low remanence material. The recoil permeability should be low for the surface mounted and high for the capped PM rotor.

Abstract

Human exploration of the hydrosphere is ever increasing as conventional industries grow and new industries emerge. A new emerging and fast-growing industry is the marine renewable energy. The last decades have been characterized by an accentuated development rate of technologies that can convert the energy contained in stream flows, waves, wind and tides. This growth benefits from the fact that human society has become notably aware of the well-being of the environment that we all live in. This brings a human desire to implement technologies which cope better with the natural environment. Yet, this environmental awareness poses difficulties in approving new renewable energy projects such as offshore wind, wave and tidal energy farms. Lessons have been learned that lack of consistent environmental data can become an impasse when consenting permits for testing and deployments marine renewable energy technologies. An example is the European Union in which a majority of the member states requires rigorous environmental monitoring programs to be in place when marine renewable energy technologies are commissioned and decommissioned. To satisfy such high demands and to simultaneously boost the marine renewable sector, long-term environmental monitoring framework that gathers multi-variable data are needed to keep providing data to technology developers, operators as well as to the general public. Technologies based on active acoustics might be the most advanced tools to monitor the subsea environment around marine manmade structures especially in murky and deep waters where divining and conventional technologies are cost.

The main objective of this PhD project has develop and test an active acoustic monitoring system for offshore renewable energy farms, by integrating a multitude of appropriate monitoring sonar, hydrophones and cameras systems to be developed with standards suitable for subsea environmental monitoring. In this project, a first task was to identify, secondly acquire and test sonar systems, then a platform was designed and built, a data acquisition device control systems were developed, finally additional instruments such as video cameras and sonars were added. This systems integration followed by calibration of devices was conducted. The sonar systems were used for quantitative measurements of the occurrence of e.g. large marine animals and schools of fish near marine renewable energy converters. The sonar systems were also used for seabed inspections, depth measurements and capitating flow observations.

So far, the combination of multibeam and dual-beam sonar systems produced good results of target detection, bottom inspection, depth measurements and biomass estimation. The multibeam sonar system was capable of resolving isolated targets located near high acoustic retroreflective objects. Panoramic acoustic images of wave and instream energy converters were acquired using a multibeam sonar operating at frequencies near 1 GHz. The Dual-beam and split-beam sonar systems produced data referent to acoustic background intensity of targets that helps to classify targets according to its size, composition and 3-Dimensional location within the water column. The next phase of this project will deploy the platform for longer periods in order to gather consistent acoustic and optical backscattering data of marine animal behaviour within marine renewable energy farms.

Abstract

Remotely Operated Vehicles (ROVs) are underwater robots that perform different kind of operations, from observation to heavier tasks like drilling, carrying and pulling cables, etc. Those ROVs are costly and require skilled personal to operate it as well as equipment for transportation and deployment (boats, cranes, etc.).

The division for electricity at Uppsala University, is developing a wave energy converter (WEC) concept. The concept is based on a point-absorbing buoy with a directly driven linear generator placed on the seabed. Several units are connected to a marine substation that is located on the seabed, whose role is to collect and smooth the power absorbed from the waves and then bring it to the shore through one single cable.

Cable connection is a big challenge in the project because the WEC concept is small and many units are necessary to create a rentable farm. Nowadays this operation is performed by divers but using Observation Class ROV (OCROV) could be an interesting alternative since they are affordable at lower costs and easier to operate. Cable connection is however a heavy task and requires force that an OCROV does not have. It will need a docking system from which the vehicle will take its force. It would then go to the station, dock itself to this support plate, grab the cables and connect them together. This procedure cannot be done by the ROV operator because it requires accurate displacement and quick adjustment of the robot’s behavior.

An autopilot was created in Matlab Simulink that consists of three units: the path following, the ROV, and the positioning unit. The first one uses the vehicle’s position and computes the speed and heading to be applied on the ROV in order to guide it on the desired path. The second one contains a controller that will adapt the thrust of each propeller to the force needed to reach the desired heading and speed from the path following unit. It also contains the model of the ROV that computes its position and speed. The last unit consists of a Kalman filter that estimates the ROV position and will be used in case of delay or failure in the communication with the positioning sensors.

The autopilot model is used with a positioning system that utilizes green lasers and image processing. Two green lasers are used as fixed points in each camera picture and from their distance on the image, the actual distance between the ROV and the docking platform can be computed. In addition, optical odometry is used. The idea behind is to estimate how the ROV is behaving by evaluating the changes between two pictures of the camera. Those two systems, laser and odometry, work together in order to get more accurate results.

The laser system has so far been tested in air. The distance measurements gave interesting results with an error inferior to 3%, and angle measurements gave less than 10% error for a distance of one meter. One advantage with the system is that it gets more accurate as the vehicle gets closer to the docking point.

In addition to the ROV project, a review study was conducted on the variability of wave energy compared with other resources such as tidal, solar, and wind power. An analysis of the different tools and models that are used to forecast the power generation of those sources was done. There is a need for collaboration between the different areas because the future will aggregate those different sources to the grid and requires a unification of the models and methods.

Abstract

Hydro-electricity plays an important role in the safe, stable and efficient operation of electric power systems. Frequency stability of power systems refers to the ability to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load. In order to suppress power grid frequency fluctuations, generating units change their power output automatically according to the change of grid frequency, to make the active power balanced again. This is the primary frequency control (PFC). PFC of electrical power grids is commonly performed by units in hydropower plants (HPPs), because of the great rapidity and amplitude of their power regulation.

A hydropower generation system is a complex nonlinear power system including hydraulic, mechanical, electrical and magnetic subsystems. Nowadays, the size of HPPs and the structure complexity of systems have been increasing, especially in China. The proportion of electricity generated by intermittent renewable energy sources have also been growing. Therefore, the performance of HPPs in terms of frequency control is more and more important. The research on control strategies and dynamic processes of HPPs is of great importance. The frequency stability of hydropower units is a critical factor of power system security and power quality. The power response time for evaluating the frequency regulation quality, is also a key indicator.

In recent years, there is a tendency that the new turbines experience fatigue to a greater extent than what seem to be the case for new runners decades ago, due to more regulation movements caused by increasingly more integration of intermittent renewable energy sources. In some countries, as in Sweden, PFC is a service that the transmission system operator buys from the power producers. In other countries, as in Norway and China, there is also an obligation for the producers to deliver this service, free of charge. However, there are costs related to this, e.g. due to design constraints and auxiliary equipment when purchasing a new unit or system, and due to wear and tear which affects the expected life time and maintenance intervals. Hence the specific research on wear and tear of hydro units due to PFC is exceedingly necessary. 

Abstract

This thesis investigates the load control of a marine current energy converter using a vertical axis turbine mounted on a permanent magnet synchronous generator. The purpose of this thesis is to show the work done in the so far relatively uncharted territory of control systems for hydro kinetic energy conversion. The work is in its early stage and is meant to serve as a guide forfuture development of the control system.

An experimental power station has been deployed and the first results are presented.

A comparison between three load control methods has been made; a fixedAC load, a fixed pulse width modulated DC load and a DC bus voltage control of a DC load. Experimental results show that the DC bus voltage control reduces the variation of rotational speed with a factor of 3.5. For all three cases, the tip speed ratio of the turbine can be kept close to the expected optimal tip speed ratio. However, for all three cases the average extracted power was significantly lower than the average power available in the turbine times the estimated maximum power coefficient. A maximum power point tracking system, with or without water velocity measurement, should increase the average extracted power.

A simulation model has been validated using experimental data. The simulated system consists of the electrical system and a hydrodynamic vortex model for the turbine. Experiments of no load operation were conducted to calibrate the drag losses of the turbine. Simulations were able to predict the behaviour in a step response for a change in tip speed ratio when the turbine was operated close to optimal tip speed ratio. The start position of the turbine was varied in the simulation to view the influence on the step response from a changed turbine position relative to the direction of the water flow.

Abstract

The increasing interest in converting energy from renewable resources into electricity has led to an increase in research covering the field of marine current energy, mainly concerning tidal currents and in-stream tidal turbines. Tides have the advantage of being predictable decades ahead. However, the tidal resource is intermittent and experiences local variations that affect the power output from a conversion system. The variability is mainly due to four aspects: the tidal regime, the tidal cycle, bathymetry at the site and weather effects. Each potential site is unique, the velocity flow field at tidal sites is highly influenced by local bathymetry and turbulence. Hence, characterizing the resource requires careful investigations and providing high quality velocity data from measurement surveys is of great importance. In this thesis, measurements of flow velocities have been performed at three kinds of sites.

A tidal site has been investigated for its resource potential in one of all of the numerous fjords in Norway. Measurements have been performed to map the spatial and temporal variability of the resource. Results show that currents in the order of 2 m/s are present in the center of the channel. Furthermore, the flow is highly bi-directional between ebb and flood flows. The site thus have potential for in-stream energy conversion. A model is proposed that predicts peak current speed from information on tidal range at the site. A corresponding model can be set up and implemented at other similar sites affected by tides, i.e. fjord inlets connecting the ocean to a fjord or a basin.

A river site serves as an experimental site for a marine current energy converter that has been designed at Uppsala University and deployed in Dalälven, Söderfors. The flow rate at the site is regulated by an upstream hydrokinetic power plant nearby, making the site suitable for experiments on the performance of the vertical axis turbine in its natural environment. The turbine has been run in uniform flow and measurements have been performed to characterize the extent of the wake.

An ocean current site was a target of investigation for its potential for providing utilizable renewable energy. A measurement campaign was conducted, mapping the flow both spatially and temporally. However, the site was shown to not be suitable for energy conversion using present technique.

Abstract

Diamond is a wide bandgap semiconductor with extreme properties such as high thermal conductivity, high breakdown field and high carrier mobilities. These properties together with the possibility to synthesize high purity Single-Crystalline (SC) diamond by Chemical Vapor Deposition (CVD), makes it a really interesting material for electronic devices. The low impurity concentration achieved when fabricating diamonds by CVD allows for a detailed study of the intrinsic electronic properties of diamond, especially at low temperatures when the carrier scattering rate is low.

During the last few years, our group has presented two new phenomena discovered in SC-CVD diamond at temperatures below 150 K. For the very first time, Negative Differential Mobility (NDM) and valley polarization have been observed in diamond. NDM occurs at a temperature range of 110 to 140 K and at an electric field range of 300 to 600 V/cm and has been explained by electron repopulation between different valleys. At temperatures below 100 K, stable valley polarization has been observed due to the low phonon scattering rate in diamond that enable electrons to reside in one valley.

This licentiate thesis will give a short review on electronic properties and charge transport in diamond. It will also present the two discovered phenomena and the methods used to observe them. There will be further discussions of how these discoveries can be used for making future devices, such as the Transferred-Electron Oscillator (TEO) and valleytronic devices.      

Abstract

Vertical axis wind turbines (VAWTs) have with time been outrivaled by the today common and economically feasible horizontal axis wind turbines (HAWTs). However, VAWTs have several advantages such as the possibility to put the drive train at ground level, lower noise emissions and better scaling behavior which still make them interesting for research.

The work within this thesis is made in collaboration between the Department of Construction and Energy Engineering at Halmstad University and the Division for Electricity at Uppsala University. A 200 kW VAWT owned by the latter and situated close to Falkenberg in the southwest of Sweden has been the main subject of the research even if most learnings has been generalized to fit a typical vertical turbine. This particular turbine has a wooden tower which is semi-guy-wired, i.e. the tower is both firmly attached to the ground and supported by guy-wires.

This thesis has two main topics both regarding VAWTs: eigenfrequency of the tower and the noise generated from the turbine. The eigenfrequency of a semi-guy-wired tower is studied and an analytical expression describing this is produced and verified by experiments and simulations. The eigenfrequency of the wire itself and how it is affected by wind load are also studied.  The noise characteristics of VAWTs have been investigated, both theoretically and by noise measurement campaigns. Both noise emission and frequency distribution of VAWTs has been studied.

The work has resulted in analytical expressions for tower and wire eigenfrequency of a semi-guy-wired tower as well as recommendations for designing future towers for VAWTs. The noise emission of VAWTs has been studied and proven low compared to HAWTs. The noise frequency distribution of the 200 kW VAWT differs significantly from that of a similar size HAWTs with for example lower levels for frequencies below 3000 Hz.

Abstract

The production and consumption of electricity on the power grid has to balance at all times. Slow balancing, over days and weeks, is governed by the electricity market and carried out through production planning.  Fast balancing, within the operational hour, is carried out by hydropower plants operating in frequency control mode. The need of balancing power is expected to increase as more varying renewable energy production is connected to the grid, and the deregulated electricity market presents a challenge to the frequency control of the grid.

The first part of this thesis suggests a method to quantify the need for balancing or energy storage induced by varying renewable energy sources. It is found that for high shares of wind and solar power in the system, the energy storage need over a two-week horizon is almost 20% of the production.

The second and third part of the thesis focus on frequency control. In the second part, measurements from three Swedish hydropower plants are compared with the behaviour expected from commonly used power system analysis hydropower models. It is found that backlash in the guide vane and runner regulating mechanisms has a large impact on the frequency control performance of the plants.

In the third part of the thesis, the parameters of the primary frequency control in the Nordic grid are optimised with respect to performance, robustness and actuator work. It is found that retuning of the controller parameters can improve the performance and robustness, with a reasonable increase of the actuator work. A floating deadband in the controller is also discussed as a means to improve performance without increasing the actuator work.

Abstract

In a world where money often is the main controlling factor, everything that can be tends to be more and more optimized. Regarding electrical machines, developers have always had the goal to make them better. The latest trend is to make machines as efficient as possible, which calls

for accurate simulation models where different designs can be tested and evaluated. The finite element method is probably the most popular approach since it makes it possible to, in an easy and accurate way, get numerical solutions to a variety of physics problems with complex geometries and non-linear materials.

This licentiate thesis includes two different projects in which finite element methods have had a central roll. In the first project, the goal was to develop a simulation model to be able to predict demagnetization of permanent magnets. It is of great importance to be able to predict if

a permanent magnet will be demagnetized or not in a certain situation. In the worst case, the permanent magnets will be completely destroyed and the machine will be completely useless. However, it is more probable that the permanent magnets will not be completely destroyed and that the machine still will be functional but not as good as before. In a time where money is more important than ever, the utilization has to be as high as possible. In this study the demagnetization risk for different rotor geometries in a 12 kW direct driven permanent magnet synchronous generator was studied with a proprietary finite element method simulation model. The demagnetization study of the different rotor geometries and magnet grades showed that here is no risk for the permanent magnets in the rotor as it is designed today to be demagnetized. The project also included experimental verification of the simulation model. The simulation model was compared with experiments and the results showed good agreement.

The second project treated the redesign of the rotor in the generator previously mentioned. The goal was to redesign the surface mounted NdFeB rotor to use a field concentrating design with ferrite permanent magnets instead. The motivation was that the price on NdFeB magnets has fluctuated a lot the last few years as well as to see if it was physically possible to fit a ferrite rotor in the same space as the NdFeB rotor. A new rotor design with ferrite permanent magnets was presented together with an electromagnetic and a mechanical design.

Abstract

Ocean wave energy can be harnessed and converted into electric energy nowadays. This provides a possibility for populations that live on islands or along coastlines to utilize the renewable and safe power produced by ocean waves. Point absorbing wave energy converter (WEC) is one example of such devices for electrical power production from ocean waves. It is composed of a floating buoy on the water surface, and a linear generator that sits on seabed and is connected with the buoy via a line. Electricity is generated when the buoy moves up and down in the waves.

The geometry and dimensions of the floating buoy have dominant influences on the energy absorption. This thesis introduces an equivalent electric circuit for modelling the  hydrodynamic interaction between the wave and a cylindrical buoy. The model allows a rapid assessment of the velocity, force in the connection line and output power, by which the system design and optimization can be performed faster and easier.

The electric circuit model is based on the WEC's dynamic force analysis, and the electric components' parameters are determined from analytical approximations of the hydrodynamic coefficients. The simulation results of the equivalent circuit for one typical wave climate in Lysekil has been presented, and the results indicate a good fitting with former experimental results.

The thesis also includes a hydrodynamic study for a torus shaped buoy, which aims at  applying a theoretical background for a force measurement experiment. A comparison has been conducted between the torus buoy and two similar cylindrical buoys. Preliminary WAMIT simulation results demonstrate that the force in the connection line will be 5% bigger by using the torus buoy. It is also found that the torus buoy is advantageous for its larger excitation force and smaller added mass. A brief introduction of the 500kN force measurement system and the communication test have been introduced as well.

Abstract

Hydrodynamic simulations of buoy movement is crucial when designing a point absorbing wave power plant. The wave energy converter (WEC) that is developed in the Lysekil project by Uppsala University is studied in this thesis. The buoy motion in response to an incoming wave can be simulated using potential linear wave theory by deriving a transfer function from the Fourier transformed equation of motion and solve it. The buoy response is found from the convolution between the incoming wave and the transfer function in time domain. It have been found that this approach shows good agreement with experimental results for normal operating conditions.The hydromechanical parameters exciting force, added mass and radiation damping have been simulated in WAMIT, using BIEM, and in COMSOL, using FEM. This was done to compare the simulation model built in COMSOL with the commercial software WAMIT. The results were comparable. The hydromechanical parameters were used to calculate buoy movement and line force in response to a 30 minutes sea state measured at the Lysekil research site. The simulated buoy movement and line force was compared to experimental results, and showed good agreement.A WEC is a complicated dynamical system. The buoy motion, and thereby the translator velocity, depends on the buoy geometry and dimensions, the mass of the moving parts of the system and on the damping force from the generator. The damping force from the generator is dependent on the translator velocity. The above mentioned approach for motion simulation can be used if the damping force from the generator is linear and described by the velocity multiplied with a constant damping coefficient. In this thesis have it been studied how the buoy draft and radius of cylinder buoys are influencing the performance of a WEC, assuming a constant generator damping coefficient. The results were compared to the experimental power from the WECs L2 and L3, two identical generators with cylinder buoys of different dimensions. It was concluded that the experimental difference in power absorption between L2 and L3 could be derived from the difference in draft and buoy mass, rather than buoy radius. The largest part of the difference have however been concluded to derive from the incorrect line lengths.

Abstract

The thesis presents results from synchronous linear wave energy converters developed at Uppsala University. A study is done on closed stator slots and a study presenting offshore data focusing on the power absorption from a wave energy converter (WEC). The first step in studying the closed slots has been done during no-load, to study the magnetic flux path from the permanent magnets and to study the forces in the linear generator. The initial studies show a reduction in cogging force and a reduction in harmonics in the magnetic flux density in the air-gap. It also shows an increase of the total flux entering the stator and an increase in flux leakage. The study has been done with FEM simulations and compared with analytical calculations.

The second study was done to investigate the power absorption of a WEC in upward and downward motion in relation to the volume of the buoy and mass of the system. The experimental results were compared with a static model focusing on the limit in the absorption. As expected from the model, the WEC absorbs more energy in the upward direction. Also indications of snatch load were observed. Within this thesis, results from a comparison study between two WECs with almost identical electrical properties and the same volume of the buoy, but with different height and diameter have been presented. Moreover, experimental studies including the conversion step between AC to DC have been done.

The work done in this thesis is a part of a larger wave power project at Uppsala University. Where everything between the energy absorption from the waves to the connection to the electrical grid is studied. The project has a test-site at the west coast of Sweden near the town Lysekil, where wave energy research has been carried out since 2004.

Abstract

At the Division of Electricity of Uppsala University, a wave power concept is being developed.It relies on wave energy converters, one buoy and one linear generator placed on the seabed, connected together to a submerged substation. The mechanical design is kept as simple as possible to ensure reliability. The submerged substation includes power electronics and different types of electric components. High reliability requirements on such subsea devices make thermal management a keyaspect in the design. Besides, no fans are used and the cooling strategy is fully-passive. The overall approach for thermal modelling of the substation is based on a thermal network atthe system level, and both analytic- and CFD- modelling at the component level. This work is focusing on the second prototype of substation developed at Uppsala University. In this thesis, this overall strategy is presented as well as a comprehensive temperature study for the inverterinstalled in the substation.In the present configuration, the inverters are limited to about 35 kW. The seawater temperature,the choice of material for the heat-sink, and the spacing of the component, were identified tohave an influence on this value. The importance of a good thermal contact between the heat-sink and the hull was also illustrated.

Abstract

Wind power can be related to natural disasters in several ways. This licentiate thesis gives some background and introduces four papers devoted to two aspects of this relation. The first section looks into how small-scale wind energy converters (WECs) could be used to generate power after a natural disaster. For this application diesel generators are the most common solution today, but there would be several advantages of replacing these systems. A study of off-grid systems with battery storage at 32 sites showed that photovoltaics (PV) were more suitable than WECs. The results were confirmed by a study for the entire globe; PV outperformed WECs at most sites when it comes to small-scale application. This is especially true for areas with a high disaster risk. Hybrid systems comprising both PV and WECs are however interesting at higher latitudes. For the Swedish case, it is shown that gridded data from a freely available meteorological model, combined with a statistical model, give good estimates of the mean wind speed at 10 meters above ground. This methodology of estimating the mean wind speed can be used when there is no time for a proper wind measurement campaign.

The second section is directed towards wind power variability and integration. The results presented in the thesis are intended as a basis for future studies on how a substantially increased wind power capacity affects the electric grid in terms of stability, grid reinforcement requirements, increased balancing needs etc. A review of variability and forecastability for non-dispatchable renewable energy sources was performed together with researchers from the solar, wave and tidal power fields. Although a lot of research is conducted in these areas, it was concluded that more studies on combinations of the sources would be desirable. The disciplines could also learn from each other and benefit from the use of more unified methods and metrics. A model of aggregated hourly wind power production has finally been developed. The model is based on reanalysis data from a meteorological model and detailed information on Swedish WECs. The model proved very successful, both in terms of low prediction errors and in the match of probability density function for power and step changes of power. 

Abstract

The wind has been used as a source of energy for a long time. Wind power in power production has had a upswing in the last decade due to its renewable and clean nature. This thesis contributes to the wind power research done at the division of Electricity at Uppsala University. The research is aimed towards increasing the understanding of vertical axis wind turbines. The thesis focuses on vertical axis wind turbines of the H-rotor type with a directly driven permanent magnet synchronous generator. In this thesis a full-scale variable speed electrical grid connection system is developed and evaluated for a 12 kW prototype turbine. The proposed electrical system consists of a diode rectifier, IGBT inverter, LCL-filter and tap transformer. The full variable speed operation is achieved by adjusting transformer taps and adjusting the modulation of the inverter. The system has been assembled in a lab environment where efficiency and harmonic content have been evaluated for the operational scheme and site specifics of the 12 kW turbine. The work also looks into transformer magnetization losses, permanent magnet generator air cooling and control system design for a similar system. The results from simulations and experiments show that the tap transformer system has a high efficiency at nominal power and that the system efficiency at nominal power is increased by going up in tap. The study also shows that the total demand distortion for the system is well below 5% for full operating range of the 12 kW turbine.

Keywords: VAWT, H-rotor, tap transformer, stall control, renewable energy, variable speed operation, transformer magnetization losses

Abstract

Wave energy conversion is a potentially clean and sustainable form ofenergy conversion with no emissions of gases or liquids during operation.There are other environmental issues that need to be taken in consideration.The deployment of a wave power farm will contribute with new surface formarine organisms to colonize. Submerged structures attract marineorganisms, both sessile and mobile. This has been observed on thegenerators in Lysekil project, this is called reef effect. Several marineorganisms have sensory systems that allows them detect electromagnetism.If the electromagnetism in the generators and sea cables will have an impacton these organisms is yet to be studied. Underwater noise is another topicthat needs to be concerned. This thesis presents a study on underwater noisefrom a full scale Wave Energy Converter (WEC) in the Lysekil Project.A hydrophone measured the underwater noise emitted from operatingdirect driven linear Wave Energy Converters in the spring of 2011. The mainpurpose was to study the emitted noise from an operating WEC, to determinecharacteristics such as spectrum levels, Sound Pressure Level (SPL), pulseduration and repetition rate.The results showed that the main noise is a transient noise with most of itsenergy in frequencies below 1 kHz. These results indicate that severalmarine organisms (fish and mammals) will be able to hear the operatingWECs. Although no behavioural reactions or injuries due to high soundpressure levels (SPLs) are expected.

Abstract

This licentiate thesis describes the design and construction of a synchronous generator test setup. Some initial measurements are also presented. A twelve pole synchronous generator has been built from a synchronous motor. Laminated poles, parallel stator circuits and a configurable damper winding were installed to adapt the machine to hydropower generator research. Equipment for various electrical and mechanical measurements has been installed. Results from some of the inital measurements are presented in the thesis. One experiment where the generator was connected to the grid investigated the importance of damper winding connections betweeen adjacent poles to damp rotor oscillations. Another study showed that parallel circuits in the stator winding under the right circumstances can contribute significantly to the reduction of eccentricity forces.

Abstract

A wave energy converter (WEC) of point absorber type has been developed and tests have been conducted outside Lysekil. The project started in 2002 and linear permanent magnet generators together with a subsea substation and buoys of various geometric shapes have been built and tested. The system is based on a low number of mechanical moving parts and the power conversion from ocean waves to electricity maintaining the quality of the national grid is handled electrically, due to the long life span of electric components. Reliability is highly prioritized in this design. To monitor the test site, measurements of electric output are done on the generators and substation. Also measurements of acceleration in heave mode are done on the buoy itself together with measurements of force between the buoy and the generator. The measurements are transmitted through the public cellular network. Also a internet based camera is set up at the site to monitor the buoys of the WECs visually. The monitoring systems, both visual and quantitative have proven to work successfully. In order for a WEC to produce electricity at competitive prices, the generator must not be larger than necessary in order to save economically on production, transport and installation. However, the WEC must be dimensioned to withstand harsh sea states. High added mass will in some cases create harsh inertia forces on the generator and large inertia forces on the buoy which might shorten the life time of the system considerably. The magnitude of the unwanted forces can be reduced by taking account for added mass when choosing a buoy geometry. A toroidal buoy is found to have less added mass than a vertical cylindrical buoy with similar excitation force.

Abstract

This licentiate thesis presents a study of the electromagnetic properties of linear synchronous permanent magnet generators, utilized in wave energy converters, and a two pole permanent magnet motor for an electrical vehicle. Both machine topologies are presented, designed with a numerical simulation tool, based on a model derived from Maxwell’s equations. Full scale prototypes of both the machines are under construction. A continued study about the impact on the magnetic circuit caused by the longitudinal ends of a linear generator is performed. The results present significant core losses in the translator and an increased cogging force caused by the longitudinal ends. Further, a new electric conversion circuit based on the electric resonance phenomena is presented. Experimental results indicate that a successful electric resonance between the generator and external circuit has been achieved. Finally, detailed analytical and numerical methods are utilized to investigate the losses in the two pole permanent magnet motor over a wide frequency interval. The results indicate that the efficiency of electrical motors in electrical vehicle system can be increased relative existing designs and argue for limiting of the gearbox. The system total efficiency and mechanical stability can thereby be increased. The work concerning the wave energy converter is a part of a larger project, the so called Lysekil Wave Power Project, whereas the work concerning the electric motor so far has been carried out as an individual project. However, a future goal is to integrate the research on the electric motor for electrical vehicle with closely related ongoing research regarding a flywheel based electric driveline for an All Electric Propulsion System.

Abstract

Wind energy is a rapidly growing energy source with vast potential. Vertical axis wind turbines could be more cost-effective than the three bladed horizontal axis turbines that dominate today. A concept based on the straight bladed Darrieus turbine together with a PMgenerator operating in direct drive is studied at Uppsala University; this thesis is a part of that project. This thesis contains a study of stator core loss at low electrical frequencies. The purpose of the study was to test the reliability of core loss simulations where the specific loss at low frequency is extrapolated using specific loss at 50Hz. The simulated loss was compared to the measured loss. Passive rectifiers can be used to connect all wind energy converters in a wind farm to a mutual DC load. This principle has been demonstrated using two PM generators with very different properties. This thesis also contains two studies of a full vertical axis wind turbine prototype. The first study concerns the power coefficient of the turbine. The power coefficient was measured at several wind speeds and turbine rotational speeds. The power coefficient attains its maximum value of 29% when the tip speed ratio is 3.3. In a second study, the armature currents of the generator were used to map the harmonic content of the turbine torque. The third harmonic of the turbine torque was found to be the strongest harmonic. This thesis also presents a wind energy converter that has been tailored for a telecom tower. Several aspects of the design are unique.

Abstract

The aerodynamics of vertical axis turbines is typically studied using streamtube, vortex or CFD models. This thesis focuses on the first two models, which are the computationally faster ones. The streamtube model is the fastest, allowing three-dimensional modeling of the turbine, but lacks a proper description of the flow through the turbine and does not include any time dependence in the solution. The vortex model used is two-dimensional, but gives a description of the flow around the turbine and can handle time dependence. Effects of a velocity profile and the inclusion of struts have been investigated using the streamtube model. Simulations with the velocity profile indicate that the vertical axis turbine should be quite insensitive to the profile (with respect to the power coefficient). If the applied profile is perpendicular to the rotational axis, the turbine generally performs better if the blade moves against the flow at the high velocity side of the profile. When including struts, the structural mechanics was included and the calculations shows that if turbines are designed for high flow velocities, additional struts are required, reducing the efficiency for lower flow velocities. Turbines in channels and turbine arrays were studied with the vortex model. The channel study included both the numerical parts of the simulations and the effects of channel width were investigated. On the numerical side, the most prominent result was that for wide channels, the number of revolutions until convergence is high. It was seen that smaller channels give higher power coefficients, as predicted by streamtube theory, but the increase in power coefficient with decreasing width was slower, than predicted by streamtube theory. Simulations on a turbine array were performed on five turbines in a row and in a zigzag pattern, where the mean power coefficients of the turbines in the array are higher than for a single turbine. The row configuration was also shown to obtain slightly higher power coefficients and being less sensitive to misalignments in flow direction than the zigzag pattern.

Abstract

Energy storage is a crucial condition for both transportation purposes and for the use of electricity. Flywheels can be used as actual energy storage but also as power handling device. Their high power capacity compared to other means of storing electric energy makes them very convenient for smoothing power transients. These occur frequently in vehicles but also in the electric grid. In both these areas there is a lot to gain by reducing the power transients and irregularities.

The research conducted at Uppsala university and described in this thesis is focused on an all-electric propulsion system based on an electric flywheel with double stator windings. The flywheel is inserted in between the main energy storage (assumed to be a battery) and the traction motor in an electric vehicle. This system has been evaluated by simulations in a Matlab model, comparing two otherwise identical drivelines, one with and one without a flywheel.

The flywheel is shown to have several advantages for an all-electric propulsion system for a vehicle. The maximum power from the battery decreases more than ten times as the flywheel absorbs and supplies all the high power fluxes occuring at acceleration and braking. The battery delivers a low and almost constant power to the flywheel. The amount of batteries needed decreases whereas the battery lifetime and efficiency increases. Another benefit the flywheel configuration brings is a higher energy efficiency and hence less need for cooling.

The model has also been used to evaluate the flywheel functionality for an electric grid application. The power from renewable intermittent energy sources such as wave, wind and current power can be smoothened by the flywheel, making these energy sources more efficient and thereby competitive with a remaining high power quality in the electric grid.

Abstract

Energy storage is a crucial condition for both transportation purposes and for the use of electricity. Flywheels can be used as actual energy storage but also as power handling device. Their high power capacity compared to other means of storing electric energy makes them very convenient for smoothing power transients. These occur frequently in vehicles but also in the electric grid. In both these areas there is a lot to gain by reducing the power transients and irregularities. The research conducted at Uppsala university and described in this thesis is focused on an all-electric propulsion system based on an electric flywheel with double stator windings. The flywheel is inserted in between the main energy storage (assumed to be a battery) and the traction motor in an electric vehicle. This system has been evaluated by simulations in a Matlab model, comparing two otherwise identical drivelines, one with and one without a flywheel. The flywheel is shown to have several advantages for an all-electric propul- sion system for a vehicle. The maximum power from the battery decreases more than ten times as the flywheel absorbs and supplies all the high power fluxes occuring at acceleration and braking. The battery delivers a low and almost constant power to the flywheel. The amount of batteries needed de- creases whereas the battery lifetime and efficiency increases. Another benefit the flywheel configuration brings is a higher energy efficiency and hence less need for cooling. The model has also been used to evaluate the flywheel functionality for an electric grid application. The power from renewable intermittent energy sources such as wave, wind and current power can be smoothened by the fly- wheel, making these energy sources more efficient and thereby competitive with a remaining high power quality in the electric grid.

Abstract

One of the main challenges in order to make electric cars competitive with gaspowered cars is in the improvement of the electric power system. Although many of the energy sources currently used in electric vehicles have sufficiently high specific energy, their applicability is limited due to low specific power. It would therefore be advantageous to create a driveline with the main energy storage separated from a smaller energy buffer, designed to have high power capabilities and to withstand frequent and deep discharge cycles. It has been found that rotating kinetic energy storage in flywheels is very well suited for this type of application. The work presented in this thesis and the included papers span a number of topcis Introductory overview - This section explains the concept of the modern flywheel, and investigates some of its properties. It illustrates the concepts with a number of examples, relevant for the usage of flywheels in vehicular applications. Experimental set-up - The construction of a complete electric driveline is ongoing within the division for Electricity at Uppsala University. An optimized electric machine has been constructed and connected with a programmable load, as well as with a DC power source through power electronics controlled by PWM. As a part of this system, an experimental set-up of an active magnetic bearing for two degrees-of-freedom has been constructed. The work with this device is described in detail and some preliminary results are presented. Self-bearing machine - The electric machine developed for the existing driveline is coreless, double wound and with a double rotor. In order to achieve magnetic bearing functionality in this device for all DOF, a novel Lorentz force self-bearing machine is suggested. The design is analyzed analytically and numerically.

Abstract

This thesis deals with the electrical system analysis of a Wave Energy Converter (WEC) for the objective of grid connection. The Linear Permanent Magnet Generator (LPMG) is modeled and validated with experimental results. The comparison studies establish that this model is useful for the subsequent system modeling and performance analysis. To simplify the generator side control, diode bridge rectifiers are used for rectification. To reduce the size and cost of energy storage elements, DC voltage regulation is done using a DC/DC converter. Since the buoy has limited control, the electrical system needs additional design requirements. Moreover, to achieve smooth and high power, many WEC are connected to a common DC link. As a result, the inverter for the DC/AC conversion should be capable to withstand the low frequency instantaneous power peaks. Conventional multilevel inverters are considered for the initial analysis. The Neutral Point Clamped (NPC) inverter is chosen for the detailed analysis due to its advantages over other topologies. Voltage balancing technique and current control method are examined. The experimental performance of NPC converter for the WEC will be the focus in future.

Abstract

Remote measurement of electric field generated by lightning has played a major role in understanding the lightning phenomenon. Even though other measurements such as optical, photographic, channel base current and thunder signatures have contributed in this regards, due to practical reasons remote measurements of electric field is considered as the most useful tool in lightning research. This thesis discussed about the remotely measured radiation field component of electric field generated by cloud flashes (ICs) and narrow bipolar pulses (NBPs). The associated HF radiation of these events at 3 MHz and 30 MHz were also discussed. To understand the initiation process of these discharges, a comparative study of the initial pulse of cloud flashes with the initial pulse of cloud to ground flashes was conducted. The result suggests that both discharges might have been initiated by similar physical process inside the thunderclouds. Comparing the features of initial pulse of cloud and ground flashes with that of pulses appeared in the later stages of cloud flashes suggests that the initiation process involved in both flashes are not very much differ from the initiation of cloud flashes at the later stage. The average spectral amplitudes of electric field for first 120 ms of cloud flashes showed f-1 frequency dependence within the interval of 100 kHz to about 2 MHz and does not follow the standard f-2 decrease (or even more) at high frequency region. It was suggested that small pulses which repeatedly appeared at the later stage of cloud flashes might have contribute to enhance the spectral amplitude at higher frequencies. Electric fields generated by Narrow Bipolar Pulses (NBPs), which are considered as one of the strongest source of HF radiation were measured in Malaysia in the tropic and their features were also studied. Result shows a good agreement with previously published observations of NBPs in other geographical regions. The pulse duration of NBPs is varied within 20 – 30μs with the normalized peak amplitude is of the order of 10 V/m, averagely 2– 3 times larger than the peak amplitude of ordinary return strokes. They were observed to emit intense burst of HF radiation at 30 MHz. Thorough analyses and observation of these pulses found previously unreported sharp, fine peaks embedded at the rising and decaying edge of the electric field change of NBPs. Therefore it was suggested that these fine peaks were most probably to be responsible for the intense HF radiation at 30 MHz.

Abstract

The SFLM Hybrid project aims to show that it is probable that a fusion-fission reactor can be constructed with a single-cell minimum B mirror machine as a neutron source. In this licentiate thesis, theoretical work has been done with the magnetic coil system of such a device and also with the overall concept. The magnetic mirror field is based on the Straight Field Line Mirror field and the device has a mirror cell length of 25 m. A fission mantle surrounds the mirror cell, catching almost all fusion neutrons. The energy multiplication of fission to fusion energy is about 150 with keff = 0.97, implying that almost all the produced energy comes from fission. Beyond each mirror end, a magnetic expander is added to the mirror cell. The expanders increase the plasma receiving “divertor” area to an almost arbitrary size, which provides tolerable average heat load on the wall materials. They also add to MHD stability and may be a mean to increase the electron temperature. The device is heated with ion cyclotron radio frequency heating and the fission mantle is cooled using liquid lead or a liquid lead-bismuth eutectic. The device is self-sufficient in tritium. The magnetic field has been optimized for flute stability, low ellipticity and low field gradients in the long-thin approximation. The optimization of the magnetic field components has been made with a local optimizer. A coil set has been calculated which reproduces the selected optimized magnetic field with satisfactory accuracy and within the preliminary geometric constraints imposed by the fission mantle and coolant influx/outflux. Circular coils produce the axisymmetric field component and quadrupolar coils similar to baseball coils produce the quadrupolar field component. The proposed device does not suffer from severe material problems (to our knowledge) or a pulsed mode of operation such as for the ITER project. The main remaining question to be answered seems to be the electron temperature in the plasma, which need to reach about 500 eV for efficient power production.

Abstract

Diamond is a promising material for high-power, high-frequency and hightemperatureelectronics applications, where its outstanding physical propertiescan be fully exploited. It exhibits an extremely high energy gap, veryhigh carrier mobilities, high breakdown field strength, and the highest thermalconductivity of any wide bandgap material. It could therefore producethe fastest switching, the highest power density, and the most efficient electronicdevices obtainable, with applications in the RF power, automotive andaerospace industries. Lightweight diamond devices, capable of high temperatureoperation in harsh environments, could also be used in radiationdetectors and particle physics applications where no other semiconductordevices would survive.The high defect and impurity concentration in natural diamond or polycrystallinehigh-pressure-high-temperature (HPHT) diamond substrates hasmade it difficult to establish reliable results when studying the electronicproperties of diamond. However, recent progress in the growth of high puritySingle-Crystal Chemical Vapor Deposited Diamond (SC-CVD) has openedthe perspective of applications under such extreme conditions based on thistype of artificial diamond.Despite the improvements, there are still many questions which must beanswered. This work will focus on electrical characterization of (SC-CVD)diamond by different measurements techniques such as internal photoemission,I-V, C-V, Hall and in particular, time-of-flight (TOF) carrier driftvelocity measurements. With the mentioned techniques, some importantproperties of diamond such as drift mobilities, lateral carrier transit velocities,compensation ratio and Schottky barrier heights have been investigated.Low compensation ratios (ND/NA) < 10-4 have been achieved in boron-dopeddiamond and a drift mobility of about 860 cm2 / V for the hole transit nearthe surface in a lateral TOF configuration could be measured.

Abstract

Vertical axis wind turbines may have mechanical advantages compared to conventional horizontal axis wind turbines, which make vertical turbines interesting to study. A 12 kW straight-bladed Darrieus turbine has been designed and constructed by the Division of Electricity at Uppsala University. The aim of the project was to build a small scale model of a turbine that was system optimized, not optimized part by part. The constructed turbine has no mechanical control systems such as blade pitching; it is instead regulated electrically by power electronics connected to the generator. A control system has been built that can run the turbine at constant optimum tip speed ratio. To start the turbine a simple robust electrical starter has been developed. It consists of a six-step modulated inverter which is feedback-controlled from the generator rotor position. The power coefficient as a function of the tip speed ratio was determined during a long measurement campaign performed in spring 2009. The top power coefficient was measured to 0.29 at a tip speed ratio of 3.3.

Abstract

Hydrokinetic energy, referring to the energy contained in moving water, is a renewable energy source that has gained much attention the past years. The energy is found in all moving water masses, but is only economical to convert for water masses moving with high velocity, i.e. likely around 1 m/s and above. This energy can for example be found in tidal, ocean and river currents which flow through narrow straits and channels. Along the west coast of Norway, there are many sites where kinetic energy conversion would be possible due to the strong current present. The driving force behind the currents is the tidal wave that progresses northward along the coast and increases in strength. The models that so far have been used for estimating the resource in Norway have been shown to be uncertain since they do not account for the fact that the velocities and the water levels are altered when energy is extracted. These effects can be simulated with numerical models. A channel in the Dal river, the Söderfors channel, is situated downstream a hydropower plant and was simulated with the numerical model MIKE. The water level alteration due to turbines was simulated. It was shown to be a lot less than the water level alteration caused by the level change in the downstream lake. Velocity profiles measured at several different locations were used to estimate how the power coefficient was changed. Four turbine configurations were studied and it was shown that changes in the power coefficient were prominent only for a vertical shear profile with a strong gradient. At the Division of Electricity, studies have been conducted on how to convert hydrokinetic energy to electricity since 2003. The main idea has been to use a system that limits the need for maintenance. The concept studied is a vertical axis turbine directly coupled to a permanent magnet generator. The Söderfors channel has, due to aspects such as the flow properties and velocity, been chosen as a site for an experimental station.

Abstract

The Licenciate Thesis presented here is a research work on numerical simulations of two different electrical phenomena: Long gap discharges under switching impulses and the lightning attachment process of positive upward leaders. The development of a positive upward leader and the process of progress of a discharge in long gaps are composed of two intertwined physical phenomena, namely the leader channel and the corona zone. The physical description and the proposed calculations of above mentioned phenomena are based on experimental tests on long gaps. In the methodology proposed here, a geometrical approximation for the representation of the corona zone is used. Furthermore, two different approaches are applied and compared to represent the leader channel. The used methodologies for the computation of the leader channel are an engineering approximation and a physics equation that takes into account the thermoequilibrium process. In order to introduce a more realistic behavior of the discharge, statistical delays for the inception and for the tortuous characteristic of the channel were brought in. A comparison between a model with or without tortuous channel was implemented. A very good agreement was found between the physical model and the test laboratory results. In addition, based on previous works related to the physics of lightning and the lightning attachment process, a new methodology is developed and tested here. The new approach refines previous calculations; the background electric field and the ionized region considered for the advance of the leader segment are computed within an alternative approach. The proposed methodology was employed to test two engineering methods that are accepted international standards, the mesh method and the electro – geometrical method. The results demonstrated that the engineering approximations are consistent with the physical approach. Besides the electrical phenomena mentioned above, one should keep in mind that there are real effects of the lightning attachment process that are not included or are avoided to simplify the calculation. In fact, when a structure is subjected to a strong electric field, it is possible to generate multiple upward leaders from the structure. This effect has not been taken into account in the different numerical models available until now. The published models consider every upward leader as an individual case. And therefore; a first approximation to the process of generation of multiple upward leaders incepted over a structure is presented here. The preliminary results have shown that it is possible to observe an influence on the background electric field when one leader develops simultaneously with other leaders.

Abstract

The large-scale conversion of the electrical energy system to incorporate other renewable sources of power, concurrently with vast refurbishment of the present machinery, brings the Swedish hydropower industry face to face with new challenges. Introduction of new technical solutions in systems originally designed for a slightly different purpose implies that fundamental conceptions need to be questioned. To be able to question prevailing conceptions regarding the mutual electromechanical interaction between hydropower generators and the interconnected power system, more advanced mathematical models are required. The presented work consists of three successive parts. The first deals with the development and initial testing of a dynamical electromagnetic field model of a single hydropower generator feeding an infinite power-system bus. The model includes rotational dynamics as well as excitation control and are treated numerically by the finite element method. Transient time stepped simulations show that the model is suitable for studying the electromechanical dynamics of a hydropower generator and that the implementation of the excitation system have a decisive impact on its behaviour. The second part suggests a method for identification of the characteristic electrical parameters of the electromagnetic-field generator model for use in equivalent circuit models of the same machine. The method is based on time stepped simulations of four common standard test procedures and the obtained parameters are compared to measured data. It is shown that the suggested method is an expedient method to obtain most of the parameters. A few improvements are suggested to enhance the agreement even further. In the third part, the dynamical behaviour of the electromagnetic field model is compared to that of two equivalent circuit based models of the same system. One of the circuit models is a simplified version of the other and they are both parametrised with the parameters extracted from the field model in the second part. It is found that the field model exhibits significantly higher stiffness and damping properties than both the circuit models which raise questions regarding several popular applications of such models. Experimental work is planned for the continuation of the doctoral work to investigate these theories more closely.

Abstract

The electrified railway system consisting of overhead lines for traction power supply, auxiliary power, return conductors for traction current and the tracks can be modeled as a system of multiconductor transmission lines (MTL) above a finitely conducting ground. There are several lumped devices connected in series or in parallel along this MTL and the response of the MTL to electromagnetic transients need to be solved along with the device response. In this licentiate thesis a method for incorporation of series and shunt devices along multiconductor transmission systems are addressed. For finding the surge voltage and current pulse propagations along the transmission line system telegrapher’s equations using the finite difference time domain method are solved. The lumped components connected along the lines are solved by means of Kirchoff’s laws for nodal voltages and currents using a circuit solver software, and the nodal voltages and currents are made available to the MTL model for each time step. The method presented is first verified by comparing simple lumped resistive and inductive components with the finite difference time domain method, where the lumped components are accounted for by representative differential equations. Later case studies on the influence of common traction transformers, i.e., booster transformers and autotransformers, and track circuits, i.e., relay and rectifier units, on a multiconductor transmission line system representative of a typical Swedish single-track electrified railway network are made. The calculations made in this work shows that the components connected along this system, which are needed for safe operation of the railway system, do affect the surge current and voltage peaks and distribution along the multiconductor transmission system. These effects are also discussed in this thesis.

Abstract

The study of a motor/generator for flywheel energy storage, meant to smooth the power transients in a vehicular power train, is expounded in this licentiate thesis. The proposed motor/generator has two sets of windings in the stator, rated at different power and voltage levels, and a significant moment of inertia to store energy in the form of rotational kinetic energy.

The need of a machine with such specific requirements is described in Section 1. The power drive presented has components operated at two different voltage levels and requires certain kinetic energy storage capability to smooth the operation under transients in power. The key component that linked both voltage levels in the same driveline and smoothes the operation is a flywheel energy storage system operated simultaneously by two sets of windings.

Flywheel energy storage systems are operated at high rotational speed to obtain high energy density. The losses in electrical machines increase rapidly with the rotational speed and unconventional solutions are required to achieve high efficiency performances. Main loss mechanisms are described in Section 2. The geometry selected for the motor/generator, in the described specific application, is the Axial-Flux, Permanent-Magnet coreless topology due to the potentially high efficiency and scalability.

Machines with two windings in the stator with different voltage and power rates but mutual induction coupling are not described in literature. Section 3 presents an equivalent circuit and the equations that describe the electric properties of the motor/generator studied.

Section 4 presents the analysis of an electrical machine. The dispersion of the magnetic flux in coreless machines advises against the accuracy of 2-D FEM methods. Therefore a 3-D FEM method has been developed specifically for coreless machines. The mechanical analysis of the machine rotor is also presented. The mechanical limitations of high speed machines impose also power rate restrictions due to the lack of space in the stator. A study to determine the power limits in high speed coreless machines is also presented.

The calculation method and the loss mechanism models presented have been validated in small scale prototypes. The tests performed are presented in Section 5. A scale prototype of a machine with two voltage levels in the stator has been tested as part of a full system. The low voltage side has been driven by a low voltage DC/AC power converter while the voltage induced in the high voltage side has been rectified and connected to a variable resistance load. The results show a steady deliver of energy from the low voltage side and abrupt power transients in the power delivered in the high voltage side, showing a promising response of this novel power drive system.

Abstract

The call for increased accuracy in the process of design and analysis of electrical machines has encouraged the use of numerical methods to calculate the electric and magnetic field distributions within this type of apparatus. The finite element method is a numerical technique that has become specifically widespread and popular, due to the relative swiftness and accuracy with which it is possible to treat complex machine geometries and non-linear material characteristics. From the calculated spatial and temporal distributions of the magnetic field inside the machine, the induced electromotive force, the cogging torque and the air-gap torque at rated load operation can be derived. Moreover, the power losses in the iron parts of the machine can be estimated if loss data for the lamination materials used in the poles and in the stator core of the machine are available. This thesis comprises two separate studies which fundamentally rely on numerical field formulations of the problems at hand. In the first study, the characteristics of a radial flux permanent magnet generator with two concentric contra-rotating rotors are explored. It is found that common core paths will give rise to a pulsating inter-rotor flux disturbance, which upsets the three-phase symmetry in such devices. The disturbance will however not be of any practical importance for synchronous generator operation, provided that there is a screening iron core between the rotors. The second study addresses the problem of core loss prediction in large hydrogenerators. Here, a special emphasis is on the importance of the additional rotational loss, attributable to bidirectional fields. It is found that model corrections introduced to account for dynamic and rotational effects typically increase the core loss prediction with about 28%. Furthermore, the magnitude of the additional rotational loss was found to be correlated to the stator slot dimensions.

Abstract

Wave energy is a renewable energy source with a large potential to contribute to the world's electricity production. There exist several techniques to convert the energy in the ocean waves into electric energy. The wave energy converter presented in this thesis is based on a linear synchronous generator. The generator is placed on the seabed and driven by a point absorbing buoy on the ocean surface. Instead of having one large unit, several smaller units are interconnected to increase the total installed power. By keeping the generator as simple as possible and minimizing the number of mechanical parts, the lifetime of the generator is believed to increase. To convert and interconnect the power from the generators, marine substations are used. The marine substation will be placed on the seabed and it will convert the AC with variable amplitude and variable frequency from the generators into an AC suitable for grid connection. The work presented in the thesis focuses on the first steps in the conversion, the rectification and filtering. The purpose with the study was to investigate how the generator will operate when it is connected to a non-linear load and to obtain guidelines on how larger systems could be designed. Another aim with the experiment was to see to which extent the voltage and power out from the generator can be smoothened. Offshore experiments and simulations have been done on a full scale generator connected to a diode rectifier and filter. The results show that a smooth DC output voltage after the rectifier has small ripple content. The kind of load and value of load will have an impact on the generators ability to absorb and produce power. The highest amount of absorbed power is achieved at the highest damping of the system. However, the highest damping function does not necessarily coincide with the maximum produced power. In connection to the installation of two new wave energy converters on the Swedish west coast, a marine substation will be launched at the site. Laboratory tests have been done on the marine substation and it worked as expected and the experimental data had a good agreement with the simulated data.

Abstract

Research within wave energy conversion has seen a large increase during the last years. Several concepts are now taking the final steps towards commercialization. Conversion from wave energy for grid connection is a multi disciplinary task involving many challenges; one being the fact that the device has to be dimensioned for a nominal power and still be able to withstand the extreme powers in some of the harshest conditions on earth. The research and development of a concept based on a point absorber converting wave energy to a direct driven linear generator were initiated in 2002 at Uppsala University. Theoretical simulations have now been compared to offshore full scale experiments at the research test site. This thesis focuses on the hydrodynamic wave/device interaction for a Wave Energy Converter subjected to Swedish west coast wave climate. First conclusions on the Wave Energy Converters dynamics and energy absorption have been drawn based on earlier results. A thorough study of the sea state of Skagerrak and Kattegat has been made to asses the potential and get design parameters for the Wave Energy Converter. Based on that, a Wave Energy Converter model using potential linear wave theory and with the generator as a linear damper, has been developed to simulate the dynamics and power capture ratio. The model has also been used to simulate how biomass accumulated on the buoy will affect the dynamics. As a next step, the model has been further developed to simulate a two component system having optimum amplitude response at frequencies coinciding with Swedish west coast conditions. Results are presented for time series of real ocean wave data collected at the research test site as well as for empirical spectra. An enhanced amplitude response for the two component system is achieved by adding supplementary inertia by use of the added mass from a submerged body. Simulations show that the increased velocity in the two component system gives an increased power capture ratio by a factor of two compared to the regular system of a point absorber. The increased velocity also leads to a decrease in optimal damping for energy absorption with 60 %. The main weakness of the hydrodynamical modeling is thought to be the neglected viscosity that might lead to unpredicted energy losses, pointing out the direction for improvement and future work.

Abstract

Flywheel systems are attractive in hybrid and electric vehicles, being able to handle the large power when accelerating and for regenerative braking. The combination of a flywheel device with a battery source has several advantages, such as higher peak power capacity, higher energy density and a decrease of the number of charging/discharging cycles in the battery. The Flywheel Energy Storage System here investigated has its novelty in the use of a double wound flywheel motor/generator to divide the system in two different voltage/power levels. High-Voltage (HV) side connects the flywheel machine to the wheel motor and Low-Voltage (LV) side connects the flywheel machine to the battery. This thesis deals with the power control systems involved in the connection between different components of the system. Bidirectional DC/DC and DC/AC converters are used to connect the flywheel motor/generator to the battery in the LV side. The DC/AC converter design and construction is the main focus of the present licentiate thesis. Two different scaled prototypes have been tested. A speed motor drive was designed and tested with the first scaled prototype, in which different filter and motor load connections were investigated. Results have shown that low harmonic distortion can be obtained in the inverter output voltage and current. The second scaled prototype is a Two Voltage Levels Machine. The bidirectional DC/AC converter has been designed so the LV side of the system could work in a fast and efficient way. Simulations have shown the dynamics of the system, which has also been tested experimentally under a drive cycle. The flywheel has shown to be able of delivering smooth power to the battery side, despite the variations in the load side. A scaling factor has been applied in order to extrapolate the results to a full-scaled system. Investigations have also been made about the battery recharging process. The main challenge is the control of the power flow despite the voltage variations of the flywheel machine as dependent on its charge state, i.e. rotational speed. A DC/DC buck/boost converter under PI control has been designed and simulated, being able to keep either the current or the voltage constant on the battery side when the machine speed is decreasing.

Abstract

Since 2002, the Division for Electricity at Uppsala University has been running the Lysekil project. The project is an attempt to construct and evaluate a technology for extracting electrical energy from the motion of ocean waves. The idea is to let this up-and-down motion drive a linear generator. A buoy moves thus in the waves, and is connected through a line to the generator at the sea floor. Three such wave energy converters, L1, L2, and L3, and a marine substation have been deployed in the ocean southwest of Lysekil on the Swedish west coast, at the Lysekil research site. Measuring equipment has also been deployed, together with a number of buoys for studying environmental impact. A measuring station has been installed on the nearby island of Hermanö, and an observation tower has been built on the islet of Klammerskär, south of the research site. This licentiate thesis describes the author's work on studying wave buoy motion and is based on five scientific papers, covering mainly two areas. Firstly, changes in water levels, and thereby changes in the equilibrium point for the buoy and generator, have been related to the ability of L1 to absorb energy. The results indicate that there is a correlation between water levels and energy absorption for L1 for the studied time period. When the water level deviates from average, the absorption values decrease. This is not unexpected, since the linear generator has a finite stroke. The effect is however noticeable primarily for water level deviations of more than 25 cm, and is only visible for those cases where either wave height or water level deviation is large. Secondly, the above mentioned observation tower has been designed and built. The tower is equipped with a network camera covering the research site, a wireless communication system and an energy system. The first acquired images of the buoy connected to L1, taken during the summer of 2008, have been analyzed, and buoy motion data has been extracted. The observation system has worked well, and the data on buoy motion (vertical motion in the range of +/-0.5 m correlates fairly well to measurements of significant wave height for the period (Hm0=0.82 m). A comparison with voltage data from the generator also indicates that the system has captured the dominating buoy motion. However, the system suffers from poor temporal resolution (about one image per second), and has not yet been synchronized with the other measurement systems at the site. Addressing these two problems is of high priority in the future.

Abstract

Electrified railways are undergoing rapid modernized in terms of increased speed, freight capacity and increased traffic. Traction systems are spread across countries with different signaling and power feeding systems. Signalling systems are undergoing modernization and now-a-days are more dependent on wireless and radio based services. Under these circumstances, there is a consistent demand on improved safety and operational reliability. Electromagnetic interference (EMI) and compatibility (EMC) issues play an important role in the overall performance on the traction power feeding and signaling systems. Some of them will be addressed in this thesis. In electrified railways sliding contact between pantograph and overhead contact wire is one of the main modes of power feeding to the electrical drives and propulsion system within the railway. The traction supply could be either AC or DC at different voltage levels. Arcing from the pantograph is a commonly observed phenomenon and is more pronounced in the winter. Pantograph arcing is one of the main reason of broadband conducted and radiated EMI within the electrified railway systems. Experience within the railway industry has shown that this source of EMI and its characteristics need to be understood properly for a deeper understanding of the associated EMI issues. In this thesis the influence of various parameters like, speed of the train, load current, power factor etc. on this sliding arc and its consequences are shown. It is found that all these parameters control the arcing phenomena and also their corresponding signature patterns. It is shown that a net DC voltage and current component originates from pantograph arcing and causes interference in both traction power and signaling systems. Parameters that influence the net DC components are also discussed. Possibilities of radiated interference to the wireless and radio based communication and signaling are also discussed in brief.

Abstract

Diamond is a semiconductor with many superior material properties such as high breakdown field, high saturation velocity, high carrier mobilities and the highest thermal conductivity of all materials. These extreme properties compared to other (wide bandgap) semiconductors make it desirable to develop single-crystalline epitaxial diamond films for many electronic device and detector applications. Future devices, such as power diodes, photoconductive switches and high-frequency field effect transistors, could in principle deliver outstanding performance due to diamond’s excellent intrinsic properties. However, such electronic applications put severe demands on the crystalline quality of the material.

Unfortunately, many fundamental electronic properties of diamond are still poorly understood, which severely holds back diamond-based electronic device and detector development. Such uncertainties are largely due to an incomplete knowledge of the types and concentrations of defects present in the material and also due to a lack of understanding of the influence that these defects have on transport properties.

The focus of this licentiate thesis is therefore the study of certain electronic properties of single-crystalline plasma-deposited (SC-CVD) diamond samples in order to gain more information about the charge creation and transport mechanisms. By measuring characteristics such as drift mobilities, saturation velocities, compensation ratios or average pair-creation energy and comparing them with theoretical predictions from simulations allows for verification of these models and improvement of the diamond growth process.

Abstract

Free-flowing water currents such as tides and unregulated water courses could contribute to world electricity production given the emergence of robust technical solutions for extracting the energy. At Uppsala University, a concept for converting water currents to electricity using a vertical axis turbine with fixed blade pitch and a direct drive permanentmagnet generator is studied. A system approach is desired, and in this thesis, a first analysis of two system components, the generator and the turbine, is presented. This thesis also deals with some issues concerning the design and construction of a low speed generator for this application. An experimental generator for verification of simulations has been designed and constructed. For the electromagnetic design, a FEM simulation tool has been used. The construction work has given valuable practical experience concerning for example handling permanent magnets and winding the generator with cable. Simulations and measurements of the experimental generator have been carried out for different speeds and loads. The generator can operate at the speeds and loads corresponding to maximum power capture for different turbines for water current velocities between approximately 0.5 and 2.5 m/s. At higher water current velocities the turbines may need to be run at a tip speed ratio that gives a lower power capture in order to limit the electrical currents in the generator, cavitation of the blades, or mechanical loads. Comparisons of measurements and simulations show an agreement. The FEM simulation tool can be used to simulate and design electrical machines with a low electrical frequency, i.e. 2–16 Hz.