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The global energy production from renewable sources is increasing, with high penetration of both wind and solar in key regions. Ocean Wave Energy is projected to contribute with an increasing share of the future power supply, and the focus of this work is to investigate the requirements for connecting wave energy to the power grid, in context of the Fred. Olsen (FO) Wave Energy Project. Most Wave Energy Converters (WECs) produce highly distorted power due to the reciprocal motion induced by the ocean waves. Some WEC systems have integrated energy storage that overcomes this limitation, but adds significant expenses. As an alternative approach, this work investigates direct power export that relies on aggregate smoothing among several WECs. By optimizing the position of the WEC devices with respect to the incoming waves, fluctuations may be mutually canceled out between the devices. FO has closely monitored the global development within wave energy for about two decades, and has worked actively on developing WECs since 2002. The latest WEC system, named Lifesaver, has been in operation since April 2012 and is the basis of this thesis work. The Lifesaver system is described in detail, and comprehensive data on operational performance is presented. The major cost driver for grid integration is the peak to average power ratio, which can be as high as 20 in the early power conversion stages. Thus, it is crucial to improve the power quality early in the conversion chain so that the downstream power system is efficiently utilized. The simulations undertaken in this work indicate that a high quality power output can be achieved at the farm level, but that significant oversize factors will be required in the intermediate power systems within the farm. Cost-benefit analysis of the system show that a grid connected system at the current technology level will return marginal profitability. Therefore, several alternative approaches are investigated that could serve as a bridge towards future large scale systems. This includes autonomous systems that could supply power to remote ocean based units such as measurement and surveillance buoys, aquaculture facilities and support systems for the off-shore oil and gas industry. In general, the findings show that the WEC system is well suited for grid integration, although it becomes clear that significant development remains before wave energy can become an important supplement in the energy mix. Moreover, there seems to be a market for autonomous systems that is economically viable at the current technology level that could allow for immediate deployment of commercial systems. PhD i elkraftteknikk PhD in Electric Power Engineering

Denne masteroppgaven har undersøkt bruken av halvleder komponenter med høy båndbredde (WBG) i høyeffektsomformere. Sammenlignet med tradisjonelle halvledere, som for eksempel silisium (Si), så har WBG halvledere som for eksempel silisiumkarbid egenskaper som gjør det mulig å produsere komponenter med høyere blokkespenning, lavere ledetap, og raskere svitsjing. Alle disse egenskapene er ønskelige når det skal produseres høyeffektsomformere, men fordi SiC-teknologi fremdeles ikke har modnet enda så er det problemer med både kostnader og pålitelighet. Disse manglene betyr at fullstendig SiC integrasjon i det industrielle markedet fremdeles ikke er gjennomførbart. Derfor kreves nytenkende løsninger, som for eksempel hybrid Si-SiC svitsjen (HyS). HyS består av en parallellkobling mellom en Si IGBT og en SiC MOSFET. Si IGBT'en tar hånd om steady-state strømoverføring, noe som gjør det mulig å bruke en laveffekt SiC MOSFET som en hjelpesvitsj som tar hånd om svitsjingen i omformeren. Ved å sette sammen de to svitsjene på denne måten blir det mulig å utnytte seg av de ønskelige egenskapene til en SiC MOSFET i en høyeffektsomformer for en fornuftig kostnad. Det er spesielt interesse for å øke svitsjehastigheten til omformeren siden det vil gjøre det mulig å redusere størrelsen på passive komponenter, som for eksempel spoler og kondensatorer, som vil bidra til å minimere massen til systemet. Av den grunn ble det sett på som nødvendig å undersøke hvordan HyS svitsjen opererte i en høyeffektsomformer. På den måten kunne det fastslås om bruk av HyS-løsningen kunne bidra til å øke den maksimalt oppnåelige svitsjefrekvensen, f_(sw,max), for en gitt omformer, og eventuelt hvor mye f_(sw,max) kunne økes med. Videre, så ble det sett på som viktig å identifisere de største utfordringene som måtte løses for å kunne realisere HyS-løsningen. For å kunne verifisere HyS-løsningen så ble både de elektriske og termiske egenskapene av HyS-svitsjen evaluert ved hjelp av simuleringer. De dynamiske elektriske egenskapene til HyS'en ble kartlagt ved å bruke Simscape-biblioteket i Simulink. Disse simuleringene gjorde det mulig å sette sammen en rekke approksimasjoner av oppførselen til HyS'en som deretter skulle brukes i simuleringene av de termiske egenskapene til HyS-løsningen. De termiske egenskapene ble evaluert ved å bruke steady-state-analyseverktøyet til PLECS. Gjennom en rekke tester av forskjellige parametere hos HyS'en, ble forholdet til disse parameterne med f_(sw,max) kartlagt. This master thesis has investigated the use of wide bandgap (WBG) semiconductors in high-powered converters. In comparison to the traditionally used semiconductors, like silicon (Si), WBG semiconductors like silicon carbide (SiC) enable the production of devices with larger breakdown voltages, lower conduction losses, and faster switching. All these attributes are desirable for use in high-powered converters, but because of the immaturity of SiC technology there are cost and reliability issues. These deficiencies makes full SiC integration a nonviable solution for the industrial market. Therefore, novel solutions, like the hybrid Si-SiC switch (HyS) needs to be employed. The HyS consists of a parallel connection of a Si IGBT and SiC MOSFET. The Si IGBT takes care of the steady-state conduction, enabling the use of a low-powered SiC MOSFET as an auxiliary switch in order to handle the hard switching actions of the converter. By using this configuration, the desirable features of SiC can be integrated into high-powered converters for a reasonable cost. Namely, increasing the switching speed of the converter is desirable as it would lead to a decrease in the size of passive components like inductors and capacitors, decreasing the bulk of the system. Therefore, it was deemed necessary to investigate the use of the HyS solution in a high-powered converter in order to establish if, and potentially by how much, the use of the HyS would increase the maximum achievable switching frequency, f_(sw,max), of the given converter. Furthermore, the most pertinent practical challenges for realizing the HyS solution needed to be identified so that proper mitigation techniques could be put into place. In order to verify the use of the HyS solution, the electrical and thermal performance of the HyS was assessed using simulations. Using the Simscape library in Simulink, the dynamic electrical performance of the HyS and its constituent components was assessed. From these simulations, several approximations of the dynamic behaviour of the HyS that would be used during the thermal simulations were established. Then, steady-state analysis of the thermal performance of the HyS was conducted using PLECS. During these simulations, a parametric sweep was conducted in order to assess how different operating conditions affected the f_(sw,max) of a converter using the HyS solution. These simulations uncovered that by using the HyS solution instead of a solo IGBT in a high-powered converter, f_(sw,max) could be increased significantly. However, this was dependent on several factors, like minimizing the stray inductance in the HyS module, optimizing the current rating of the SiC MOSFETs, and maximizing the cooling efficiency of the system. Therefore, this thesis proposes a design process for an HyS custom module in order to combat the most pressing of these practical challenges.

Lifesaver is a point-absorber wave energy converter developed by Fred. Olsen. She is currently deployed off the coast of England for pre-commercial testing. Lifesaver consists of a toroidal floater supporting three Power-Take Off (PTO) units moored separately to the seabed. The mooring lines are kept taut by electrical generators.Large force oscillations have been encountered in the mooring lines during testing. The source of force oscillations is identified as velocity fluctuations in the PTO drive train due to sudden saturation of generator torque. The unfavorable transient response is a result of low stiffness in the mooring line combined with large inertia in the drive train. A numerical model of the mooring line and PTO unit is developed for use in control system development. The system dynamics are identified through a frequency analysis of a linearized model. Based on these findings a Kalman filter observer is developed to estimate force and force gradient from the angular velocity of the electrical generator. Three different controllers are proposed to mitigate the undesirable force oscillations. The first prevents sudden saturation by limiting the generator force gradient, and should be straight-forward to implement. Proportional-derivative (PD) feedback of the rope force is enabled by the Kalman filter, and is shown to effectively mitigate the unwanted behavior. However, PD feedback reduces the stability margin of the controller and must be implemented with care. A control algorithm using hysteresis is explored. The hysteretic controller behaves in a predictable and similar manner independent of the incoming wave, and has the potential of increasing robustness during high sea states. Real-time optimization of the main control parameter using extremum seeking is explored. The algorithm may be used to track a time-varying optimum due to changes in sea state. The adaptation rate is limited by the high peak-to-average power ratio of Lifesaver.