<p>Offshore power transmission is becoming an increasingly important issue. To moderate climate change, world leaders have set environmental goals that will be very difficult to reach without renewable power production and the removal of production units with high emissions. Wind power and electrification have been the focus in this report. Plans for the expensive wind power are already moving offshore. This report has made an attempt at suggesting a guideline for well-suited transmission systems, for wind power projects located at a distance in order to make them more economically attractive. Another emphasis has been to find the most suitable transmission system for gas turbines at offshore installations. As expected, the use of alternating current is best suited at shorter distances. At longer distances this system is still feasible up to 350 km, but losses will be high and there will be limited power available. A conventional thyristor-based direct current system will therefore be an attractive option for high power ratings and long distances. On the other hand, direct current based on voltage-source converters is considered more expensive, but has an improved control of reactive power and is therefore preferable to the conventional direct current system. To determine which system has the best design, one has to consider each case individually.</p>

<p>A direct driven permanent magnet synchronous machine with concentrated windings is optimized with respect to system efficiency. The goal is to win the European Shell Eco Marathon Urban Concept group using a hydrogen fuel cell and an electric motor. Considerations such as on-board energy storage, a freewheel for coasting, winding design and connections are taken into account. The result is a machine with higher efficiency at all loads and an optimal operation point at cruising speed, obtaining 93% efficiency. Considerations for further improvements in both power electronics and motor design are presented, along with a new philosophy for making very slow PMSM CW machines with multiple phases, both yielding higher efficiency and smaller requirements to structural stiffness.</p>

<p>This master thesis is written at the Department of Electric Power Engineering at the Norwegian University of Science and Technology. The work has been carried out at NTNU in Trondheim. The thesis deals with configuration of large offshore wind farms and transmission systems, and is a continuation of the project written during the autumn 2008. Today several plans on 1000 MW offshore wind farms exists. The size of the wind farms has led to a challenge of how to find an efficient and secure design of the overall system. The system has to be cost-effective in order to compete with other forms of power generation. In this study, costs is not considered. The purpose of this thesis was to study different transmission systems and configuration of an 1000 MW wind farm located 75 km from shore. The optimal distance between the turbines is a compromise between wake effect, wind farm are and cable lengths. To perform a detailed study of wake effects and optimal spacing, computer programs like WindSim would be necessary. Three common wind farm configurations is radial, star and ring layout. The selection of layout depends on costs, wind data and the wind farm area. Various wind turbine systems have been developed and different wind generators have been built. According to the survey of different wind generator system and considering the grid connection requirements on wind turbines, the developing trends of wind turbine generator systems shows that variable speed is very attractive and concepts with full-scale power converters will become more attractive. In this thesis two wind farm configurations with different transmission system were further studied. AC/AC, AC/DC and DC/DC are possible transmission systems. In this thesis AC/AC and AC/DC were compared. The selected layout of the wind farm was the radial layout. Number of strings was 35, with eight turbines in each string. Each wind turbine could produce 3.6 MW, which gives a total generation of 1008 MW. The two configurations were modeled in PSS/E. Siemens has made a model called WT3 that was developed to simulate performance of a wind turbine employing a doubly fed induction generator (DFIG). The model was developed in close cooperation with the GE Energy modeling team. This model was used in this thesis. For the dc transmission the HVDC Light from ABB was used. Two different disturbances were applied. One at the connection point at shore, and one at the connection point for all the radials. The load flow results shows that the losses are 5.8$%$ higher in the AC/DC system. The dynamical result shows that both of the systems were stable, and fulfill the grid code requirements. The results indicates that the short-circuit MVA is higher in the ac system than in the dc system. After a fault the voltage recovery was more smoother in the dc system, and the voltage recovery time were shorter.</p>

<p>This master thesis has been written at the Department of Electric Power Engineering at the Norwegian University of Science and Technology. The work has been carried out at the Royal Institute of Technology in Stockholm, where the author spent the last year of his studies as an exchange student. In the thesis, six different designs of the electrical grid of a 540 MW offshore wind farm, placed 100km off the Norwegian coast, have been studied and compared. At this distance, AC cable transmission might be difficult because of the reactive power production in the cables. Taking this into consideration, two options for the transmission system to shore have been studied. In addition to the AC cable transmission, voltage source converter based HVDC transmission, in the form of HVDC Light, has been studied, giving a total of 12 models. The main scope of the thesis was to study the load flow situation and power system performance of the different offshore wind farm layouts. Two load flow cases were run for each model; the first studying the model when the active power transmission to shore was maximized, the second studying the model under a contingency situation. The reliability of the six designs was compared by calculating the expected number of cable failures during the life time of the wind farm for each design, and what consequence the disconnection of any cable would have on the power losses. In order to study the effect of the offshore grid design and transmission system design on the offshore power system stability, dynamic simulations have also been executed, and the voltage response and rotor speed response following a fault have been studied. All simulations have been executed in version 31 of the program PSS/E. The wind farm was modeled full scale, consisting of 108 wind turbines rated at 5MW. The wind turbines were modeled as doubly fed induction generators, using the generic wind model that comes with the program. The load flow simulations showed that an AC cable connection to shore gave lower total system losses than a DC connection for all designs. The lowest losses were found at the n-sided ring design in the AC/AC system, and the highest losses were found for the star design in the AC/DC system. These losses were 2.33% and 8.19% of the total installed capacity, respectively. In the dynamic simulations, a three phase short circuit fault, lasting 150ms, was applied at three different places in the system. The simulations showed that except from at the wind turbines that were islanded as a result of a fault, all dynamic responses were stable. The HVDC Light transmission to shore gave the highest voltage drops and the lowest voltage peaks offshore. Also, the maximum speed deviation was found to be larger when using HVDC Light transmission compared to using AC cables, with two exceptions; the radial and star designs when a fault was applied to the transmission system. A comparison of the six different grid designs showed that the results were varying. Based on the results in this thesis it has not been concluded that one of the offshore designs have better dynamic qualities than the other. The simulation results indicated that this is case specific, and more dependent on where in the offshore grid the fault occurs rather than the design of the offshore grid.</p>

<p>The European Union has set a binding target saying 20 per cent of their energy consumption shall come from renewable energy sources within 2020. Around 4 per cent of the total amount is planned to come from offshore installations (40 GW). There total amount of planned offshore wind capacity is as of today 37 GW, mainly installations in the North Sea. The technologies that will be used for transporting the power to the shore are either HVAC technology using XLPE cables, transistor or thyristor based HVDC systems or HVAC Gas Insulated Line (GIL) technology. However, as the different technical solutions all have advantages and disadvantages compared to the other, the size of the power plants, distances from the shore and closeness to other wind parks will decide what technology will be used for the different cases.</p>

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In this work, a STAtic synchronous COMpensator (STATCOM) with energy storage system for wind power application has been treated. This device was proposed as a mean to improve voltage stability and power transmission by offering reactive as well as active power compensation. The work focuses on the converter topology of the STATCOM part and the control system. Further on, the energy storage system needed for this application was designed, including the choice of energy storage, its size and the interface/control system. The STATCOM, reactive part of the compensator was based on a voltage source converter (VSC), using a vector control. Its purpose was to maintain a stable grid voltage. For active compensation of wind power, a bank of super capacitors for energy storage system, SCESS, was used in this thesis. The super capacitor bank size was estimated, based upon the short term fluctuations in wind power. These fluctuations are results of contstructional factors of the turbines, variations and turbulence in the wind. The super capacitor bank was interfaced with the DC-bus of the STATCOM with a normal half-bridge buck-boost converter, to control the voltage level of the bank while maintaing a constant DC-bus voltage for good switching operation in the VSC. The control system for the active power compensation part was implented as a cascaded PI-control, compromising an inner current control loop, and an outer power control loop. The outermost loop included a dynamical power reference, based on the actual power transfer in the grid. This reference is supposed to assure that the controller is only compensating small fluctuations, while larger changes are left for other means, for instance controlled hydro power. The designed system was implemented in EMTDC/PSCAD. A small model, including one wind turbine, a weak grid and the STATCOM/SCESS was used in the simulations. With regards to the reactive- and active power compensation, the results were promising. However, the dynamical power reference could be of a better quality, as it does not take into account the losses in the STATCOM/SCESS, and thereby is inacurate regarding the amount power fed to/from the super capacitor bank. In addition, a small STATCOM model was realised in the laboratory. The results from the practical work showed the same general patterns as the simulations.

Artificial intelligence (AI) is a collective term for several computing techniques. They have in common that they use non-linear algorithms and that they are inspired from different processes in the nature, in particular how human beings make decisions. The use of AI for optimal operation of a stand-alone power plant has been investigated. This includes prediction, estimation, optimization and control. A presentation of some relevant AI techniques are given. A comparison with classical approaches such as for example PI control was made. The new techniques that were investigated proved to be very powerful and should be used more frequently than it is used today. AI techniques are especially promising for supervisorial control, but can also be used to control converters directly. A controller for a DC/DC boost converter was developed. It proved to be significantly better than a classical PI controller. Whether the computing time is shorter or faster than for classical approaches depends on the application. Compared to PI controllers the AI algorithms have a long computing time. Compared to classical wind power prediction techniques on the other hand AI techniques are very fast. A disadvantage with AI is the lack of rules for deciding the inner structure of the algorithms.