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Wave energy converters (WECs) are still at an earlier stage of development when compared to variable renewable energy systems based on wind or solar power. Indeed, only a few WECs have exported power to electric grids until recently. Thus, the development of mathematical models able to represent essential aspects of the system and its connection to the grid becomes fundamental to assess the impact of integrating wave power to grids. This work develops a fully integrated wave-to-wire model, where the electrical model has re-configurable dynamic models of rotary and linear generators (with controllers) to accommodate different types of oscillating-body systems. Such an electrical model is interfaced with the WEC hydrodynamic and mechanical models. A complete wave-to-grid model is presented by integrating the generator system model, an electrical grid interface unit and a network equivalent for the receiving grid in a unified simulation environment with the WEC-Sim, an open-source tool for simulating the dynamic behaviour of WECs. Numerical simulation studies are presented considering different operating conditions for the grid integration of a floating body that is connected to either an hydraulic power take-off system or a direct-drive system. A wave-to-wire model for grid integration studies of oscillating-body wave energy converters

The real-world interaction of an offshore wind farm with an HVDC is complex due issues such as noise, randomness of event timings, and hardware design issues that are not well explored. Numerical simulations are widely accepted and cost effective tool to test a wide variety of different cases, however, the fidelity of the results is difficult to assess. Higher fidelity can be obtained with scale-down experiments, however, this solution has a limited test coverage. The power-hardware-in-the-loop (PHIL) option offers a good trade-off between test fidelity and test coverage. The hardware part allows a high fidelity of the results whereas, the software simulation part allows an extensive study of different cases at a reasonable cost. This paper presents a PHIL implementation of an offshore wind farm connected with a voltage source converter (VSC)-based HVDC. The offshore wind farm is emulated in a real time simulation environment, and a grid emulator is used as power interface between the emulated system and the hardware system, i.e. VSC-based HVDC substation. The interaction of real time models and hardware is demonstrated with experimental results

This report contains an overview over the most relevant technologies for realising an offshore power system in the North Sea. Also electric energy storage technologies are being considered, as these may be built offshore and integrated into the offshore power system. Special focus is given on the future perspectives and potentials of the technologies. The technologies are regarded both on component and system level. This technology assessment is concluded with a gap analysis and recommendations for the ways forward toward the realisation of the North Sea Super Grid.

This thesis focuses on power sharing in low-voltage, islanded ac microgrids. It is motivated by the challenges related to climate change. To achieve a successful integration of renewable energy resources in the power grid, it is likely with an increased use of small distributed generation units. A promising way of organizing these units is by the use of microgrids. With microgrids disconnected from the main grid, several challenges related to current, voltage and power flow control arise. These challenges are studied in this thesis, where a microgrid with two distributed generation units and one load is investigated while operating in island mode. The two distributed generation units in the system are implemented with significantly different output impedances, in order to examine the unequal power distribution. Distributed control systems are developed for this system, especially focusing on the use of virtual impedances for improved power sharing. Proposed virtual impedance methods are presented. These methods reduce the effect the virtual impedances have on the voltage level. Three different cases are considered, each of which divided into two subcases. In case 1, the microgrid system is assumed to have predominantly resistive output impedances. The subcases 1 a) and 1 b) consider the use of opposite- and conventional droop control algorithms for controlling the voltage and frequency levels. The microgrid considered in case 2 is similar to the one in case 1, but with predominantly inductive output impedances. As in case 1, the conventional and opposite droop control algorithms are utilized in case 2 a) and 2 b), respectively. Case 3 is a reproduction of case 2, but in the selection of virtual impedances, the physical output impedances are estimated to be 25 % lower than the actual values. Simulation models are carried out in all of the cases by utilizing the software MATLAB/Simulink. Each simulation case considers the use of physical output impedance only, an existing virtual impedance method and a proposed virtual impedance method. The proposed virtual impedance methods show promising results in all cases, where the power sharing performances are the same as for the existing methods. The voltage drops due to the use of virtual impedances are however significantly reduced. All of the simulation cases show a reduction of at least 90 %. This master's thesis is a part of the research project FME CINELDI, and the results of this thesis are planned to be verified through laboratory experiments.

Today, the world faces major challenges regarding our global environment. The most pressing challenge is global warming, and the scientific community is well aware of the catastrophic consequences an increase in global temperature will have. There is a global understanding that fossil fuels must be phased out as soon as possible, and renewable energy sources must replace them. Wind power is an important energy source which is mostly exploited onshore. Today, the number of offshore wind farms is increasing because they have some advantages over onshore installations like the high wind speeds and low turbulence. A challenge for offshore wind farm developers, are the transportation of power over long distances. HVDC-transmission technology is the preferred solution for offshore transportation, but it is dependent on an offshore converter station. It is expensive to install the converter station at deep water, therefore possible solutions for reducing the cost must be investigated. This Master's thesis has investigated a hybrid converter topology which will reduce the power losses and the cost. The hybrid converter consists of a diode rectifier connected in series with a voltage source converter. The two converters complement each other in a way such that the power losses and the total cost are kept low. The hybrid solution has few active power devices and will therefore be a reliable and robust converter station. A model-based controller for the hybrid converter was proposed. The controller makes the solution flexible and adaptable to the system dynamics. The hybrid topology was compared with other converter topologies to see the benefits of using the hybrid solution. The work on the hybrid converter began by reviewing proposals on alternative solutions for the converter platform. The control theory was studied to create a foundation of knowledge for the control design. The control objectives for the system was determined in the beginning because the objectives decide which parameters are to be controlled. Several control objectives makes the controller more complex and difficult to implement. A model-based controller was developed by utilizing a procedure from the passivity-based control theory. A model of the system was implemented in the simulation program PSCAD/EMTDC and the control system was simulated. Several approaches for the control design were tested before a final solution for a system with two control objectives was found. A dynamic performance assessment was performed on the final controller to validate that it performed well. A comparison between the different solutions showed that the hybrid topology has the advantages of lower losses and total cost. The hybrid topology showed promising results for further research and a detailed plan for future work was given in this thesis.