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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.  

The imperative goal of reducing carbon emissions and reaching net zero emissions in electricity generation has driven various renewable technologies, including wind energy, solar energy, and wave energy, among others. Even though wave energy is a recent addition to the already existing renewable energy technologies, it offers distinct advantages, such as a high power density and lower inter-annual variability compared to wind energy. In addition to supplying electric power to the grid, wave energy has the potential to supply power to other applications, such as remote islands or desalination plants. However, one of the primary challenges of wave energy lies in its inherent variability, which poses difficulties in terms of grid integration and can lead to increased current harmonics at its grid connection point. With advancements in power electronic converters and energy storage systems, it has now become feasible to control and manage these variabilities, thereby ensuring higher power quality for grid integration of wave energy. This thesis delves into different power electronic converter controls employed within the electrical network of wave energy converters. It explores the utilization of a hybrid energy storage system comprising a battery and supercapacitor integrated into the DC bus of the electrical network. The wave energy converter electrical network is simulated in MATLAB/Simulink. The results illustrate notable enhancements in power quality at the grid connection point and reduced stress on the battery. Additionally, the system effectively mitigates the intermittent nature of wave power, enabling the provision of constant power to the grid. Furthermore, the thesis delves into the design of grid forming control, specifically designed to electrify islanded loads utilizing a wave energy converter. A multimode converter control is developed for electrifying a remote island modelled as a 10 kW and 1 kVAR load using a wave power park with three wave energy converters. The inverter is controlled in three different modes: grid following, grid supporting and grid forming, which is modelled using MATLAB/Simulink. One of the main issues of using grid following control is the voltage dip at the island’s point of common coupling. A grid support control is implemented with the multimode converter to counteract the voltage dip and restore the voltage to its nominal value. The converter was able to follow the IEEE 519-2022 standard for voltage harmonics and IEEE std 1547 for current harmonics at PCC while supplying the load.

In the past decade, the global share of fossil-based electricity generation has decreased from 67% to 61% in favor of renewable alternatives. To achieve global goals, a continued extensive expansion of electricity generation from renewable energy sources is necessary. Offshore wind power is expected to constitute a significant portion of this additional generation capability. However, intermittent energy generation like wind or solar power has negative impacts on the electricity grid due to its inherently variable and non-dispatchable nature. Furthermore, energy generation from renewable energy sources is characterized by low utilization and requirement of large geographical areas.

One way to mitigate several of these negative aspects is by co-locating energy sources with complementary characteristics. Combining different types of complementary renewable energy sources can reduce overall variability, increase transmission system utilization, and decrease land use. This thesis addresses several aspects of grid integration of offshore co-located energy sources, primarily, offshore wind power, floating solar power, and wave power. One question analyzed in several of the included studies is the optimal combination of energy sources for co-location to achieve the lowest variability.

Another aspect investigated is the capacity credit for a hybrid park consisting of co-located energy generation compared to the capacity credit for a wind farm. In a case study for the Netherlands, the capacity credit for combined wave and wind power is higher than for wind power alone. Additionally, the complementarity of renewable energy sources is analyzed and explained.

A substantial increase in renewable energy sources connected to the electrical grid is imperative to achieve net-zero emissions from the electricity sector. Marine energy sources, like marine current power and wave power, have the potential to significantly contribute to the increase of electricity from renewable energy sources. A crucial aspect of enabling marine energy utilization is the development of electrical systems for offshore renewable energy. Hence, this thesis addresses challenges regarding the grid connection of offshore renewable energy.

Two important questions for offshore renewable energy are how to construct the offshore electrical grid and how to transmit the power to the shore. This thesis provides a review of AC and DC collection grid topologies and compares HVAC and HVDC transmission for offshore applications. It is concluded that HVDC is the preferred technology for transmission distances exceeding 50 to 100 km.

Regardless of the configuration of the offshore collection grid, the energy converters must be connected to the collection and distribution grid. Uppsala University has deployed a marine current energy converter in the river Dalälven in Söderfors, Sweden. The grid connection system at the test site is based on a back-to-back converter technology. In the thesis, a simulation model of the grid connection system of the energy converter is presented. The simulation model is used to evaluate MPPT methods for marine current power. An advanced hydrodynamic model based on a two-dimensional free vortex method is utilized for this purpose. Additionally, a low-complexity hydrodynamic model is incorporated into the simulation model to assess electrical grids for marine current energy. One AC and one DC collection grid, each comprising five marine current energy converters, are compared. Furthermore, three DC collection grids, each with ten marine current energy converters, are assessed and compared.

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 accurately captures the frequency response of the experimental system.

Net-zero emissions from electricity production and their effect on global warming have led to an increased focus on power production from different renewable energy resources. Wind energy, solar energy, and hydroelectric power currently lead this effort. However, newer technologies, such as wave energy for electricity generation, have significant potential. This thesis investigates the usability and integration of wave energy systems into the electricity grid. This form of energy also has substantial potential in applications such as remote island electrification, which historically has higher carbon emissions due to its reliance on fossil fuels for energy.

This thesis focuses on Uppsala University’s developed point-absorber-based wave energy converter connected to the grid via power electronics converters. This thesis investigates various grid-side power electronics controls to safely connect the fluctuating frequency and voltage from the wave energy converter to the fixed 50 Hz grid. Additionally, a hybrid energy storage system consisting of a battery and a supercapacitor reduces the effect of variability and increases the reliability. The results illustrate the increased controllability of power flow to the grid and improved power quality. Additionally, the use of supercapacitors also increased the battery's performance.

The other part of the thesis explores the use of wave energy for remote island electrification. A novel multimode converter control (grid-feeding, grid-support, isolated grid-forming) approach is modelled in MATLAB/Simulink in a grid-connected system. These control modes are switched based on the requirement and scenario of the island load. The result shows the seamless transition between different modes, restoration of the island’s load voltage, and the constant power supply in the case of a blackout at acceptable power quality. An experimental study using a wave energy system for island electrification in an isolated grid-forming mode is also performed. The result illustrates the formation of the required load voltage at 50 Hz frequency, along with the functionality of black-start. A novel experimental approach of using a SiC-based converter in a wave energy system for improved power quality is also performed. The load voltage and current harmonics are reduced in all the experimented switching frequencies and comply with the grid code requirements.