Logo: to the web site of Uppsala University

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.

As the world continues to deal with the effects of climate change, the need for carbon neu-trality becomes increasingly urgent. To achieve this goal, many countries are exploring the potential of distributed energy resources to reduce their dependence on fossil fuels and transition to renewable sources of energy. A microgrid is a small, independent energy system that can operate on its own or in connection with the main power grid. It integrates different energy sources like solar panels and batteries. Inverters are crucial in microgrids as they facilitate the seamless integration of various energy sources and contribute to grid stability. These inverters can be categorized into three distinct groups: grid-feeding, grid-supporting, and grid-forming. Each category serves a unique purpose, from synchronizing power with the main grid to providing support during grid disturbances and even enabling autonomous grid operation. These varying inverter functionalities contribute to the adaptability and resilience of microgrids, ensuring they can meet diverse energy needs and operate effectively in a range of scenarios. The thesis provides a comprehensive background of critical aspects of power systems and distributed energy resources, specifically focusing on microgrids and their significance in the evolving energy landscape. A particular emphasis is placed on the crucial functions served by inverters within microgrid architectures. Additionally, the thesis delves into fault analysis and mitigation strategies to ensure system resilience. Furthermore, the study highlights the importance of hybrid energy storage systems in enhancing the power quality of wave energy converters, achieved through the mitigation of power fluctuations. The outcomes and results of this thesis were developed and simulated using two platforms: MATLAB/Simulink and PSCAD. It delves into five distinct scenarios, each examining microgrid inverters from different perspectives in terms of circuit topology and control structures. The first one, shows the integration of a hybrid energy storage system as a key factor in elevating system efficiency, mitigating power fluctuations, and optimizing battery performance within the context of a wave energy system. In the second scenario, the thesis shifts its attention to grid-feeding and grid-forming inverters connected to a three-phase four-wire power system. The results showed the effectiveness of the suggested control strategy with smooth synchronization where the grid-forming inverter was able to form a network with an unbalanced factor lower than 2%, sinusoidal voltage, and frequency within standard limits. The third scenario places its emphasis on grid-supporting inverter, showcasing adaptability, and robust response to fault conditions by injecting or absorbing power, helping to mitigate voltage dips and fluctuations. The fourth, grid-forming inverter successfully formed a network with an unbalanced degree lower than standard regulations, maintaining sinusoidal voltage and frequency within standard limits. The fifth scenario explores the potential benefits and challenges of combining grid-feeding, grid-supporting, and grid-forming inverters as multi-functional inverters in the context of grid integration of wave energy converters. The multifunctional inverter configuration offers increased operational flexibility and resilience, effectively addressing a wider range of grid and microgrid possibilities.

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.