Integrating renewable offshore wind generation into global power grids is a critical issue in the energy industry. Modular Multilevel Converter (MMC)-based High Voltage Direct Current (HVDC) grids are the most effective and promising technical solution for the new offshore wind energy power system connections. These grids offer scalability, controllability, and reliability advantages, but their stable and compliant operation requires implementing MMC control strategies and controllers. This thesis investigates the intricacies of HVDC technologies, focusing on MMC control strategies and controllers. The research explores grid following and grid forming converter control strategies, essential for ensuring stable and compliant operation of MMC-based HVDC grids.
The research elaborates on the Model Predictive Control (MPC), which is a promising control strategy for MMC-based HVDC grids. The MPC controller makes the control strategies respond faster, emphasizing constraints
and cost functions. Furthermore, a comparative analysis of Proportional Integral (PI)-based and MPC-based
controllers is conducted across various scenarios, highlighting the advantages of MPC-based controllers over
PI-based controllers regarding response time and stability.
Lastly, a large stability analysis is conducted using the direct Lyapunov method to evaluate the effectiveness of the control strategies and controllers during transient events. This analysis provides insights into the stability and performance of MMC-based HVDC grids under different operating conditions and disturbances.
This thesis illustrates the capabilities of grid-forming control strategy through Model Predictive controllers for
FPGA-based MMC-MTDC networks on RSCAD/RTDS®. The outcomes of this thesis contribute to advancing
HVDC technologies and control strategies, with implications for enhancing the stability and efficiency of MMC-based HVDC grids. The findings of this thesis have significant potential in the energy industry, particularly in integrating renewable offshore wind generation into global power grids. ... Read more

Given the urgency to shift to a cleaner energy mix, offshore wind energy is bound to be quickly deployed on a large scale in the next years. In order to meet the Paris Agreement, the current role-out rate of wind energy in the North Sea, that is 2 GW/year, needs to be increased to an average value of 7 GW/year over the period 2023-2040. This rapid increase of the share of uncontrollable, renewable sources in the energy mix raises a certain number of challenges, among which sustaining energy balance and transient stability. To organise the large-scale deployment of offshore wind in a strategic manner, the concept of hub-and-spoke has been introduced. It envisages modular offshore renewable power generation clusters (a.k.a. offshore hubs) designed to be easily and safely operated and interconnected.

Dealing with large power fluctuations constitutes a major challenge for each offshore hub. The variability of power supply needs to be mitigated through the integration of storage solutions. Offshore power-to-gas conversion for storage purposes has been widely investigated in the literature, under techno-economic or power balance points of view. However, detailed analysis of the effective connection of large scale electrolysers in offshore hubs are missing.

This thesis proposes a real-time simulation model of a 2 GW offshore hub integrating electrolysers in order to investigate their impact on the dynamic active power management of the hub. Two locations for the connection of the large-size electrolysers are studied: i) connection of an electrolyser to the AC common bus of the hub and ii) connection of electrolysers to the DC link of the back-to-back converter of each Type-4 wind turbine of the hub. An Electromagnetic Transient (EMT) simulation based analysis, necessary to capture the fast dynamics of the zero-inertia hub, is performed under different severe disturbances, such as a three phase fault and sudden large wind speed fluctuations.

In a second part, the thesis investigates the connection of the hub to the shore via a bipolar High-Voltage Direct Current (HVDC) link. It has been shown in the literature that bipolar links are more reliable than monopolar ones, as they can still be operated to transfer half of their nominal capacity in case of failure of one cable or converter. The work of this thesis is focused on developing and implementing a power sharing strategy between the two Modular Multi-level Converters (MMCs) constituting the offshore terminal of the link.

It was found in this thesis that both investigated locations for the connection of the electrolysers can be beneficial in different ways. Connected at the AC common bus, the electrolyser can provide centralized ancillary services, while an electrolyser connected locally at the wind turbine can consume the power generated during an islanding event. Regarding the power sharing in the HVDC link, two different strategies were developed, both enabling to unload one of the poles of the link. Further studies should be carried out to provide in-depth understanding of the potentialities of the two methods.
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