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The electrification of societies and the decarbonisation of electricity production are changing energy systems worldwide. A fast transition towards the replacement of fossil fuels by intermittent renewable energy sources is expected in the next decades to combat climate change. A significant share of the produced electricity is likely to be generated from offshore wind farms, due to the abundant wind resources in the offshore regions and the lack of available onshore sites. However, increased electricity dependence in combination with expanded offshore wind power generation introduce new vulnerabilities to the society. Specifically, the effects of high impact low probability (HILP) events are considered as potential threats to the power system, not least because of the increasing number of extreme weather events. Therefore, research on power grid vulnerability and power system resilience to HILP events are of significant interest.

This thesis presents results of studies investigating power grid vulnerability from a topological perspective, and resilience to storm conditions of power systems with varying dependencies on offshore wind. To achieve this, methods based on complex network theory and AC power flow analysis have been developed, tested, and evaluated. Further, geospatial wind data from historical extreme storm events have been used to generate realistic power production profiles from hypothetical offshore wind farms.

The results strengthen that complex network concepts can be used successfully in the context of power grid vulnerability analysis. Further, the results show that the resilience of power systems with large dependencies on offshore wind differ vastly depending on the grid properties and control strategies, which are further discussed in this thesis.

Energy systems worldwide are undergoing fundamental changes driven by efforts to reduce carbon dioxide emissions. Fossil fuels are expected to be gradually replaced by intermittent renewable energy sources such as wind, solar, and wave power in order to reduce the greenhouse gas emissions. In parallel, substantial investments in power grid infrastructure are being made to meet the growing demand resulting from the electrification of industrial processes, transportation, and digitalisation, among other sectors. At the same time, power systems are facing challenges related to external threats, including extreme weather events and malicious attacks, which are increasing in frequency due to climate change and an uncertain geopolitical security landscape. Consequently, research on power system vulnerability and resilience to external threats, and how an increasing share of renewable energy sources affects these characteristics, is of high interest.

In this thesis, results from studies evaluating power system vulnerability and resilience are presented. Vulnerability is analysed from a topological perspective using methods based on complex network theory. Resilience to extreme weather events and malicious attacks is assessed by applying methods based on AC power flow models. Also, to provide a broader perspective on renewable power generation, a techno-economic assessment of offshore hybrid power parks is presented.

The findings of the studies strengthen the applicability of complex network theory for analysing power grid vulnerability. Moreover, the results show that the resilience of power systems with a high dependence on offshore wind power varies substantially based on grid characteristics and control strategies. The results also show that replacing conventional generators with wind power can increase the system's resilience to transmission line outages, which may result from extreme weather or malicious attacks. Furthermore, the results show that co-location of offshore renewable energy technologies can yield cost efficiencies even when the negative correlation between their generation profiles is weak.