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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.