HISTORICAL RELIABILITY AND AVAILABILITY ANALYSIS OF HVDC SYSTEM OF OFFSHORE WIND FARMS IN EUROPE

Sammanfattning

Offshore wind farms in Europe are increasingly connected to onshore power systems through high‑voltage direct current (HVDC) transmission, making the availability of these systems critical for reliable energy export. Despite their growing importance, there is limited research that focuses specifically on the long‑term availability behaviour of offshore wind farm HVDC transmission systems using empirical operational data. This thesis addresses this gap by analysing historical outage data and developing a probabilistic framework to project long‑term availability under conditions of data scarcity. The study compiles and harmonizes publicly available outage records from TenneT and ENTSO‑E (European Network of Transmission System Operators for Electricity) for offshore wind farms connected via HVDC systems. Outage events are categorized into AC (Alternating Current) link and DC (Direct Current) and analyzed in terms of frequency, duration, intensity and seasonal behavior. Key reliability indicators, including Failure Rate, MTBF (Mean Time Between Failure) and MTTR (Mean Time To Repair) are derived and compared across assets. The results show that unplanned outages occur more frequently, while planned outages dominate overall downtime and impact. To overcome limited historical availability data, two complementary reconstruction techniques - Normalized Pattern Scaling and Cross‑Asset Linear Regression are applied to generate a complete asset‑year availability dataset. Long‑term availability is then projected over a 30‑year operational horizon using a Correlation‑Weighted Transfer approach based on CIGRÉ (Conseil International des Grands Réseaux Électriques) HVDC survey data, combined with Monte Carlo simulation to project availability. The projected availability demonstrates a three‑phase behavior, with lower availability during early operation, stabilization during mid‑life and increased variability in later years. Overall, the results demonstrate that meaningful long‑term availability projections for offshore wind HVDC systems are achievable when empirical analysis, statistical reconstruction and stochastic modelling are combined. The proposed framework offers an initial basis for availability assessment and can assist in estimating energy export for future offshore wind farm HVDC transmission systems.