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  • Cost and Efficiency analysis of the Secondary electric machine in a CRAFT wind turbine
    Floating offshore wind turbines are ideal for deeper waters, providing access to stronger and more stable winds. The Counter-Rotating Axis Floating Tilting (CRAFT) turbine features a unique design with two counter-rotating turbines on a tilted vertical shaft and two independent electrical machines submerged below sea level. The primary generator, connected to both turbines, includes counter-rotation which doubles the relative torque, while the secondary machine controls the upper turbine. This study examines the impact of primary and secondary machine efficiency on electricity generation. The findings indicate that the primary generator’s efficiency is crucial for system stability, whereas the secondary machine’s efficiency is less critical. Reducing the secondary machine’s efficiency from 97% to 83% resulted in a 0.1% reduction in annual electricity generation. Despite the asynchronous machine’s lower efficiency, it is the economically favorable choice as the secondary machine over its synchronous counterpart due to its reduced design complexity and lower magnet costs, leading to lower overall expenses. Future research investigate how turbulent flow effects and airflow interactions between the turbines influences the model. Incorporating the cooling factor, a more comprehensive cost model and a refined dynamic stall model will also further improve the simulation’s accuracy and robustness.

  • Preface
    This Special Issue presents selected papers from EERA DeepWind conference, 17 – 19 January. This was the 21th Deep Sea Offshore Wind R&D Conference. The conference was hosted by SINTEF and NTNU and organized in cooperation with NorthWind and the European Energy Research Alliance (EERA) joint programme on wind energy. A total of 50 papers are included addressing the research topics of the conference: List of Acknowledgements, Conference chairs and guest editors are available in this pdf.

  • Peer Review Statement
    All papers published in this volume have been reviewed through processes administered by the Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing. • Type of peer review: Single Anonymous • Conference submission management system: Morressier • Number of submissions received: 62 • Number of submissions sent for review: 59 • Number of submissions accepted: 50 • Acceptance Rate (Submissions Accepted / Submissions Received × 100): 80.6 • Average number of reviews per paper: 2 • Total number of reviewers involved: 22 • Contact person for queries: Name: Randi H. Aukan Email: randi.h.aukan@sintef.no Affiliation: Sintef Energi AS - Energisystemer

  • Influence of wind farm fault current control strategies on distance protection during unsymmetrical faults
    This paper evaluates the performance of distance protection near a wind farm with type IV wind turbines with various basic fault current control strategies implemented in the power converters. The considered fault control strategies include control of purely positive sequence currents or combined positive and negative sequence currents for reactive power support of various magnitudes. A multi-variable study is performed in PSCAD to show performance of a basic algorithm of distance protection at a 420 kV line in a vicinity of the wind farm. The results show that the majority of relay failures occur for phase-phase-ground faults, that reactive current control has less influence in strong grid than in weak grid, and that standard wind turbine fault control procedure of providing balanced current might result in fault currents much smaller than maximum allowed (usually 1.2 pu).

  • Comparing fatigue and ultimate loads of two- and three-bladed 20 MW floating offshore wind turbines
    Two-bladed turbines could reduce costs in the whole turbine’s life cycle. Yet, the wind loads are less well distributed, and the rotor does not have the calm inertia of a rotating plate, like a three-bladed turbine. This paper should serve as a numerical basis to understand how different loads of large 20MW floating two- and three-bladed turbines actually are to enable a better estimation of implications from these loads. The most surprising finding is that the tower base bending loads do not increase for the floating two-bladed turbine compared to the floating three-bladed reference. The main tower excitation, known as the blade-passing frequency, happens two- instead of three times per revolution for a two-bladed turbine. For bottom-fixed turbines, an operation with the tower eigenfrequency close to this excitation causes severe loads, which is more likely for a two-bladed turbine. For most floating turbines, the tower eigenfrequency is much higher and happens to be in a bandwidth that serves two-bladed turbines better than three-bladed ones. However, it was also observed that the issue of tower resonance might, in general, be less critical for floating turbines due to a vast increase in tower damping. The highest increase in loads has been found at the tower top if no load alleviation concept, e.g. a teetering hinge or free-yaw, is utilized. Yaw and main bearing loads did not show any significant increase. The unique parked T-position exhibited a major benefit in storm conditions. The final results indicate that large floating two-bladed wind turbines may offer a valuable economic advantage when compared to three-bladed turbines of equal design maturity.