Modelling and analysis of electro-mechanical interactions in wind turbines

Abstract

The contribution of wind energy to the energy transition is steadily increasing. This growing contribution is driven by two aspects: the construction of new turbines and the increase in turbine size. In particular, for offshore sites, the nominal power of new turbines is now up to 16 MW and rising. Two main drive-train concepts are used, the geared and the direct-drive. Direct-drives, the focus of this work, have a characteristically low speed in order to limit the blades' tip speed. This results in large generator diameters, which have reached around 10 m. Scaling laws show that structural support mass grows faster than active mass, contributing to power generation. Therefore, new design methods for mass reduction are desired. The generator design is optimised based on given input loads and must maintain the air gap between the rotor and stator at all times. Typically, this is achieved by very high main bearing and generator support structure stiffness requirements, limiting mass reduction potential. In this work, it is assumed that designing wind turbines based on component optimisation does not ensure the best system design. However, moving to a more system-oriented approach requires new, holistic modelling techniques to simulate the wind turbine system, including electromagnetic forces from the generator. The required system model is derived in this thesis by adding a radial degree of freedom to the state-of-the-art wind turbine model. Two generator models of different fidelity are coupled to the wind turbine model, an analytical and a finite element model. Influences of the model adaptations on the system behaviour are identified. Structural component interactions are analysed and the effects of modelling on interactions with the aerodynamic solver and controller are investigated. The results show that lower system modes can be affected in their natural frequency by the modelling. Furthermore, a new system mode is introduced which is related to the new degree of freedom. The controller shows a high excitation of the new system mode for specific parameter combinations. Frequency-dependent feedbacks into the aerodynamics are also identified. Based on the comparison of both generator models, the analytical generator model promises a good trade-off between accuracy and computation time. In a second step, the effects of the identified interactions on turbine loads inside and outside the drive-train are analysed at the main bearing, the tower top, the tower base and the blade root. For this purpose, the equivalent loads of both models are compared. The load comparison shows that components inside and outside the drive-train are affected by the modelling. In particular, the main bearings and the tower show significant changes in load. However, loads can be increased as well as decreased compared to the state-of-the-art model. This supports the hypothesis of this work that a system design optimisation will differ from the component optimisation result. Furthermore, it can be shown that the added radial degrees of freedom and the electromagnetic forces add up in some cases and cancel each other out in other cases for load level changes. Based on the results of this work, follow-up questions arise, including lifetime estimation. Overall, this work contributes to a better understanding of the electro-mechanical interactions in direct-drive wind turbines and provides insight into the modelling approaches required for their analysis. It thus promotes the way towards system-oriented design of wind turbines.