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Mastering electro-mechanical dynamics of direct drive generators for large wind turbines

Introduction

The project “Mastering Electro-Mechanical Dynamics of Direct Drive Generators for Large Wind Turbine” aims at reducing the cost of energy of wind turbines with large direct drive generators. This project mainly focusses on reducing the weight of the generator structure of large direct drive generators in order to reduce the capital expenditures for manufacturing and installing these turbines.

The goals of this project can be broken down as follows:

1. Identify Appropriate Modelling Techniques

• Determine the appropriate modelling approach for the magneto-mechanical dynamics in generators in order to design for the lowest weight of the structure.
• Develop a model that can describe the electromagnetic and  the structural dynamics in an integrated (coupled) way, because the dynamic deformation of the structure changes the electromagnetic forces, which in turn changes the structural dynamics. The model should accurately describe the phenomena most relevant for performance, such as the forces and resulting deformations. The model must be validated.
• Identify modelling techniques and calculation methods, that make the modelling of coupled dynamics possible for future larger generator sizes and more complex geometries.

2. Improve the Design of Direct drive Wind Turbine Generators

• Investigate the role of the magneto-mechanical behavior of the direct drive generator and its structure to determine the design trade-offs of the overall turbine design.
• Identify and develop methods to optimize the design of direct drive generators.
• Develop innovative ideas for the design of future direct drive generator systems.

Summary of results

The project results can be separated in three themes.

1. Modelling Techniques
Modelling techniques were developed that can accurately predict the dynamics of coupled mechanical and magnetic systems, such as generators of wind turbines. The techniques were validated by measurements conducted on laboratory test rig.

2. Application to XD-115
After developing the modelling techniques, they are applied to the XD-115, a multi-megawatt wind turbine. A thorough dynamic analysis of the XD-115 generator showed that the generator is well designed, dynamically seen. Although the magnetic forces show a significant fluctuation locally, the resonances of the structure are not significantly excited. The dynamic forces created by space harmonics and torque cogging were identified as crucial, as they are in the same frequency range as global resonance frequencies. The model of the generator could be validated by in-situ vibration measurements.

3. Design Considerations
Finally, conclusions are drawn and design considerations assembled from the insights gained. An optimization of the generator structure was conducted yielding significant weight reduction and dynamic performance improvements of the generator structure. Furthermore, important effects and influences on the generator’s dynamic performance were identified. Possible challenges during the design phase of the generator and considerations to avoid deterioration of the dynamic performance of the generator were discussed.

Contribution to FLOW Targets

According to the FLOW cost model, the results of this project could reduce the levelised cost of energy by up to 0.96 %. This cost reduction is facilitated in the following ways:

Reduced Logistic Costs
Firstly, the optimized design of the generator structure is likely to decrease the mass of the generator. Because this mass contributes significantly to the head mass of direct drive turbines, a reduction of generator mass will entail reduced loads on the tower and other parts of the turbine. This will lead to less structural material needed in those parts multiplying the effect of the reduced generator mass. Eventually, the reduced weight of the turbine will lead to reduced logistics cost during the installation of the turbine.

Reduced Risk by Reduced Uncertainty
The better insights into the dynamics of the generator and the increased accuracy of the modal analysis reduces the uncertainty during the design phase of the generator. This reduced uncertainty leads to the possibility to use smaller safety factors. This will reduce the weight of the design, as the structure will be less over engineered.

Reduced Cost for Material
Using less material will also reduce the cost of the turbine, as less material is used at a certain price.