Dynamic structural design of offshore direct-drive wind turbine electrical generators

Abstract

Direct-drive permanent magnet generators for multi-MW wind turbines are low speed high torque electrical machines requiring large, heavy and robust structures to maintain the airgap clearance open and stable. The structural mass of radial-flux generators can be estimated at an early phase of the design process. Using distinct approaches, the integrity of a 3MW electrical machine structure has been addressed from a dynamic perspective by carrying out modal analyses with a view to minimize its mass. A versatile tool has also been developed to help the engineers with the dynamic design of generator disc structures. Conical structures have been analysed and compared with the baseline disc model obtaining very promising results. Potential improvements have been proposed for more ambitious structural designs.

Parametric lightweight design of a direct-drive wind turbine electrical generator supporting structure for minimising dynamic response

“…The drop in deflection allowed a further reduction of the thickness of one of the sub-structures. As stated in Jaen-Sola et al (2018), the reduction of cone thickness supposes a fall in the first mode shapes. Then, the cone thickness could be brought down to 58mm and with it the natural frequencies were reduced by almost 4Hz.…”

“…The total mass of the structure was 33,524kg. This represents more than 6.5 times the weight given by Jaen-Sola et al (2018) for the structure under Mode 0. It is important to highlight that thanks to the thermal loading the stress was well spread across the cone sub-structure.…”

“…Stiffeners of triangular shape and ribs are added to the structure in order to increase its stiffness in particular directions and modify its natural frequencies (See Figure 6(a)). In this study, the authors continue the work carried out by Jaen-Sola et al (2018) to come up with a welldefined methodology that can be tracked when approaching the static and dynamic structural design of the machine. The authors minimised the weight of a 3MW generator rotor conical structure using the ANSYS Design Explorer Optimisation tool.…”

“…For this study, the authors considered the application of magnetic attraction loading (Mode 3), shear (30kPa) and gravitational (9.81m/ s 2 ) loading, as well as thermal (constant temperature of 55.69°C on the whole structure) and centrifugal (corresponding to 14rpm) loadings. Shear and gravitational loadings are the same as used by Jaen-Sola et al (2018), whereas the temperature for the thermal loading was taken from the SCADA data presented by ORE Catapult (2020) for a 7MW offshore machine and the centrifugal loading was applied based on the turbine's maximum assumed rotational speed of 14 rpm. This ensured that the authors were able to study the model working under the static worst-case scenario.…”

“…A density-based topology optimisation approach was used by Kirschneck (2016) to study potential mass savings of the XD-115 offshore direct-drive wind turbine electrical machine rotor taking its dynamic behaviour into account. A more generic methodology was developed by Jaen-Sola et al (2018), where a series of contour plots allowing the engineer to accurately estimate the mass and the natural frequencies of the structure were presented. The consequences of implementing distinct types of stiffeners in a disc structure were analysed.…”