15. In-Situ Experimental Modal Analysis of a Direct-Drive Wind Turbine Generator

The purpose of the support structure of wind turbine generators is to ensure a constant air gap length throughout the circumference of the generator. It is supposed to be light weight and at the same time stiff enough to minimize deformation of the rotor outer surface caused by electro-magnetic forces in the air gap. Reducing weight without compromising the stability and stiffness of the generator requires an in-depth analysis of the dynamic behaviour. In a previous study a model and a calculation method for estimating this behaviour was developed. For validation of this model an in-situ modal testing of a large off-shore wind turbine generator is necessary. The challenges encountered during in-situ dynamic measurements of such a generator are the coupling with the rest of the turbine, the limited accessibility of the structure within the nacelle as well as the large mass of the structure. Various excitation methods were used for the measurements including hammering tests as well as output only measurements during operation and idling of the turbine. This paper presents the challenges encounter during the measurement campaign as well as the results of these measurements.