Novel Sensor Concept for Monitoring of Wind Turbine Blades

Development in wind turbine blade design has accelerated impressively over the past decade as rated output power has increased up to 6 MW today. State of the art turbine blades have reached dimensions up to 65 meters, which only seems to be limited in terms of transportation. For this reason, German manufacturer Enercon has started to build hybrid blades, made of both, a composite and a steel section which are fit together at the construction site. Still blade fabrication is hardly automized which can result in divergence of quality. Even though design standards have improved, damages due to fabrication inaccuracy still are present. Regarding the consequences of one blade failure, the average inspection and repair time must be estimated taking five days of down time [1]. Hence, the load carrying structure should be monitored in order to minimize costs of maintenance and repair and to extend lifetime. Today, there are only three products on the market other than visual inspection [7][8]. None of these products have been certified for reliable damage detection. For this purpose an automated blade monitoring system for early damage detection is currently developed. It is based on an approach using the proportionality of maximum dynamic stress and maximum vibration velocity related to the first bending modes, in the edge- and flapwise direction. This approach already could be proven to indicate starting damage much earlier than a change in eigenfrequency. Both numerical calculations and laboratory testing could reveal that dependent on the location where damage occurs, the proportionality method can show tremendous changes compared to the initial level [6]. A smart technical platform is supplied by the project partner HBM, offering sensor technology fully orientated on the challenging environment of a turbine blade. Quickly changing climate conditions combined with a high risk of lightning strike which mostly is affecting the entire measurement system have been included into the development of a new deflection sensor which is absolutely immune to the influences mentioned. In 2009 this system could be tested successfully during an edgewise fatigue test at the blade test center in Aalborg, Denmark. Currently, a long-term testing inside a 50.8 m blade of a 3.3 MW REpower turbine is performed, operating near the city of Husum, Germany.