A Concept for the Estimation of Displacement Fields in Flexible Wind Turbine Structures

The design of wind turbines is based on conservative load assumptions compared to the actual load conditions at the designated erection site. As a consequence, structural reserves are very likely at the end of the turbine’s approved lifetime. Driven by this, a research collaboration between different institutions from wind industry and research facilities was initiated to exploit these structural reserves using continuous, model-based fatigue monitoring of individual wind turbines. This contribution focuses on the dynamic state estimator as a central part of the monitoring process chain. Arising challenges are the generally unknown system excitation, e.g. by wind loads, and the usually small set of available structural responses due to the high installation effort and maintenance costs of adequate sensors. On that account, the authors propose a Modal Decomposition and Expansion (MDE) approach for the displacement field reconstruction utilizing detailed Finite Element (FE) models of the structures under consideration in combination with inertial measurements. As a limiting factor, the MDE approach can only be applied to linear dynamic systems. To also account for the generally large nonlinear motion of wind turbines, the authors propose to subdivide the full system into a number of substructures mutually connected by interface definitions in the first stage, followed by a sequential implementation of single state estimators for each substructure. The quality of this implementation framework will be demonstrated in on a detailed flexible multibody model of a small-scale wind turbine test stand.