Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Methods for Experimental Modal Analysis of Wind Turbines in Operation

Operational Modal Analysis is used to extract the real modal frequencies and damping of the same tower modes and the two first edgewise-whirling modes.

The OMA method was chosen as a supplement to the exciter method to reach the objective of measuring the modal damping of all lower order turbine modes. This method only requires that long time-series of vibrational turbine responses are measured under almost steady operational conditions. The time-series selected for the analysis were signals from strain gauges mounted on the tower, main shaft, and rotor blades and used for load assessment measurements. The natural frequencies and modal damping extracted from strain-gauge signals in the fixed frame of reference showed to be most robust, and this frame of reference has furthermore the advantage that the extracted natural frequencies are directly comparable to the theoretical predictions with the new stability tools.

The OMA results gave new estimates of the first two tower modes, and estimates of the first two whirling modes that involve edgewise blade vibrations. These four modes are sufficiently low damped by the aerodynamic forces, whereas modes involving flapwise blade vibrations are highly damped by the aerodynamic forces. The latter do therefore not appear in the strain-gauge signals, which disables their extraction.

The advantage of OMA is that it does not require an extensive investigation of the turbine prior to the measurements as described in the Task-2 Report. Most importantly, the results of OMA are real estimates of both natural frequencies and modal damping of the aeroelastic turbine modes. The disadvantage is that it requires long-term measurements, which means that the controller has to be active in closed-loop operation, and this can affect the estimated modal properties.

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