44 B. Moynihan et al. Fig. 1 Sensor location at blade roots This paper details a method to analyze the forces on the blades of a Clipper Liberty C96 2.5 MW turbine instrumented with strain sensors at the blade roots. SCADA data from the turbine is also available and used in this analysis. This study computes the forces and bending moments on the blades from the strain measurements in the absence of all the detailed design specifications. The reference WT detailed by the National Renewable Energy Laboratory (NREL) is used to infer certain structural properties of the WT blades where proprietary information from manufacturers is unavailable [9]. In these specifications, NREL also provides descriptions of resulting forces and other parameters to be used in the design of WT. Results of this method are compared to these forces and parameters described by NREL as a function of wind speed. 2 Methodology The data used in this paper includes axial strain measurements from the roots of each of the blades, where four sensors are attached. This data is collected in hour long periods spanning 4 months of operation and over 1500 h of data is available. Supervisory Control and Data Acquisition (SCADA) data is available from the same time period and the data are synchronized with the sensor readings. The strain sensors used are optical intereferometers with corresponding temperature sensors as a way to compensate for temperature. The sensors are positioned in the four locations at each blade root schematically depicted in Fig. 1. A rigid model for the considered section wind turbine was used due to its high stiffness [10] and displayed in Fig. 2. Moments and forces on each of the blades are calculated in the local axes of the blade, which can be resolved onto the rotor in the global axes system. The local axes of blades are labeled as x, y, z, while the global axes of turbine are denoted as X, Y, Z as shown in Fig. 2. Thex-axis is perpendicular to span of blade, and points in the same direction as the nacelle, running parallel to and coincident with wind direction (when there is no veer). The y-axis is in the axial direction and along the blade, and the z-axis is in the plane of the blades’ rotation. Two key bending moments in flapwise and edgewise directions can be calculated directly from the strain data due to the placement of the sensors at the High Pressure, Low Pressure, Leading Edge, and Trailing Edge positions. The flapwise and edgewise moments are calculated from the temperature-compensated strain data as described by Eqs. (2) and (3). These moments are later resolved into the local blade axes defined for our analysis. MFlapwise =(εHP–εLP) ∗k1 (1) MEdgewise =(εLE–εTE) ∗k2 (2)
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