Centre for Renewable Energy Sources

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WT Load Measurements A Comparison between
Load-Based and Analytical Calibration

• D. J. Lekou, F. Mouzakis

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Load Measurements

• Tower (Bending Torsion)
• Main Shaft (Bending Torsion)
• Blades (Bending)
• Supporting Systems
• Hydraulic pistons
• Coupling axles
• Bearing plates

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Recommended Sensors

• Bending Moments Tower type T
• Bending Moments Shaft type II
• Torsion type V
• Axial Loads type T

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Load-based Calibration

• Apply load
• Determine influence coefficient

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Analytical Calibration

• Electrical calibration
• Knowledge of material geometry
• Definition of influence coefficient

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Calibration uncertainty

• Measured quantity y
• Normal (measured value, standard uncertainty)
• Type A analysis of repeated measurements
• Type B based on scientific judgment

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Load-based Calibration Uncertainty

• Load
• Magnitude of load
• Load direction Load introduction point
• Sensors
• Location of sensors
• Data Acquisition System
• Other parameters

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Analytical Calibration Uncertainty

• Material properties
• E-modulus, Poisson Ratio
• Geometrical Data
• Diameter, Thickness
• Sensors
• Location, orientation
• Data Acquisition System
• Gauge factor, Resistor, etc.
• Other parameters

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Tension of Plate Load based

• Load Magnitude
• 0.2 full scale
• Load direction
• 1
• Data Acquisition System
• uV  2.89mV

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Tension of Plate - Analytical

• Material Properties
• E-modulus
• Poisson Ratio
• Geometrical data
• Width
• Thickness

• Electrical calibration (C)
• S.g. factor
• S.g. calibrator
• Type A (linear regression)
• Data Acquisition System
• S.g. misalignment (neglected)

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Tension of Plate - Results

• Influence Coefficient uncertainty

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Tension of Plate - Sensitivity

• Influence of material properties on coefficient
• E-modulus
• Poisson ratio

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Wind Turbine Tower Load based

• Bending moment
• Applied load
• Distance measurement
• Load Direction
• Indication of s.g. bridge output
• Nacelle position
• Prevailing wind

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Wind Turbine Tower - Analytical

• Electrical Calibration (C)
• Material properties
• Geometrical Properties
• Data Acquisition system
• Strain gauge misalignment (neglected)
• Strain gauge positioning (neglected)

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Wind Turbine Tower - Results

• Load calibration
• 2.25
• Analytical calibration
• 3.56
• Target (IEC) uncertainty
• 3

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Wind Turbine Main Shaft

• Similar approach to previous
• Load calibration
• Blade position
• Analytical calibration
• Only E-modulus

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Wind Turbine Main Shaft - Results

• Load based calibration
• 1.26
• Analytical calibration
• 3.15
• Target (IEC) uncertainty
• 3

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Conclusions - Recommendations

• Both methods have limitations
• Load calibration needs high loads
• Load calibration not always feasible
• Analytical calibration results to higher
  uncertainties
• Combination of methods preferred

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Thank you for your attention