BS EN 61400-13:2016 pdf free.Wind turbines Measurement of mechanical loads.
BS EN 61400-13 first torsional frequency and damping can be estimated using strain measured witha half or full torsion strain-gauge bridge through operational modal analysis.Since neither thefrequency nor the damping rely on absolute magnitude of the measurement, no calibration ofthis signal is required.
Measurement of rotor yaw and tilt moment
Asymmetrical rotor loads shall be measured on the primary load path as close to the rotor aspossible.
The preferred method for the measurement of rotor yaw and tilt moments is through twoperpendicular bending bridges on main shaft along with the hub azimuth positionmeasurement. lt is recommended that the main shaft strain gauges be positioned in line withthe instrumented rotor blade.
On some machines, it may not be possible to apply strain gauges to the shaft. In such cases,it is required to install gauges for bending moments in the non-rotating system,either on theshaft-support or tower top.
Measurement of the rotor torque
The strain gauges for measuring the torque of the main shaft shall consist of a full bridge withpairs of gauges on opposite sides of the shaft.Using the bridge only on one spot of the shaftsurface,shear due to bending and transverse load will be interpreted as torque. lf torquemeasurements on the shaft are not possible，tower top bending gauges and nacelle yawposition are permitted.Power and rotor speed shall not be used as a substitute to measurerotor torque.
Measurement of tower base bending
The bending moments at the tower base shall be measured in two perpendicular directions fora tubular tower.Tower base bending moments shall be measured using full strain gaugebridges mounted in the lower 20 % of the tower height as close to the base flange as possiblewhile avoiding disturbances from load introduction effects for the base flange,door,etc.As arule of thumb，tower base gauges should be placed at least one tower diameter from anyflange.
In the case of a guy-wire supported tower,the tower base measurements may be replacedwith measurements above the guy-wire attachment point. If the guy ropes are the mainstructural elements then the force in each rope should be monitored.
A lattice tower will require strain measurements in all supporting legs to arrive at the resultanttower base load for all wind directions. A special assessment of the strain pattern in the latticetower and the consequence for the measurements shall be performed.BS EN 61400-13 pdf download.