Method of in situ calibrating load sensors of a wind turbine blade

A method of in situ calibrating load sensors of a horizontal axis wind turbine, the wind turbine comprising:

• a rotor including a hub and a plurality of wind turbine blades, which extend radially from the hub, the hub being connected to a drive train including a generator, which converts mechanical rotational motion of the rotor into electrical energy, wherein
• said plurality of blades comprises at least a first wind turbine blade provided with a plurality of first load sensors positioned in a first cross-section of the first wind turbine blade, and wherein
• the method comprises the steps of:
• a) determining a rotor azimuth angle of the first wind turbine blade,
• b) determining a pitch angle of the first wind turbine blade,
• c) measuring loads in the first cross-section of the first wind turbine blade using the first load sensors,
• d) calculating theoretical loads based on at least the rotor azimuth angle and the pitch angle of the blade determined in steps a) and b),
• e) comparing the loads measured in step c) with the theoretical loads calculated in step d), and
• f) calibrating the first load sensors based on the comparison of step e), wherein the calibration are based only on measurements carried out, when the generator is cut out, wherein values collected in steps a)-d) are repeatedly sampled and calculated for each sample;
• wherein values determined in steps a)-e) are repeatedly sampled;
• and
• wherein step f) is carried out after a predetermined number of distinct samples have been logged, alternatively that a number of distinct samples exceeds a minimum threshold value, which is determined by a function depending on sampling frequency and spread of data;
• wherein the rotor azimuth angle and the pitch angle are continuously sampled;
• and
• wherein the rotor azimuth angle and the pitch angle may change values between subsequent determinations.