Load sensors in wind turbines

A method for determining reliability of one or more load sensors in a wind turbine, comprising:

• measuring loads with the load sensors during operation of the wind turbine;
• determining in-plane moments with rotor rotational speed frequency of one or more blades based on the measured loads;
• comparing the in-plane moments with rotor rotational speed frequency with theoretical in-plane moments due to a mass of the blades;
• and
• determining that the load sensors have reduced reliability if the in-plane moments with rotor rotational speed frequency deviate from the theoretical in-plane moments by more than a first threshold value.

A wind turbine system comprising:

• a wind turbine including a wind turbine rotor with a plurality of blades,
• a plurality of load sensors for measuring loads on the blades, and
• a control system configured to:
• receive signals from the load sensors during operation;
• determine in-plane moments on one or more of the blades;
• select the in-plane moments with rotor rotational speed frequency for the blades;
• compare the selected in-plane moments with theoretical in-plane moments due to a mass of the one or more blades;
• and
• generate a warning signal when the selected in-plane moments deviate from the theoretical in-plane moments by more than a predefined threshold.

A method for on-line determination of correct functioning of load sensors mounted on a wind turbine blade, comprising:

• measuring edge-wise and flap-wise strains in the wind turbine blade;
• determining edge-wise and flap-wise bending moments in the wind turbine blade based on the measured strains;
• converting the edge-wise and flap-wise bending moments to measured in-plane moments and measured out-of-plane moments on the wind turbine blade;
• applying a peak filter to determine the measured in-plane moments with rotor rotational speed frequency;
• determining an azimuth angle of the rotor blade;
• based on the azimuth angle, determining a theoretical variation of a moment due to the mass of the rotor blade;
• and
• comparing the theoretical variation of the moment due to the mass of the rotor blade with the in-plane moment with rotor rotational speed frequency.