Blade Unbalance Calculator

The mechanical unbalance of a wind turbine rotor can exert considerable forces on the drivetrain and the tower, thereby driving "fatigue". Typically the specification requires a value barely better than G16 1). In practice, a much better tolerance can be achieved. This aspect is all the more important because unbalance can change over time due to blade erosion/damage, and ingression of moisture into the structure of one or more blades. To correct this, operators must arrange for mechanical balancing of the turbines, which requires access to the blades and the consequent loss of availability.

To optimize the on-site work, some method of identifying those turbines in need of attention regarding unbalance anomalies would be useful. Although mass unbalance provides a clear 1/rev signal, on a wind turbine this option is complicated by ...

• variable speed operation
• natural frequency of the tower
• an aerodynamic imbalance of the rotations can just as well originate from the axial force.

The necessary differentiation between mechanical and other other causes of unbalance requires software that can can differentiate between the triggers of rotational vibrations and display them. 

The "Blade Unbalance Calculator" – a plugin for the CMSSTD software from Bachmann Monitoring – allows the calculation of the mechanical rotor blade unbalance and a regular determination of the rotor mass unbalance. This allows you to identify those turbines which require mechanical balancing. An examination of the entire wind farm operation with corresponding productivity losses is no longer necessary when using the "Blade Unbalance Calculator".

The mechanical balance is calculated by a model-based algorithm using basic build data. The model is built once and is valid for all the similar turbines on your farm. Using real-time information from a 2D MEMS sensor in the centre of the nacelle, the output from the module is a measure of the actual mechanical unbalance in kgm. With the addition of an extra position sensor on the main shaft the module will also identify the angular location of the required correction mass..

The plugin also displays whether a unbalance results from aerodynamic or mechanical effects. Observing the change of the turbine record – for example a continuous deterioration of the balance – allows you to plan the appropriate corrective action in advance. There is also full visibility of whether rotor blade balance is degrading.

The "Blade Unbalance Calculator" is based on Bachmann Monitorings 20 years of experience in the field of Condition Monitoring. Numerous case analyses have been incorporated into the development of the algorithm. This gives the software a high degree of accuracy when it comes to diagnosing the condition of turbine rotors.

_{1) For a 30 t rotor revolving at 20 rpm, G16 is equivalent to an unbalance of ~230 kgm.}