Mechanical unbalance of a wind turbine is typically specified to G16 tolerance1), which can cause significant forces on the drivetrain and the tower. In practice, a much better tolerance can be achieved, and often is, but this is not a requirement. In addition, 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 optimise the on-site work, some method of identifying those turbines in need of attention would be useful. Although mass unbalance provides a clear 1/rev signal, on a wind turbine this is complicated by: variable speed operation; natural frequency of the tower and aerodynamic imbalance – also 1/rev but principally an axial force.
The Blade Unbalance Calculator for Bachmann Monitoring’s CMSSTD software, which drives the CMS modules, provides calculated mass unbalance results for your rotor, without stopping the turbine. This allows you to identify those turbines which require mechanical balancing.
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 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.
You can also confirm whether a rotor 1/rev results from aerodynamic or mechanical effects, thus allowing you to plan the appropriate corrective action for a time that suits you. There is also full visibility of whether turbine balance is degrading.
With Bachmann Monitoring’s 20 years of Condition Monitoring experience underpinning this software and the diagnoses it provides, you can be confident that you will have better knowledge of the state of your turbine rotor thanks to the Blade Unbalance Calculator.
1) For a 30 t rotor revolving at 20 rpm, G16 is equivalent to an unbalance of ~230 kgm