Relative Blade Pitch Misalignment Detection

Case study: Dynamic and Static Visual Inspection Measurements.

Campaign details
Alt Objective:
Detect relative blade pitch misalignment between the wind turbine blades
Alt Wind turbine:
Senvion M114
3.2MW
Rotor 114m
Alt Commissioning year:
2017
Alt Measurements date:
March 2019
Alt Campaign outcome:
Relative blade pitch misalignment of 1.28° from dynamic and 0.61° from static measurements
General overview

Key facts regarding relative blade pitch misalignment:

  • Leads to a significant power production loss. It is a general assumption in the industry that, even a 1° pitching angle difference between two blades could lead to up to 5% of power production loss.
  • Causes aerodynamic rotor imbalance, which leads to increased loads on different components of the wind turbine, reduces lifetime and leads to consequential damages on the wind turbine structure and main components.

Key benefits from correcting relative dynamic blade pitch misalignment:

  • Increase energy production
  • Avoid premature wear due to excessive stress and loads
  • Prevent critical component failures and extend turbine lifetime
Measurement principle and set-up

In order to detect relative blade pitch misalignment between the wind turbine blades, visual inspections of the wind turbine are performed while in operation (Dynamic) and in still stand (Static). The captured images are then filtered and analysed, using different methods.
The camera set-up is located on the ground, targeting on the wind turbine blades. For the static measurement the camera is located at 6 o’clock rotor position, while for the dynamic measurement the camera is located at 4 o’ clock rotor position, in order to avoid the tower influence while the blade is passing from it.

Results

Static measurement

Alt
Fig. 1: Blade 1
Alt
Fig. 2: Blade 2
Alt
Fig. 3: Blade 3
Static results
Blade 1 - Blade 2
-0.43°
Blade 2 - Blade 3
-0.18°
Blade 1 - Blade 3
-0.61°

Dynamic measurement

Alt
Fig. 4: Blade 1
Alt
Fig. 5: Blade 2
Alt
Fig. 6: Blade 3
Dynamic results
Blade 1 - Blade 2
0.71°
Blade 2 - Blade 3
0.57°
Blade 1 - Blade 3
1.28°
Conclusion and Recommendation
  • The static and dynamic results have the same trend, meaning that the same blade is misaligned significantly in comparison to the other two blades. An important finding is that they do not have the same magnitude. In the wind industry, relative blade pitch misalignment detection is mainly done based on static measurements, but we recommend that it should be detected while the wind turbine is in operation.
  • Blade 3 is the blade with the most misalignment detected and the maximum amount of blade pitch misalignment is 1.28 °, between Blade 1 and Blade 3. The acceptable threshold of relative blade pitch misalignment is ± 0.3°. Relative dynamic blade pitch misalignment of a wind turbine over ± 0.3°, like in this case, should be corrected as soon as possible.
  • We recommend that further measurements should be performed on each blade separately, in order to figure out which blade has the highest aerodynamic efficiency. The optimal pitch angle of this blade is recommended to be used as a reference angle for the other two blades.
  • It can also be noted that, there is a difference in bending between the three blades. We recommend that this relation, between dynamic relative blade pitch misalignment and relative blade bending difference, should be investigated further.
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