An empirical data set of laser-optical pitch angle misalignment measurements on wind turbines was analyzed, and showed that 38 % of the turbines have been operating outside the accepted aerodynamic imbalance range. This imbalance results from deviations between the working pitch angle and the design angle set point. Several studies have focused on the consequences of this imbalance for the annual energy production (AEP) loss and mention a possible decrease in fatigue budget, i.e., remaining useful life (RUL). This research, however, quantifies the effect of the individual blade pitch angle misalignment and the resulting aerodynamic imbalance on the RUL of a wind turbine. To this end, several imbalance scenarios were derived from the empirical data representing various individual pitch misalignment configurations of the three blades. As the use case, a commercial 1.5 MW turbine was investigated, which provided a good representation of the sites and the turbine types in the empirical data set. Aeroelastic load simulations were conducted to determine the RUL of the turbine components. It was found that the RUL decreased in most scenarios, while the non-rotating wind turbine components were affected most by an aerodynamic imbalance.
During the manufacture of wind turbine blades, pitch bearings, and hubs, a reference mark is positioned at the bolt circle diameter, which is used to position the blade at the hub with respect to the rotor plane. Manufacturing tolerances of the bolt positions at the blade root, the pitch bearing, and the hub flange
Aerodynamic imbalance causes not only a loss in energy yield, but also an increase in vibration and rotor speed fluctuations, as well as loads on most turbine components and, in turn, a shorter turbine life
Wind farm operators are motivated to reduce the levelized cost of energy (LCoE) of their wind turbines. First, the loss in energy yield due to an aerodynamic imbalance increases LCoE. Second, on most sites, the fatigue budget is not fully utilized during the design lifetime of a turbine. The LCoE can be decreased when the energy yield over the lifetime is increased. To achieve this, the lifetime of the turbine can be extended until the fatigue budget is exhausted. Therefore, the quantitative effect of the aerodynamic imbalance on the remaining useful life (RUL) is of economic importance for the operator.
The pitch angle misalignment can be measured by photometric means, where a camera is placed directly below a blade pointing downwards to the ground, i.e., six o'clock position
To the authors' knowledge, the effect of the aerodynamic imbalance on the RUL has been mentioned in literature but not been quantified. To this end, this research analyzes empirical data of pitch angle misalignments encountered in the field. This analysis is then used to derive representative aerodynamic imbalance scenarios. As a use case, a commercial turbine sited in northern Germany is chosen for the assessment of the RUL of the turbine.
This article is structured as follows: Sect.
The blade pitch angle measurements were carried out based on a laser-optical method where the three-dimensional shape of the blade was captured with reference to a coordinate system at the center of the hub
Measurement setup: tachymeter and turbine at a standstill.
Identification of chord shape from measured coordinates.
Example of measured shapes of the three blades within the rotor; view from the blade tip.
Empirical data on blade pitch angle measurements obtained between 2013 and 2021 in Europe and North and South America were analyzed. The data set of more than 1100 turbines was filtered to 195 turbines representing a fleet of turbine types that need to be assessed for a lifetime extension today or within the next few years and are located mainly in northern Europe. The set contained 22 combinations of turbine and blade types in the power range between 0.9 and 2.5 MW.
Figure
Measured minimum and maximum pitch angle misalignment across analyzed wind turbines. A positive pitch misalignment
To assess the impact of aerodynamic imbalance on the RUL, the design imbalance was compared to several imbalance scenarios, which were derived from the pitch angle measurement data set.
For the design situation, the aerodynamic imbalance is derived from relevant design standards and guidelines. While more recent standards such as IEC 61400-1 Ed. 2 to Ed. 4
Figure
Aerodynamic imbalance derived from the measurement data set.
Seven imbalance scenarios were defined to represent the blue and red groups.
The combinations of blade angle misalignments in each scenario are summarized in Table
Imbalance scenarios simulated.
A wind turbine located at Bremervörde-Iselersheim, Germany, was modeled as an example. The wind turbine of the type Südwind S70 is a variable-speed, pitch-controlled wind turbine with a rated power of 1.5 MW. The chosen wind site and turbine type are a good representation of the average of the sites and turbine types analyzed in the empirical data set of pitch angle misalignment measurements; see above. This turbine type was certified for a large number of blade types, rotor diameters, and hub heights. The design loads were assumed for the turbine class IIIA according to IEC 61400-1 Ed. 2
Design parameters of the Südwind S70 turbine type.
The wind conditions on site were estimated according to Eurocode 1
The use case turbine was modeled in the wind turbine simulation software openFAST v2.3.0
The tower structure was modeled taking into account parameters from the type certificate and related documents of the S70 turbine
The load simulations were conducted for the design and the site wind conditions. In both simulation sets, the fatigue loads were generated from Design Load Case (DLC) 1.2 and DLC 6.4 according to IEC 61400-1
The time series from the load simulation were post-processed with a rainflow counting algorithm
The remaining useful life is determined for the turbine components that are critical to the structural integrity of the wind turbine, i.e., the blade root, blade bolts, hub, rotor shaft, main frame, and tower base. In general, the component with the shortest
First, we consider the RUL in the design scenario D (Fig.
Remaining useful life (RUL) of components in design (D) situation and imbalance scenarios (S).
Second, we consider the effect of the aerodynamic imbalance scenarios, which represent the blue and red groups, on the RUL. Here, we assume that the aerodynamic imbalance is not corrected during the entire lifetime of the turbine. Scenario S2.1 shows the largest decrease in RUL. Three component groups show similar RULs when compared to each other. The first group contains components in the vicinity of the blade connection, i.e., the blade root, the blade bolts, and the hub. Here, the smallest RUL ranges between 2.9 and 5.9 years. The second group contains the rotor shaft, which also includes the bolted connection to the hub. For these components, the RUL ranges from 6.2 to 11.1 years. The main frame and tower base represent the third group with RUL values between 3.4 and 26.1 years.
The third group is affected the most in the scenarios representing the red group with
Moreover, we observe that the RUL for the blade root and the rotor shaft increases in the two scenarios S2.4 and S2.5.
The annual energy production (AEP) of the wind turbine is calculated using the power curve specified by the turbine manufacturer and the wind speed distribution. The effect of imbalance on the AEP is assessed by comparing scenario D with the imbalance scenarios (Fig.
Annual energy production in imbalance scenarios (AEP
The previous section shows that different component groups are affected to a different degree by the aerodynamic imbalance. This observation is explained by the type of loading to which the different components are subjected. The components rotating through the gravity field, i.e., in the vicinity of the blade root connection and the rotor shaft, are mainly designed to withstand the alternating inertial loads. The edge-wise blade root bending moment, for example, is to a large extent dominated by the blade mass. Aerodynamic loads and thus an aerodynamic imbalance do therefore not significantly affect the RUL of the components rotating through the gravity field. In addition, the aerodynamic imbalance results from the loading of all three blades and therefore has no significant effect on the root connection of a single blade.
Both aerodynamic loads and the aerodynamic imbalance, in particular, have a larger effect on the rotor shaft and the components in the hub-to-shaft connection than on the blade root. For the non-rotating components, i.e., the main frame, and tower base, the effect of the aerodynamic imbalance is observed clearly. The imbalance excites the turbine in the rotational frequency and its harmonics. Hence, the decrease in RUL is greatest for the non-rotating components.
The largest decrease in RUL is observed in scenarios S2.1, S2.2, S2.4, and S2.5 where either one or two blades are misaligned collectively with an imbalance of
The difference between S2.2 and S2.4 is not as obvious but becomes clear at the tower base, where the effect is amplified by the lever arm of the tower. The equal distribution scenario S2.3 is overall the least severe scenario as the differences between aerodynamic loads of the three blades are not as large as in the other scenarios.
The effect that the imbalance severity has on the different component types can also be quantified by comparing the situations for
In all cases, the decrease in RUL for the critical components is accompanied by a decrease in AEP. In scenario S2.1, the largest losses in AEP coincide with the largest losses in RUL.
In the two scenarios S2.2 and S2.5, the AEP losses amount to 0.6 %.
In S2.5, two of the rotor blades have a misalignment of
Annual energy production per wind speed bin in imbalance scenarios (AEP
The scenarios simulated represent the derived aerodynamic imbalance of the measured data (Fig.
The scenarios simulated represent an absolute pitch angle misalignment of one blade of
The pitch angle misalignment data were determined from the pressure side of the airfoil at the maximum chord cross section. Considering the maximum chord position has two advantages: (i) the accuracy of the angle calculation is higher and (ii) the effect on the blade deflection and twist due to gravity loads and aerodynamic loads during the measurement is lower when compared to a cross section further outboard with a shorter chord length.
It must be noted, however, that most of the aerodynamic loads are generated at the outboard blade portion. Thus, the measured absolute twist angle relative to the target at this blade portion is of interest for the optimum absolute blade pitch angle alignment. This task would be rather challenging due to there being other causes for the deviation between the actual in-field blade geometry and the target blade design geometry, e.g., manufacture-induced blade distortions
The individual blade pitch angle misalignments of 195 wind turbines of different types were measured using a laser-optical method. From the empirical data, aerodynamic imbalance scenarios, which represent 35.3 % of the measured imbalance situations not accepted by the standards and guideline, were derived and assessed by means of aeroelastic simulations.
The RUL of the turbine served as a metric to quantify the effect of the aerodynamic imbalance. We have shown that the aerodynamic imbalance reduced the RUL in most imbalance scenarios compared to the design situation. Rotating components, i.e., in the vicinity of the blade root and shaft-to-hub connection, were affected less by the imbalance than non-rotating components, i.e., main frame and tower base, which became limiting for the RUL of the use case turbine in an imbalance situation toward stall. Pitch angle misalignment toward stall had a more severe impact on RUL than misalignment toward feather. Depending on the turbine component, we can observe different non-linear relationships between imbalance severity and loss in RUL.
The AEP can increase or decrease depending on the wind speed and the direction of misalignment. The total energy production across all wind speeds was always negative, however. The scenario leading to the highest loss in RUL also led to the highest loss in AEP.
The data presented in the figures are available at
BR and MS modeled the use case turbine and its controller and conducted the aeroelastic load simulations and the fatigue analysis of the turbine components. MR initiated this research and wrote the paper together with MS. TK analyzed the measurement data. The four authors assessed the lifetime extension of the turbine together.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We acknowledge the support of P. E. Concepts GmbH. Moreover, we would like to thank WIND-consult GmbH for sharing their measurement data for this research.
This paper was edited by Amir R. Nejad and reviewed by two anonymous referees.