Standard blade element momentum (BEM) codes use Prandtl's tip-loss correction which relies on simplified vortex theory under the assumption of optimal operating condition and no wake expansion. The various tip-loss functions found in the literature are listed. A simple comparison between them shows important differences in Annual Energy Production which reveal a large uncertainty in current BEM-based computations. A new tip-loss correction for implementation in BEM codes has been developed using a lifting-line code to account for the effect of wake expansion, roll-up and distortion under many operating conditions. A database of tip-loss corrections is established for further use in BEM codes. This model is closer to the physics of the flow and hence, a better assessment of the performance of wind turbines by this method is expected.

Recent developments in LiDAR technology have led to much interest in the possibility of improving turbine control by using a turbine-mounted LiDAR, to provide advance information about the approaching wind field. This could significantly reduce turbine loads, bringing improved cost-effectiveness, especially for large turbines. There have also been claims of direct increases in energy capture as a result of using such preview information. This study reports on an independent study employing detailed analytical methods to evaluate the likely benefits of LiDAR-assisted control and advise LiDAR manufacturers about the characteristics of their systems, which are most likely to be useful for this application. Accurate simulation models are vital for assessing the performance of LiDARs and controllers which use them. Current models use Taylor's frozen turbulence hypothesis, but this is not strictly valid when LiDAR is used to measure upstream wind speeds, as the measured wind cannot be assumed to convect unchanged to the turbine. A method for avoiding the frozen turbulence assumption is proposed, and simulation results are presented to illustrate the effect on fatigue load reductions which LiDAR-assisted control might achieve. A detailed assessment of possible LiDAR benefits is made using the UPWIND generic 5 MW turbine as an example.

This study investigates wound rotor induction machine bearing fault detection by stator current analysis. The research first establishes an analytic machine model that enables high fidelity simulation of a range of machine bearing defects. The time-stepped simulation results are then used to examine stator current spectral signatures of typical bearing faults. The calculations indicate that a number of low magnitude fault specific frequency components appear in the current signal as a result of air-gap variations produced by an incipient bearing fault. However, the considerably low magnitude levels at which these components are exhibited would make the detection of bearing fault using conventional current signature analysis techniques challenging. An alternative technique based on spectral analysis of complex current signals is therefore proposed in order to improve fault detection. The validity of the findings of this work is confirmed by analysis of measured data obtained on a 30 kW commercial machine test rig that can be configured to introduce a range of different bearing fault severities.

Certain parts of a wind turbine, for example, the gearbox require significant time and heavy lifting equipment in the event of catastrophic failure necessitating replacement. Continuous condition monitoring has the potential to catch problems early, enable scheduled preventative maintenance and thereby reduce turbine downtime, reduce the number of site visits and prevent secondary damage. Accelerometers applied to mechanical components of the drive train are traditionally used for condition monitoring, but require their own data acquisition system and analysis software. In contrast, the electrical current and voltage are continuously measured and could also be used for condition monitoring more cheaply. An experimental data acquisition system has been installed on a small (25 kW) onshore turbine in Leicestershire, UK to compare three-phase currents and voltages on the stator windings with six accelerometer signals. Data have been recorded before and after a gearbox failure and replacement. Data were analysed using both Fourier transform and Morlet continuous wavelet transform methods. Results show that the stator voltages show the same radial and axial mode vibration frequencies as the accelerometers, and could therefore be used for condition monitoring. Furthermore, the stator currents show torsional modes of vibration not picked up by the accelerometers.

Offshore wind energy is catching increasing worldwide interest. However, access and maintenance offshore can be difficult and will be more costly than onshore, and hence, availability is correspondingly lower. As a result, there is a growing interest in wind turbine condition monitoring with condition-based rather than responsive and scheduled maintenance. A non-linear state estimation technique (NSET) model is presented here to model a healthy wind turbine gearbox using stored historical data. These data capture the inter-relationship between the model input and output parameters. The state vectors comprising the data should cover as much as turbine operational range, including the extreme conditions in order to obtain an accurate model performance. A model so constructed can be applied to assess the operational data. Welch's t-test is employed in the fault detection algorithm, together with suitable time series filtering, to identify incipient anomalies in the turbine gearbox before they develop into catastrophic faults. Two case studies based on 10-minute supervisory control and data acquisition data from a commercial wind farm are presented to demonstrate the model's effectiveness. Comparison is made with neural network modelling, and the NSET approach is demonstrated to be superior.

The Gearbox Reliability Collaborative has conducted extensive field and dynamometer test campaigns on two heavily instrumented wind turbine gearboxes. In this study, the load sharing behaviour between six bearings in the planetary stage is described using a combined approach of measurement and simulation. First, planet-bearing data are analysed to characterise planetary stage behaviour in different environments. Second, a method is described for integrating the measured responses of the planetary stage into an advanced model of the bearing life that significantly changes the life prediction. Third, a sensitivity study of the planet bearings is conducted using multibody gearbox models. Various levels of gearbox flexibility and different planet assembly fits are investigated and compared with experimental observations. Measurements in the dynamometer and field show that bearing loading differs significantly between the six planet bearings. The relative loading behaviour of the planetary stage bearings is directly influenced by boundary conditions of the planet carrier pins. Assembly differences between two identically designed gearboxes cause different load sharing behaviour. Simulations are used to quantify the effect of different component flexibilities. Reduced order models are developed to accurately predict bearing loading in a cost-efficient manner.

Control algorithms for the rotor- and grid-side power converters of a double-fed induction generator (DFIG)-based wind turbine under non-ideal grid voltage conditions are proposed, and guidelines for tuning the controller parameters are presented. The control schemes are based on sliding-mode control (SMC) theory. Apart from directly controlling the DFIG's average active and reactive powers, the proposed methods also fulfil two additional control targets during voltage unbalance and harmonic distortion, that is, the rotor-side converter (RSC) eliminating electromagnetic torque fluctuations and the grid-side converter (GSC) compensating for the stator current harmonics to ensure a sinusoidal total current from the overall generating unit. The described control strategies are proved to be robust against parameter deviations and of fast dynamic response. In spite of the discontinuous nature of the standard SMC, constant converter switching frequency is achieved. Besides, the RSC control algorithm does not require a phase-locked loop and, furthermore, there is no need for decomposing the grid voltage and different currents into symmetrical sequences or harmonic components in any of the converters’ control systems. Finally, the excellent performance of the system, as well as its robustness, is verified by means of simulation results under different grid voltage conditions.

This study presents wind turbine converter stability analysis of wind farms in frequency domain. The interaction between the wind turbine control system and the wind farm structure in wind farms is deeply investigated. Two wind farms (i.e. Horns Rev II and Karnice) are taken into consideration in this study. It is shown that wind farm components, such as long high-voltage alternating current cables and park transformers, can introduce significant low-frequency series resonances seen from the wind turbine terminals that can affect wind turbine control system operation and overall wind farm stability. The same wind turbine converter control strategy is evaluated in two different wind farms. It is emphasised that the grid-side converter controller should be characterised by sufficient harmonic/noise rejection and adjusted depending on wind farms to which it is connected. Various stability indices such as gain margin, vector gain margin and phase margin are used in order to emphasise the differences between the two wind farms.

Costs and losses have been calculated for several different network topologies, which centralise the turbine power electronic converters, in order to improve access for maintenance. These are divided into star topologies, where each turbine is connected individually to its own converter on a platform housing many converters, and cluster topologies, where multiple turbines are connected through a single large converter. Both AC and DC topologies were considered, along with standard string topologies for comparison. Star and cluster topologies were both found to have higher costs and losses than the string topology. In the case of the star topology, this is due to the longer cable length and higher component count. In the case of the cluster topology, this is due to the reduced energy capture from controlling turbine speeds in clusters rather than individually. DC topologies were generally found to have a lower cost and loss than AC, but the fact that the converters are not commercially available makes this advantage less certain.

The work presented in this study describes a comparative study between different techniques aimed at identifying the damping values of an offshore wind turbine on a monopile foundation. It will be shown that damping ratios can directly be obtained from vibrations of the tower under ambient excitation from wave and wind loading. The results will be compared with the damping values obtained from a commonly used overspeed stop. Ambient vibration tests have the strong advantage of being more practical and less demanding for the wind turbine in comparison with the overspeed stop. Several identification algorithms, the standard exponential decay method, alternative procedures in the time domain as well as more advanced operational modal analysis techniques in the frequency domain will be applied to the experimental data. These data have been obtained during a short measurement campaign on an offshore wind turbine in the Belgian North Sea. The results of the used methods for estimating the modal damping of a wind turbine excited by ambient excitation will be discussed and compared. This study also presents some aspects related to the practical implementation of the measurements.

Applying a land-based designed pitch controller on a floating wind turbine may cause severe instability. A common strategy to overcome this problem is to reduce the closed-loop bandwidth of the pitch control system. In doing so, the generator speed variation increases possibly leading to shutdowns because of overspeed. This study uses a parallel path modification to avoid instability without increasing the generator speed variation. The results of comprehensive simulations and load calculations carried out on a benchmark wind turbine are presented. These demonstrate that by using the proposed method it is possible to apply the land-based designed pitch controller on its floater-based equivalent.