Full-scale Wind Research Articles

A full-scale wind turbine blade monitoring campaign: detection of damage initiation and progression using medium-frequency active vibrations

This work is concerned with a structural health monitoring campaign of a 52-m wind turbine blade. Multiple artificial damages are introduced in the blade sequentially, and fatigue testing is conducted with each damage in sequence. Progressive fatigue-driven damage propagation is achieved, enabling investigations concerning detection of initiation and propagation of damage in the blade. Using distributed accelerometers, operational modal analysis is performed to extract the lower-order natural vibration modes of the blade, which are shown to not be sensitive to small damages in the blade. To enable monitoring of small damages, an active vibration monitoring system is used, comprised of an electrodynamic vibration shaker and distributed accelerometers. From the accelerometer data, frequency domain methods are used to extract features. Using the extracted features, outlier detection is performed to investigate changes in the measurements resulting from the introduced damages. Capabilities of using features based on the active vibration data for detection of initiation and progression of damage in a wind turbine blade during fatigue testing are investigated, showing good correlation between the observed damage progression and the calculated changes in the damage index.

A QUASI-Z-SOURCE Converter Using a Single-Stage Switched-Inductor Boost DC-DC Converter

Abstract: This paper introduces a new topology, yet simple and efficient, for a grid-connected wind-solar cogeneration system. A permanent magnet synchronous generator-based full-scale wind turbine is interconnected to the utility grid. The dc-link capacitor has been utilized to directly interface a photovoltaic solar generator. No dc/dc conversion stages are required, and hence, the hybrid system is simple and efficient. Moreover, the proposed topology features an independent maximum power point tracking for both the wind and the solar generators to maximize the extraction of renewable energy. This paper presents a quasiZ-source inverter (qZSI) that is a new topology derived from the traditional Z-source inverter (ZSI). The qZSI inherits all the advantages of the ZSI, which can realize buck/boost, inversion and power conditioning in a single stage with improved reliability. In addition, the proposed qZSI has the unique advantages of lower component ratings and constant dc current from the source. All of the boost control methods that have been developed for the ZSI can be used by the qZSI. One of the more commonly used techniques is the incremental conductance method. In this paper, an improved particle swarm optimization- (IPSO-) based MPPT technique for photovoltaic system operating under varying environmental conditions is proposed. Basically, an Artificial Neural Network (ANN) based approach is utilized to automatically detect the global maximum power point of the PV array by using a preselected number of power measurements of the PV system. The approach of linearly decreasing scheme for weighting factors and cognitive and social parameters is modified. The proposed control scheme can overcome deficiency and accelerate convergence of the IPSO-based MPPT algorithm. Detailed small-signal models for the system components are developed to characterize the overall stability. The influence of the utility-grid faults on the performance of the proposed system is also investigated. Nonlinear time-domain simulation results under different operating conditions are presented to validate the effectiveness of the proposed topology.

Impact of DC link brake chopper design on the LVRT behavior of full-scale converter wind turbines

Wind turbines (WTs) are loaded by the wind and by the electrical grid. A low voltage ride through (LVRT) is a special load case for WTs, which can be critical for their drivetrain. They are usually less severe for WTs with a full-scale converter (FSC); however, in some cases there are still loads observable in the mechanical drivetrains of WTs with FSCs. Different aspects like the brake chopper design, the control and specifications of the grid side converter (GSC) as well as the grid and fault parameters affect these loads. In this paper, three different LVRT behaviors of a WT with a squirrel-cage induction generator (SCIG) and a FSC were investigated via simulations. In the worst case, a sharp peak in the electromagnetic torque of the generator after fault clearance is possible. The cause lies in the GSC control, especially the phase-locked loop (PLL), and can also be influenced by adjusting the control parameters, brake chopper and filter design. The resulting mechanical drivetrain torque, on the other hand, is increased by only 2.9% for a very short time. Thus, the loading of the mechanical drivetrain components is only slightly increased. Therefore, the risk of damage to the mechanical components of the FSC WT due to grid faults is relatively low.

Experimental and Numerical Investigation of the Aerodynamic Ventilation Drag of Heavy-Duty Vehicle Wheels

Due to current EU regulations, constant-speed testing on test tracks is used for aerodynamic certification of heavy-duty vehicles (HDV). However, the aerodynamic development of HDVs is performed using wind tunnels and computational fluid dynamics (CFD). Both techniques commonly neglect the rotational aerodynamic losses of the wheels—the so-called ventilation drag—that are present when driving on the road. This is due to the fact that there is no full-scale wind tunnel for this type of vehicle with a suitable belt system for the simulation of the wheel rotation. Furthermore, the ventilation drag of HDV wheels has been neglected in CFD due to their almost completely closed rim design. These constraints lead to an underprediction of the aerodynamic forces in comparison to the results under on-road conditions when performing constant-speed tests. In order to investigate the ventilation drag of HDV wheels, measurements were carried out on a 1:4.5 scale generic tractor-trailer model in the Model Scale Wind Tunnel of the University of Stuttgart. The measured aerodynamic forces as well as the measured flow field data provide the basis for the definition and validation of a procedure for analyzing the ventilation drag in CFD. Accordingly, the ventilation drag of a full scale HDV was investigated in CFD. The results show that the tire treading and rim geometry have a significant influence on ventilation drag that contributes to the total aerodynamic drag of the HDV. The present work shows that the ventilation drag has a relevant impact on the total aerodynamic drag of HDVs and should therefore not be neglected. The presented CFD approach thus allows to assess the aerodynamic drag under real on-road conditions in an early stage of the vehicle development.

Extended scaling approach for droplet flow and glaze ice accretion on a rotating wind turbine blade

This paper presents and analyzes a non-dimensional model of the flow field and droplet trajectories with glaze ice accretion on a rotating wind turbine blade. The formulation leads to similitude relationships for glaze ice accretion to evaluate new scaling parameters corresponding to the rotation of the blade. New scaling parameters are developed to determine ice scaling conditions based on the geometry scale factor of a wind turbine blade. The scaling methodology can be used to determine alternative test conditions and predict glaze icing conditions on a full-scale wind turbine blade. Numerical CFD icing simulations are performed using ANSYS FENSAP ICE software. A wind blade model is developed using blade element momentum theory (BEM). Each blade size is tested at specific flow conditions using the scaling equations. Scaled conditions for velocity (streamwise and rotational), droplet size, and icing time are tested. CFD solutions for the flow field are obtained for comparisons of the velocity, droplet trajectories, pressure coefficient distributions, ice thickness, and ice shapes. Recommended parameters to be used for glaze ice scaling on a rotating blade are presented and numerical results are reported to support these recommendations. The paper provides new insights into modeling of glaze ice accretion on large wind turbine blades based on smaller scaled blade sections tested in a laboratory setting.

Experimental and two-scale numerical studies on the behavior of prestressed concrete-steel hybrid wind turbine tower models

Prestressed concrete-steel hybrid (PCSH) wind turbine tower has been recognized as an efficient alternative to traditional steel tube supporting towers for wind farms in harsh sites such as mountain regions. To investigate the global and local mechanical behavior including failure patterns of a full scale PCSH wind turbine towers, an experimental study on a substructure of a scaled PCSH tower is carried out firstly. Then, a material subroutine for three-dimensional (3D) fiber beam elements suitable for implicit analysis on the mechanical behavior of the substructure of the scaled PCSH tower is developed. A two-scale numerical model of the substructure of the scaled PCSH wind turbine tower employing the developed 3D fiber beam elements and solid elements is developed. The accuracy and efficiency of the developed two-scale numerical model for the behavior simulation of the substructure of the scaled PCSH wind turbine tower is illustrated by comparing the numerical results with the experimental measurements. Additional comparison of the simulation results using the developed two-scale numerical model is made with them of a model using sole 3D fiber beam elements and a model using sole solid elements, respectively. Results show that the force–displacement curve determined with the developed two-scale model is close to the test result and coincident well with that of the two models using 3D fiber beam elements and the solid elements. The developed two-scale model can describe the global behavior, the local failure patterns and the stress distribution of the tested specimen accurately with improved simulation efficiency. Finally, the validated two-scale modelling approach is employed to analyze the static behavior, modal parameter, and fatigue of the optimized full-scale PCSH wind turbine tower previously developed by the authors. The simulation results show that the two-scale model can reflect the stress of the PCSH wind turbine tower at the critical position as well as the global mechanical behavior with higher computational efficiency. The fiber beam model can be used for predicting only global, but not local, behavior.

On the generation of full-scale urban pedestrian level wind in the wind tunnel using passive and active turbulence generators

The utilization of outdoor spaces is affected by their thermal environment. Wind, which determines the convective and evaporative heat loss from the human body, is one of the most influencing factors of thermal comfort. In the prevailing outdoor thermal comfort models, the convective heat transfer coefficients were pre-measured on a thermal manikin in a wind tunnel, where the wind speed was mostly stable. However, field measurements suggested that a typical outdoor wind process over complex terrains, like urban landscapes, are generally non-stationary. There is no experimental data on how the misrepresentation of wind may affect the convective heat transfer prediction for outdoor thermal comfort prediction. This study simulates urban pedestrian-level wind patterns in a wind tunnel by employing active and passive turbulence generators. Results show the turbulence scale through an active gust generator is approximately one to two orders of magnitude larger than the turbulence scale through a passive grid system. The eddy length scale can be adjusted through the rotation speed of the shutters. This opens the possibility of performing studies on measuring convective heat transfer coefficient under unsteady wind conditions, similar to the real outdoor environments.

Comparative study between field measurements of wind pressures on a 600-m-high skyscraper during Super Typhoon Mangkhut and wind tunnel test

Wind pressures on cladding of a 600-m-high skyscraper during Super Typhoon Mangkhut were monitored by a synchronous monitoring system including 48 pressure transducers. Then, simultaneous wind pressure measurements on a 1:500 scale model of the skyscraper were conducted in wind tunnel test. The field measurements of wind pressures are further analyzed to investigate the wind pressure characteristics on the skyscraper under extreme windstorm condition and utilized to validate the wind tunnel test results. The full-scale and model-scale wind pressure coefficients (mean, root mean square, maximum and minimum) are compared with each other and analyzed in detail. Moreover, to explore the deviations between the full-scale and model-scale results, the Reynolds number effect (5.5 × 104 to 1.8 × 108) on the wind pressure coefficients, probability distributions, power spectral densities, spatial correlations and coherences of wind pressures are investigated. The comparative study shows that the wind tunnel test can reproduce the wind pressures on the windward face. While for the other faces, the results from the wind tunnel test generally deviate from those from the field measurements. The reasons that caused the discrepancies, especially the Reynolds number effect, are discussed. The objectives of this paper are to examine the accuracy of wind tunnel testing for predicting wind pressures on high-rise buildings under severe wind condition and enhance the understanding of wind pressures on high-rise buildings during strong windstorms, and therefore provide useful information for the wind-resistant design of skyscrapers.

Wind Tunnel Tests of Wake Characteristics for a Scaled Wind Turbine Model Based on Dynamic Similarity

This wind tunnel study was conducted to investigate the similarity laws involved in the reasonable simulation of the wake characteristics of a full-scale wind turbine. A 5 MW scaled wind turbine model was designed using an optimization method based on the blade element momentum (BEM) theory. Subsequently, wind tunnel tests were carried out on the geometrically similar model and the thrust-optimized model, with different yaw angles and under various upstream flow conditions. The results indicated that the wake development of the wind turbine model was closely related to the thrust forces of the wind turbine, and both kinematic and dynamic similarity laws should be observed to achieve wake characteristics that are reasonably similar to those of a full-scale wind turbine. This study investigated the aerodynamic similarity principles of small-scale wind turbine models to develop a more effective method for simulating full-scale turbine wake characteristics in wind tunnel tests. The outcomes of this study revealed the limitations of the anomalously low thrust coefficients in geometrically similar wind turbine models and present reasonable model design methodologies for small-scale wind turbine models in wind tunnel tests.

Improved resonance method for fatigue test of full-scale wind turbine blades

To further reduce the power of exciter in the common fatigue testing methods and increase the testing frequency to decrease the fatigue testing time, this paper proposes an improved fatigue testing method—Tug Fatigue Testing system (TFTs). The advantages of this new fatigue testing method are low power and high testing frequency of its exciter due to no exciter with moving masses attached to the blade. In TFTs, exciters mounted on the ground or fixed bracket can be used to excite the blade in the uniaxial or biaxial fatigue test. In this paper, the mechanical model of TFTs is established to compare the motor power required by exciters in TFTs and the inertial exciters and the shear load on the blades in both ways. Furthermore, a test of a 56.5 m blade will be performed to verify the feasibility of the new method. In addition, the bending moment distribution of an 80 m blade excited by TFTs was measured and compared with the bending moment distribution of the same blade excited by inertial resonance excitation to evaluate its excitation effect. The test results prove that this improved method needs lower power of exciter, produces smaller shear loads, and provides a higher test frequency in the flap-wise test direction than common inertial resonance excitation. Biaxial fatigue tests can also be conducted by this new method.

Investigation of Tyre Pattern Effect on the Aerodynamics of a Passenger Vehicle

Abstract The wheels of a passenger vehicle are one of the major contributors to the total aerodynamic drag, making their aerodynamic performance a considerable factor for the overall energy efficiency of the vehicle. Previous studies have shown that the complex flow field created by the wheels is sensitive to small geometrical variations of the tyre and that features such as shoulder profile and tread pattern can have a significant impact on drag and lift. In this study, the DrivAer model is used to evaluate the flow fields and aerodynamics of four tyre tread patterns with two rim designs. Full-scale wind tunnel tests were conducted where forces, surface pressures and flow fields were measured. Numerical simulations were also performed to aid the analysis. Using a slick tyre as the reference, it was found that rain grooves typically reduced the drag, whereas the effect of lateral grooves was dependent on the rim configuration. For the lift forces, the largest lift variations were obtained for the front lift which, in general, was reduced by rain grooves and increased by lateral grooves, most notably for the closed rim. The importance of considering the parasitic lift force acting on the wheel drive units when comparing experiments and simulations was demonstrated.

On the Investigation of the Effect of Tower and Hub Exclusion on the Numerical Results of a Horizontal Axis Wind Turbine

In order to construct a 3D rotor model, periodicity and Multiple Reference Frame (MRF) strategies were applied to address full Computational Fluid Dynamics (CFD) that incorporate the Reynolds-averaged Navier–Stokes (RANS) equations. A simulation model was run to examine the power of the wind turbines, both including and excluding the hub and tower. The purpose of this was to identify an approach that would decrease the computational time cost of wind turbine simulation. Firstly, a full-scale horizontal axis wind turbine simulation was carried out, and subsequently, comparisons were made with the results of the wind tunnel experiment utilizing the K-omega SST and Sparlat Allmaras viscosity models. The results were compared to determine the optimum model with the lowest simulation time and highest accuracy to the experimental results. Following this, the complete models’ simulation was compared to the model excluding the hub and tower in order to check the disparity in the results of the two. Additionally, to ascertain the computational cost saving, the ratio of the two simulation times was established. The findings show that the two models produce results that correspond well to the experimental results, and that the power coefficient had a greater value for the complete model than the simple model. Furthermore, it was found that the power coefficient values of the full model have significantly higher levels of similarity to the experimental values. The Sparlat Allmaras and K-omega SST viscosity models returned error percentages of 0.52% and 12.6% respectively. For the partial model utilizing the same computer device, a 5.7% error rate was recorded, with a computational time saving of approximately 34%.

Life cycle assessment of a wind farm in Turkey.

The aim of this study is to investigate the environmental impacts of a full-scale wind farm using life cycle assessment methodology. The facility in question is an onshore wind farm located in Turkey with a total installed capacity of 47.5 MW consisting of 2.5 MW Nordex wind turbines. Hub height and rotor diameter of the wind turbines are 100 m. The system boundary is defined as material extraction, part production, construction, operation and maintenance and decommissioning phases of the wind farm. The functional unit is 1-kWh electricity produced. Environmental impacts are mainly generated by manufacturing and installation operations. Steel sheet usage in tower manufacturing is the main contributor to abiotic depletion of fossil resources, acidification, eutrophication, global warming and marine aquatic ecotoxicity potentials. Apart from ozone layer depletion, end-of-life phase decreases the environmental impacts due to metal recycling. Metal recycling ratio scenario results show that when the recycling ratio decreases from 90 to 20%; increases of 110%, 102%, 92% and 87% are observed in acidification, terrestrial ecotoxicity, marine aquatic ecotoxicity and global warming potentials, respectively. In the baseline, the main parts which are manufactured in Germany are transported by sea to Turkey. Transportation scenario involves shifting the manufacturing of main parts to Turkey then transporting these parts by trucks to the farm. This conversion causes increases of 31%, 35% and 27% in abiotic depletion of fossil resources, freshwater aquatic ecotoxicity and global warming potentials, respectively, while causing decreases of 11% and 4% in acidification and eutrophication potentials generated by transportation activities, respectively.

System identification of cropped delta UAVs from flight test methods using particle Swarm-Optimisation-based estimation

AbstractIn the era of Unmanned Aerial Systems (UAS), an onboard autopilot occupies a prominent place and is inevitable for many of their modern applications. The efficacy of autopilot heavily relies upon the accuracy of the sensors employed and the capability of the onboard flight controller. In general, aerodynamic behaviour and flight dynamic capabilities of Unmanned Aerial Vehicles (UAVs) govern the selection and the design of flight controllers. Precise modeling of linear aerodynamic characteristics from flight data can be achieved using many of the existing classical parameter estimation techniques such as Output Error Method (OEM), Equation Error Method (EEM), and Filter Error Method (FEM). However, all the classical methods may not be readily applicable for aerodynamic modeling in nonlinear flight envelopes. The current manuscript is an attempt to exploit the capabilities of the Artificial Intelligence (AI) technique, named Particle Swarm Optimisation (PSO), in combination with Least Squares (LS) cost function to perform linear as well as nonlinear aerodynamic parameter estimation. The aforementioned task is accomplished by considering flight data from manoeuvers pertaining to linear angles of attack, moderate and near stall flight envelopes of two different UAVs with cropped delta planform geometry. Parameters estimated using the proposed LS-PSO method are consistent with minimum standard deviation and are on a par with OEM estimates. The proposed LS-PSO method enhances the capabilities of LS-based EEM while estimating stall characteristic parameters, which was not possible with LS alone. The longitudinal and lateral-directional static parameters estimated from the full-scale wind tunnel testing of the two UAVs were also used to corroborate the results obtained from the flight data using the LS-PSO method.

Evaluation of dynamic testing of full-scale wind turbine drivetrains with hardware-in-the-loop

Dynamometer testing of full-scale wind turbine drivetrains involves subjecting integrated components to extreme static loads, fatigue loads, or dynamic loads to replicate field conditions. This paper provides a detailed evaluation of the uncertainty and errors in applied loads and measured responses, and their magnitude relative to established repeatability bounds for a drivetrain subjected to static and dynamic loads on the 7.5 MW test bench at the Clemson University Wind Turbine Drivetrain Testing Facility. An ideal drivetrain simulation model is utilized to isolate the influence of load tracking errors on test article responses. It is shown that the test article response variations are mainly driven by run out and clearances inherent to the drivetrain. For the dynamic profiles, the load tracking errors are within acceptable limits. A frequency analysis is used to show that the test bench controller tracking performance is acceptable for profiles with frequency content up to 1.5 Hz.

Thermographic data analytics-based damage characterization in a large-scale composite structure under cyclic loading

Large-scale composite structures such as aircraft wings and wind turbine blades undergo cyclic loading in operation. In-service damage often generates excessive heat due to material frictions and it could be detected by thermography. This study develops a methodology to quantitatively analyze such structural damage based on thermographic data analytics. A full-scale composite wind turbine blade is inspected using passive thermography when it is subject to cyclic loads in laboratory. The damage region is identified from thermographic images and it is tracked automatically using image processing. The damage region is subsequently characterized on both overall and detailed levels. The change of the damage status versus fatigue cycle number is analyzed and the information regarding the growth of the damage area and the damage severity is provided. The initiation and the progress of damage are investigated based on the temperature and the enthalpy change in the damage region. This study provides a viable solution for efficient structural health monitoring and damage prognosis of large-scale composite structures under cyclic loading.

Development of scheme to evaluate the performance of parachute canopy fabrics under tensile impact

The present research is an extension of the earlier work wherein a wider scheme of small-scale tensile impact testing is proposed, to characterise the whole parachute canopy instead of full-scale wind tunnel testing. The drop height for equivalent tensile impact on the rectangular stitched specimens is chosen as 300 mm, and accordingly, the dimension (Length × Width) has been decided (300 mm × 100 mm). To evaluate the performance of stitched specimen (at 45° bias angle), ungripped width of 20 mm on both sides of the specimen has been worked out. In the work, the actual payload responsible for parachute inflation has been addressed for the calculation of the exact opening shock force. Subsequently, to apply impact load on the specimen, dead weights are predicted based on the different levels of opening shock forces of the C-9 parachute canopy under different aerodynamic conditions. Considering a specific case, the behaviour of the ripstop and plain-woven fabric sample in terms of load versus extension curve and work done at the time of impact have been analysed under different loads using a high-speed digital camera. This study revealed that the plain-woven fabric is to be preferred for human dropping because it has a higher extension and higher impact duration than ripstop fabric. As a result, the plain-woven fabric will produce a lower impact or shock force in the parachute canopy which is to be transferred to the skydiver. On the other hand, ripstop fabric possesses higher strength, therefore can be preferred for materials dropping.

Modal analysis of full-scale wind turbine blade structural fatigue tests based on particle swarm optimization

Abstract Modal analysis is the vital prerequisite of the full-scale fatigue test of the wind turbine blade. In order to resolve the difficulty in solving the characteristic equation of the transfer matrix method in modal analysis, a method of solving the characteristic equation based on particle swarm optimization is proposed. Firstly, the beam model is used to simplify and discrete the blade along the spanwise of the blade. A concept of average bending stiffness is used for the elasticity assignment with beam segment, and the blade kinematics model is established. Subsequently, a designed particle swarm optimization is used to solve the characteristic equation. Finally, the proposed method is verified by modal analysis of an MW-scale blade. On this basis, the effect of the mass and position of the counterweight on the mode shape is investigated. The results indicate that the proposed method can provide a theoretical reference for the fatigue test technology of wind turbine blades.

Research on negative sequence current coordinated control strategy for severe asymmetric offshore ac faults of wind power transmission system based on mmchvdc

When there is a severe asymmetric offshore ac fault in the grid-connected system for full-scale type-4 wind turbines via the high voltage direct current (HVDC) transmission system based on modular multilevel converters (MMC), there will be transient overcurrent in the MMC and the grid side converters (GSC) of wind turbines. In order to prevent transient overcurrent, a negative sequence coordinated current control strategy is proposed in this paper. The method of decoupling circuit characteristics and control characteristics is adopted to study the asymmetric fault characteristics of this system. The levels of the negative sequence current during severe asymmetric ac faults are analysed. The key influencing factor of the negative sequence currents is extracted. And the feature that the negative sequence currents of the MMC and the GSC cannot be suppressed to 0 simultaneously is revealed. Based on the above reasons, the control strategy that the negative sequence current of the MMC is suppressed to zero and the GSC cooperate to reduce the positive sequence current thus reducing the current stress of the bridge arm is proposed. Finally, an electromagnetic transient model was built on PSCAD to verify the effectiveness of the control strategy.

Numerical Simulation of Low Frequency Pulsations in the Mixing Layer of a Jet of a Full-Scale Wind Tunnel and Experiment of Modeling a Jet Actuator for Their Suppression

Numerical Simulation of Low Frequency Pulsations in the Mixing Layer of a Jet of a Full-Scale Wind Tunnel and Experiment of Modeling a Jet Actuator for Their Suppression