Wind Farm Applications Research Articles

Modeling of the atmospheric boundary layer under stability stratification for wind turbine wake production

The assessment of climatological site conditions, airflow characteristics, and the turbulence affecting wind turbines is an important phase in developing wake engineering models. A method of modeling atmospheric boundary layer structure under atmospheric stability effects is crucial for accurate evaluation of the spatial scale of modern wind turbines, but by themselves, they are incapable to account for the varying large-scale weather conditions. As a result, combining lower atmospheric models with mesoscale models is required. In order to realize a reasonable approximation of initial atmospheric inflow condition used for wake identification behind an NREL 5-MW wind turbine, different vertical wind profile models on equilibrium conditions are tested and evaluated in this article. Wind farm simulator solvers require massive computing resources and forcing mechanisms tendencies inputs from weather forecast models. A three-dimensional Flow Redirection and Induction in Steady-state engineering model was developed for simulating and optimizing the wake losses of different rows of wind turbines under different stability stratifications. The obtained results were compared to high-fidelity simulation data generated by the famous Simulator for Wind Farm Applications. This work showed that a significant improvement related to atmospheric boundary layer structure can be made to develop accurate engineering wake models in order to reduce wake losses.

A Two-Stage Fault Detection and Classification Scheme for Electrical Pitch Drives in Offshore Wind Farms Using Support Vector Machine

Pitch systems are one of the components with the most frequent failure in wind turbines. This paper presents a two-stage fault detection and classification scheme for electric motor drives in wind turbine pitch systems. The presented approach is suitable for application in offshore wind farms with electric pitch systems driven by induction motors as well as permanent magnet synchronous motors. The adopted strategy utilizes three-phase motor current sensing at the pitch drives for fault detection and only when a fault condition is detected at this stage, features extracted from the current signals are transmitted to a support vector machine classifier located centrally to the wind farm. The proposed method is validated in an in-house setup of the pitch drive.

Large eddy simulations of floating offshore wind turbine wakes with coupled platform motion

The growing prospect for large farms of floating offshore wind turbines requires a better understanding of wake effects for floating turbines, particularly the differences when compared to fixed-bottom turbine wakes. The increased range of motion of floating platforms can influence wake characteristics, affecting downstream turbines. In this work, large eddy simulations with an actuator line model are used to study downstream wake characteristics of the NREL 5 MW reference turbine mounted on the OC3-UMaine spar platform for several different metocean conditions. The simulations are carried out in the Simulator fOr Wind Farm Applications (SOWFA) coupled with OpenFAST for the platform and turbine motion. The downstream wake characteristics of the floating platform are compared to equivalent fixed-bottom cases for different wind speeds, wave heights, wind-wave alignments, and turbine yaw angles. Overall, the differences in wake shape between floating and fixed platforms are associated with mean platform displacements, while differences in turbulence are associated with time-varying platform motion. However, these observed wake differences between fixed and floating platforms are small, especially for higher wind speeds and lower wave heights.

Multi-Criteria Decision Making Approach for Hybrid Operation of Wind Farms

Hybrid operation of wind farms has been in the limelight in recent years wherein the stochastic nature of wind causes market operators to choose an optimal strategy to maximize profit. The current work deals with a multi-criteria decision making approach to choose the best possible alternatives for a hybrid wind farm operation. A set of three, non-beneficial criteria, namely wind wakes, wind curtailment, and forced outages, were chosen to evaluate the best alternative. Three methods, (i) Simple Additive Weighting (SAW), (ii) the Technique for Order or Preference by Similarity to Ideal Solution (TOPSIS) and (iii) Complex Proportional Assessment (COPRAS), were applied to identify the best alternative, and the results revealed that for all three methods, borrowing deficit power from a neighboring wind farm is the best alternative. Comparative analyses in terms of the data requirement, the effect of dynamic decision matrices, and rank reversal in wind farm application have also been pioneered.

Bidirectional marx DC–DC converter for offshore wind farm application

The bidirectional DC-DC converter explained in this paper is based on the Marx principle, and is capable of achieving step-up and step-down voltage transformations at kV level and is able to handle MW level power transfers in both directions. The main features of this topology is the absence of a high frequency transformer, reduced weight, volume, and soft switching to reduce the switching losses. In the boost mode, five capacitors are charged in parallel and discharged in series to achieve the step-up action, and in the buck mode the converse action takes place. The operating principle is explained, and the steady-state analysis of the converter is given. Matlab/Simulink simulation of a 50MW converter, interfacing 6kV, and 30kV systems supports and validates the theoretical analysis, and enables positive supporting conclusions to be made.

Wind farm simulations using an overset hp-adaptive approach with blade-resolved turbine models

Blade-resolved numerical simulations of wind energy applications using full blade and tower models are presented. The computational methodology combines solution technologies in a multi-mesh, multi-solver paradigm through a dynamic overset framework. The coupling of a finite volume solver and a high-order, hp-adaptive finite element solver is utilized. Additional technologies including in-situ visualization and atmospheric microscale modeling are incorporated into the analysis environment. Validation of the computational framework is performed on the National Renewable Energy Laboratory (NREL) 5MW baseline wind turbine, the unsteady aerodynamics experimental NREL Phase VI turbine, and the Siemens SWT-2.3-93 wind turbine. The power and thrust results of all single turbine simulations agree well with low-fidelity model simulation results and field experiments when available. Scalability of the computational framework is demonstrated using 6, 12, 24, 48, and 96 wind turbine setups including the 48 turbine wind plant known as Lillgrund. The largest case consisting of 96 wind turbines and a total of 385 overset grids are run on 44,928 cores at a weak scaling efficiency of 86%. Demonstration of the coupling of atmospheric microscale and Computational Fluid Dynamics (CFD) solvers is presented using the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting Model (WRF) solver and the NREL Simulator fOr Wind Farm Applications (SOWFA) solver.

Model Predictive Control of a Multilevel CHB STATCOM in Wind Farm Application Using Diophantine Equations

The finite control set model predictive control (FCS-MPC) for power electronic converters provides high dynamic performance, based on the limited number of inputs and accurate model of the converter. By applying this algorithm to multilevel converters such as a cascaded-H-bridge-based static var compensator (CHB STATCOM), the dynamic performance is degraded, because the optimized input is achieved by searching among a large set of switching combinations and redundancies. This paper proposes an FCS-MPC algorithm, which benefits high dynamic performance for the CHB STATCOM, despite the large set of inputs. The proposed FCS-MPC replaces the time-consuming optimization algorithm by solving Diophantine equations over the large set of switching combinations. The desired switching combination and all its redundancies are determined in a minimum execution time. The proposed FCS-MPC performance is validated by applying to two configurations: 1) a 15-level CHB STATCOM with energy storage capability for a short-term active power smoothing and reactive power compensation of a 10 MW fixed speed wind farm at medium voltage; and 2) an experimental seven-level CHB STATCOM at low voltage.

Near-wake analysis of actuator line method immersed in turbulent flow using large-eddy simulations

Abstract. The interaction between wind turbines through their wakes is an important aspect of the conception and operation of a wind farm. Wakes are characterized by an elevated turbulence level and a noticeable velocity deficit, which causes a decrease in energy output and fatigue on downstream turbines. In order to gain a better understanding of this phenomenon this work uses large-eddy simulations together with an actuator line model and different ambient turbulence imposed as boundary conditions. This is achieved by using the Simulator fOr Wind Farm Applications (SOWFA) framework from the National Renewable Energy Laboratory (NREL) (USA), which is first validated against another popular Computational Fluid Dynamics (CFD) framework for wind energy, EllipSys3D, and then verified against the experimental results from the Model Experiment in Controlled Conditions (MEXICO) and New Model Experiment in Controlled Conditions (NEW MEXICO) wind tunnel experiments. By using the predicted torque as a global indicator, the optimal width of the distribution kernel for the actuator line is determined for different grid resolutions. Then, the rotor is immersed in homogeneous isotropic turbulence and a shear layer turbulence with different turbulence intensities, allowing us to determine how far downstream the effect of the distinct blades is discernible. This can be used as an indicator of the extents of the near wake for different flow conditions.

A Passivity-Based Stability Analysis of the Active Damping Technique in the Offshore Wind Farm Applications

The LCL -based filter has been widely applied to mitigate the size of the inductor in the high-power converter, but it usually leads to resonance in the system. Therefore, an active damping technique based on the virtual resistor is provided in this paper for the LCL -filter system. The literature papers only suppressed the resonance of the LCL -filter and focused on the stability of the internal current control loop with an inductive grid impedance. Therefore, these previous stability analysis methods cannot be suitable for offshore wind farm applications due to the multiple resonance frequency characteristic of the transmission cable and multiparalleled converters. Therefore, this paper analyzes the control stability with both of the internal resonance ( LCL -filter) and external resonance (between the grid impedance and current controller). Besides, the behavior of multiparalleled converter and the multiple resonance frequency of long transmission cable are discussed and analyzed for the offshore wind farm applications. Finally, the laboratory and simulation results are for the proposed method verification.

Current Error Based Compensations for VSC Current Control in Weak Grids for Wind Farm Applications

A novel current control strategy is proposed for voltage source converters connected to weak grids using conventional current vector control with additional current error based voltage angle and magnitude compensations. For connecting to very weak ac network, the combination of vector control and grid synchronization with a conventional phase-locked loop is proved to be unstable, whereas the proposed current error based compensations can significantly improve system stability. In this way, the proposed control can still benefit from the presence of closed-loop current control without the need for control switching during large ac voltage variations. A comprehensive frequency domain model is established to analyze stability performance. Comprehensive time domain simulations are further carried out to validate its effectiveness and robustness by demonstrating its current control performance during a three-phase fault, multiple-converter situation, and various grid strength conditions.

Limitations of HVAC Offshore Cables in Large Scale Offshore Wind Farm Applications

Limitations of HVAC Offshore Cables in Large Scale Offshore Wind Farm Applications

A review of design consideration for Doubly Fed Induction Generator based wind energy system

In the design of a Doubly Fed Induction Generator (DFIG), the electrical, dielectric, magnetic, thermal, and mechanical considerations are essential in the design. The generator speed, rotor and stator windings, core, rotor slots, and the insulation system must also be considered for practical designs as wind turbines in the range of 2MW. In this paper, the conflicting design requirements reconciled for the construction DFIGs with high reliability and efficiency in the wind farm applications.

Operation and control design of an input‐series–input‐parallel–output‐series conversion scheme for offshore DC wind systems

High-power converters for high-voltage direct current transmission systems and collecting networks are attracting increasing interest for application in large offshore wind farms. Offshore wind farms are capable of generating more electric energy at lower cost when compared with onshore wind systems. In this study, DC/DC voltage conversion should be achieved with a power converter that uses readily available semiconductor devices. A modular DC/DC converter can achieve the required system currents and voltages without exceeding semiconductor ratings. In this study, the operation and control strategy for an input-series–input-parallel–output-series (ISIPOS) energy conversion system for wind systems are presented. The ISIPOS system allows the direct connection of wind turbines to the DC grid. In this research, the design process to control the input and output currents and voltages is explained. In addition, a new method to ensure voltage and current sharing between the different modules is presented and explained. The basic structure, control design, and system performance are tested using MATLAB/SIMULINK. Practical results validate the control design flexibility of the ISIPOS topology when controlled by a TMSF280335 DSP.

Wind Tunnel Wake Measurements of Floating Offshore Wind Turbines

This paper reports the results of a wake measurement campaign carried out at Politecnico di Milano wind tunnel to support current studies of the authors about unsteady aerodynamics of floating offshore wind turbines (FOWT) as well as evaluating experimental evidence for wind farm applications of FOWT. The wind turbine scale model adopted for these tests is the 1/75 DTU 10 MW wind turbine developed for the LIFES50+ project. In this study, imposed surge motions, at different frequencies and amplitudes, were provided at the base of the turbine’s tower, to investigate the influence of motion on the wake. Results reported show good agreement with the state-of-the-art of wind turbines wake aerodynamics in steady conditions; concerning the unsteady conditions (imposed motion) interesting data are reported and collected with respect to a ”wake reduced frequency” parameter which is, according to the authors, a straightforward value to estimate the ”level” of unsteadiness connected to a specific dynamic condition due to floating motion. Interesting and consistent phenomena are observed and commented, although opening to the need of further experimental and numerical investigations.

Control of HVDC systems based on diode rectifier for offshore wind farm applications

The transfer of energy from offshore wind farms using HVDC technology is highly dependant on the converter technology used. The conversion through HVDC has conventionally been done by Line Commutated Converters (LCC) or Voltage Source Converters (VSC), by using thyristor rectifiers or IGBTs, respectively. However, 12-pulse diode rectifier at the offshore converter station can be a promising option due to lower conduction losses, reduced installation costs and converter size, and higher reliability. The main drawback is that the overall control strategy needs to adapt to the uncontrollable diode rectifier. This paper describes the system topology and control solutions proposed with the use of diode rectifier. In addition the paper proposes a shifted voltage and frequency droop control strategy related to the Phase Locked Loop (PLL) to improve the control system as an original contribution. The control strategies are validated in Matlab/Simulink.

Techno-economic analysis of energy storage systems for application in wind farms

The objective of this paper is to analyse reduction in wind power variability through aggregation and use of energy storage systems. A key focus is to evaluate the impact of regulatory framework in addition to the capital expenditure to ascertain techno-economic feasibility of energy storage systems in wind farm applications. A generic techno-economic is developed which takes into account the effects of regulatory framework in addition to the technical and economic features of storage options. Existing wind farms from South Australia are used as test cases. First, a detailed quantitative analysis is performed to establish the variability associated with individual wind farms and the aggregations of their power outputs. Then, the appropriateness of a number of existing energy storage types are evaluated using the developed techno-economic model. Relationships between wind farm sizes, wind farm variability levels, storage capacity requirements, storage costs and storage payback times are determined and discussed for both current and potential future economic and regulatory scenarios. It is found that regulatory framework can be of paramount importance in ascertaining the economic feasibility of energy storage. For example, if the ramp-rate violation penalty (determined to be $8.89/MW/min) is doubled, then the payback time of energy storage capital investment is found to reduce from 5.32 years to 2.52 years. It is also found that larger wind farms require smaller energy storage capacity and smaller wind farms generally results in a shorter energy storage system payback times.

Lidar-based wake tracking for closed-loop wind farm control

Abstract. This work presents two advancements towards closed-loop wake redirection of a wind turbine. First, a model-based wake-tracking approach is presented, which uses a nacelle-based lidar system facing downwind to obtain information about the wake. The method uses a reduced-order wake model to track the wake. The wake tracking is demonstrated with lidar measurement data from an offshore campaign and with simulated lidar data from a simulation with the Simulator fOr Wind Farm Applications (SOWFA). Second, a controller for closed-loop wake steering is presented. It uses the wake-tracking information to set the yaw actuator of the wind turbine to redirect the wake to a desired position. Altogether, the two approaches enable a closed-loop wake redirection.

Field test of wake steering at an offshore wind farm

Abstract. In this paper, a field test of wake-steering control is presented. The field test is the result of a collaboration between the National Renewable Energy Laboratory (NREL) and Envision Energy, a smart energy management company and turbine manufacturer. In the campaign, an array of turbines within an operating commercial offshore wind farm in China have the normal yaw controller modified to implement wake steering according to a yaw control strategy. The strategy was designed using NREL wind farm models, including a computational fluid dynamics model, Simulator fOr Wind Farm Applications (SOWFA), for understanding wake dynamics and an engineering model, FLOw Redirection and Induction in Steady State (FLORIS), for yaw control optimization. Results indicate that, within the certainty afforded by the data, the wake-steering controller was successful in increasing power capture, by amounts similar to those predicted from the models.

A control strategy for a multi-terminal HVDC network integrating wind farms to the AC grid

This paper describes a novel control strategy of voltage source converters (VSC) in large-scale wind farm applications, in order to achieve a constant DC voltage for connecting to the long transmission system based on multi-terminal high voltage direct current (HVDC) technology. The control strategy is based on a multi-loop current and voltage control for tracking the predetermined value of the DC link voltage in the rectifier side station to achieve proper performance in the irregular circumstances of wind farm operation. In addition, the control strategy is able to transmit the maximum input power to the consumption side in different situations in grid side converter. Grid connection of the HVDC system is analysed under two conditions: balanced AC grid, and connection of unbalanced nonlinear load into the AC grid. The control strategy guarantees injection of minimum harmonic current components from the grid to the loads. MATLAB simulation results are presented to demonstrate the effectiveness of the proposed strategy for different types of variations in AC voltage amplitude and frequency of wind turbines output voltage.

Wind farm wake modeling using NWTC design codes

Wind turbines located in a wind farm are subject to a wind field that is substantially modified when compared to the ambient wind field due to wake effects. It is clear that in addition to the power, a tool that could model the loads of the waked turbine is needed. In this work, a wind farm wake model is created using a single wake model based on the dynamic wake meandering model to systematically model the wake effects and thus the power and loads of an entire wind farm with an arbitrary wind turbine layout and wind condition. This wind farm wake model is incorporated into the National Wind Technology Center design codes. Preliminary results demonstrate that this wind farm tool yields satisfactory results when compared with Simulator for Wind Farm Applications, a high fidelity computational fluid dynamics Large eddy simulation model, and with experimentally-obtained field data. The integration of the wind farm model with the National Wind Technology Center design codes is described in detail and validation results of the tool are provided.