MW Machine Research Articles

Field validation of a yaw misalignment observer for wind farm control

In this work, a load-based yaw misalignment observer was tested and validated with turbine and mast data collected during a wake steering and characterization campaign. A shallow feed-forward neural network was used to map the relation between the yaw misalignment and the in-and out-of-plane blade load harmonics for a 3.5 MW machine, and its performance was analysed over about 108 full days of useful data. Confirming previous findings, this simple neural network was able to accurately estimate the yaw misalignment, with an average 10-minute absolute error of at most 4°.The performance of the yaw misalignment observer was compared to the one of the standard onboard wind vane during the wake steering campaign. Results indicate that the wind vane may significantly overestimate the misalignment for large angles, possibly on account of the wake rotation and flow distortion effects caused by the nacelle. The observer on the other hand, sampling the flow at the rotor disk and not behind it, is not affected by such phenomena and could therefore provide a more accurate measurement of the misalignment angle, possibly improving the performance of wake steering. When the turbine is already equipped with load sensors, this is obtained without the need to install and maintain extra hardware, which instead is the case with spinner-mounted anemometers or lidars.

Superconducting Synchronous Motor Development for Airplane Applications - Mechanical and Electrical Design of a Prototype 100 kW Motor

A 100 kW, 4500 RPM synchronous superconducting motor is being built at the Paihau-Robinson Research Institute, Victoria University of Wellington, New Zealand. The motor has superconducting excitation field coils on the rotor and a conventional stator winding. To minimize mass the motor is an air-core type, meaning no ferromagnetic materials are employed inside the motor. Some of the salient features of this motor include: 1) field winding coils employing REBCO CORC conductor; 2) field coils powered with a contactless superconducting dynamo exciter; 3) an EM shield on the rotor for protecting the field coils from high frequency magnetic fields due to stator harmonics and the switching of the drive electronics; 4) a unique single-layer winding design for the stator winding, where the stator coils are made with copper Litz wire and are liquid-cooled. This paper describes the electrical design and 3D simulation of the motor including the superconducting dynamo, and highlights stator and rotor coil developments that will be validated in the prototype. Sensitivity to critical design parameters has been assessed in simulation and initial sub-system prototypes manufactured for testing. Preliminary design is projecting this motor diameter and axial length as 360 mm and 550 mm respectively, with an efficiency of 96.4 % at the rated load. These numbers may be comparable with permanent magnet motors of similar rating, but this motor is being built to serve as a test-bed for evaluating different technologies for rotor and stator components. The ultimate goal for this program is a 3.0 MW machine with superconducting armature and field windings. In comparison with the permanent magnet motor in 2.5 - 3.0 MW range, the superconducting motors are expected to be 4 to 5 times smaller in size and mass with efficiency exceeding 99 %.

Wind inflow observation from load harmonics via neural networks: A simulation and field study

In this work, feed-forward neural networks are used to estimate the inflow of a wind turbine in real time during operation. A previously published method achieved this capability by correlating the turbine response to the wind inflow via a piecewise-linear model. Specifically, the existing formulation used the once-per-revolution harmonics of both in- and out-of-plane blade bending moments to estimate four wind parameters: the vertical and horizontal shears, and the vertical and horizontal misalignment angles. The novel method presented here builds on the same concept, but replaces the linear model with neural networks. Compared to the previous approach, the neural-based one is simpler to tune and use, because it decouples the wind parameters, which can be observed each one independently from the others. The proposed formulation is tested using both aeroservoelastic simulations and field data obtained on a 3.5 MW machine. In the field, independent measurements of the inflow are provided by a nearby hub-tall met mast, providing data for the training and verification of the wind observer. Notwithstanding some limitations of the experimental setup, results indicate that the neural networks produce estimates of the shears and yaw misalignment angle with accuracies compatible with typical use cases in wind turbine and wind farm control. Additionally, the neural-based observer appears to slightly outperform the original formulation in the lower wind speed range, where stronger nonlinearities in the load-wind mapping can occur.

Large wind turbine edge instability field validation

Wind turbines can exhibit flutter-like edgewise instabilities past a critical rotor speed. These edgewise instabilities are dominated by an edge-torsion coupling due to flapwise blade deflection. This paper experimentally validates the predicted stability limit of a 7 MW machine. State of the art simulation software is used to compare against recorded test data and characterize the observed flutter mechanism. The critical rotor speed at which the edgewise instabilities occur at in field measurements can be predicted by time domain simulations and stability analysis with very good agreement.

Homopolar superconducting AC machines, with HTS dynamo driven field coils, for aerospace applications

There is worldwide interest in high-speed motors and generators with characteristics of compactness, light weight and high efficiency for aerospace applications. Several options are under consideration. However, machines employing high temperature superconductors (HTS) look promising for enabling machines with the desired characteristics. Machines employing excitation field windings on the rotor are constrained by the stress limit of rotor teeth and mechanisms for holding the winding at very high speed. Homopolar AC synchronous machines characteristically employ both the DC field excitation winding and AC armature windings in the stator. The rotor is merely a magnetic iron forging with salient pole lumps, which could be rotated at very high speeds up to the stress limit of the rotor materials. Rotational speeds of 50,000 RPM and higher are achievable. The high rotational speed enables more compact lightweight machines.This paper describes a 2 MW 25,000 RPM concept designs for machines employing HTS field excitation windings. The AC armature winding is made of actively cooled copper Litz conductor. The field winding consists of a small turn-count HTS coil that could be ramped up or down with a contactless HTS dynamo. This eliminates current leads spanning room-temperature and cryogenic regions and are major source for thermal conduction into the cryogenic region and thereby increase thermal load to be removed with refrigerators. For early adaption of this technology for the aerospace applications, this 2 MW machine weighing 380 kg with an efficiency > 99% represents an attractive option.

Optimal yaw strategy for optimized power and load in various wake situations

The interaction between nearby wind turbines in a wind farm modifies the power and loads compared to their stand-alone values. The increased turbulence intensity and the modified turbulence structure at the downstream turbines creates higher fatigue loading, which can be mitigated by wind farm and/or wind turbine control. To alleviate loads and maximize power possible strategies such as wake steering, where the turbine is yawed to redirect the wake such that it does not impinge the downstream turbine, have been studied. The work presented here focuses on situations where the wake is nevertheless affecting the downstream turbine, and more specifically how high loads can be avoided by yawing the wake-affected turbine. The analysis is conducted on a 2.3 MW machine, and the flow field is simulated using the Dynamic Wake Meandering model. The study investigates the impact on power and loads for different longitudinal interspacing and turbulence intensities. Optimal yaw strategies are defined for above rated regions where no power loss occurs. The potential load alleviation for different load sensors are studied, but the presentation is focussed on the blade root flapwise fatigue loading. For full wake at 3D interspacing and turbulence intensity of 5 %, around 35 % of load reduction on the 1 Hz Damage Equivalent Loads can be achieved at high wind speeds. Smaller reductions are achieved for higher atmospheric turbulence; the analogue case with 15 % turbulence intensity shows 17 % potential alleviation. The alleviation on the wind turbine lifetime is also calculated and compared for different turbulence intensities and mean wind speeds. Small reductions are achieved for sites with low mean wind speed and high turbulence intensity, but high reductions, of around 19 %, are accomplished in low turbulence intensity with high mean wind speed.

Virtual Voltage Source Control for 2 × 27 MVA Machine Test Bench

In this paper, a special control solution called virtual voltage source (VVS) is developed for no-load tests of induction machines. This concept is applied to a 2 × 27 MVA high-power machine test bench where the test bench is requested to precisely characterize, e.g., 3 MW machines at no-load currents. The developed VVS thoroughly solves the stability issue of induction machines at low-speed ranges. A set of optimizations is presented here that systematically improves the voltage precision of the test bench. Experimental results prove that the proposed concept achieves 0.08-0.12% voltage unbalance, less than 3% current unbalance, with less than 0.3% harmonic-voltage-factor, and verified by more than 100 induction machines.

A validation and code-to-code verification of FAST for a megawatt-scale wind turbine with aeroelastically tailored blades

Abstract. This paper presents validation and code-to-code verification of the latest version of the U.S. Department of Energy, National Renewable Energy Laboratory wind turbine aeroelastic engineering simulation tool, FAST v8. A set of 1141 test cases, for which experimental data from a Siemens 2.3 MW machine have been made available and were in accordance with the International Electrotechnical Commission 61400-13 guidelines, were identified. These conditions were simulated using FAST as well as the Siemens in-house aeroelastic code, BHawC. This paper presents a detailed analysis comparing results from FAST with those from BHawC as well as experimental measurements, using statistics including the means and the standard deviations along with the power spectral densities of select turbine parameters and loads. Results indicate good agreement among the predictions using FAST, BHawC, and experimental measurements. These agreements are discussed in detail in this paper, along with some comments regarding the differences seen in these comparisons relative to the inherent uncertainties in such a model-based analysis.

Design of the OffWindChina 5 MW Wind Turbine Rotor

The current article describes the conceptual design of a rotor for a 5 MW machine situated at an offshore site in China (OffWindChina). The OffWindChina 5 MW rotor design work was divided into two parts between the Technical University of Denmark (DTU) and the Chong Qing University (CQU). The two parts consist of the aeroelastic and structural design phases. The aeroelastic part determines the optimal outer blade shape in terms of cost of energy (COE), while the structural part determines the internal laminate layup to achieve a minimum blade mass. Each part is performed sequentially using in-house optimization tools developed at DTU and CQU. The designed blade yields a high energy output while maintaining the structural feasibility with respect to international standards.

Fluctuating loads on a tidal turbine due to velocity shear and turbulence: Comparison of CFD with field data

Fluctuating loads on tidal turbines are important for fatigue analysis and there is limited information or simulation available for full-scale conditions. Here, CFD simulations have been performed for a geometry-resolved full-scale tidal-stream turbine and compared with experimental data from a 1 MW machine deployed at the EMEC test site. Initially, Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulations (LES) were performed using an inflow mean velocity profile representative of the site but low inflow turbulence. Mean blade pressures were similar for the two types of turbulence closure and yielded mean power coefficients comparable with measurements. Then, to simulate the effect of turbulence on loads, LES with synthetic turbulence prescribed at inlet was employed. For these simulations, inflow profiles of mean velocity, Reynolds stresses and length scales were determined from a precursor channel-flow simulation, with additional factoring of stresses and length scales to match hub-height conditions measured on site. Fluctuations in thrust, power and blade bending moment arise cyclically from onset mean velocity shear and the blocking effect of the support tower and over continuous spectral ranges from blade-generated turbulence, approach-flow turbulence and waves. LES simulations with realistic inflow turbulence satisfactorily reproduced the relative spectral distribution of blade bending moments in low-wave conditions.

Analytical modeling of air-gap field distributions in permanent magnet embedded salient pole wind generator

A simple magnetic circuit method (MCM) is developed for analytical calculation of the air-gap magnetic flux density waveform of a permanent magnet embedded salient pole wind generator (PMESPWG), which can consider the saturation in the side frame and the upper core of its pole shoe. In the MCM, the magnetic circuit is divided into a series of sub-circuits according to the flux paths to cope with different lengths of air-gap and different saturation levels of the pole shoe upper core. In order to evaluate the validity of the proposed method, air-gap flux densities of a 3.3 MW machine prototype are calculated and validated by the finite-element method (FEM) results. By utilizing the developed MCM, the air-gap flux distribution of the PMESPWG is optimized to obtain the maximum fundamental and minimum total harmonic distortion.

Fatigue Analysis of Wind Turbine

This article combines with finite element method and the rain flow count method, which is using the nominal stress method developed a wind turbine used for structural fatigue analysis method, with 2.0 MW wind and machine hub to analyze the fatigue, calculated the operation lifespan fatigue load of wind turbine in the 20 years. By choosing the hot point of hub, analyzes the wheels fatigue damage characteristics. The calculation results show that: the selection of hot point wheels meet with the requirements of GL standard Fatigue design. This paper can be used in large wind turbine development design and in the process of production design authentication.

3-D Finite Element Method Analysis of Additional Load Losses in the End Region of Permanent-Magnet Generators

Machine design typically concentrates on the main magnetic and electric circuits while various additional losses are evaluated only by simple empirical equations. The objective of this paper is to examine the importance of the additional losses in the stator end region and in the housing of a 3.1 MW permanent-magnet wind generator. Stator stack clamping ring and finger plate losses are investigated using a 3-D finite element method. 2-D FEM analysis of the stator supporting housing losses is also performed. Two common mechanical component materials are analyzed in the 3-D studies; construction steel and stainless steel. The results show that failure to consider stator end areas and stator supporting constructions at the design stage can result in unwanted additional losses and affect machine efficiency. Indirect verification of the results is provided by efficiency measurements of different versions of the real 3.1 MW machine.

Power curve tracking in the presence of a tip speed constraint

This paper considers the problem of power regulation for a variable speed wind turbine in the presence of a blade tip speed constraint, for example to limit noise emissions. The main contribution of the paper is the formulation of a policy for the regulation of the machine in the transition region between the classical regions II and III that accommodates the tip speed constraint, and the derivation of associated wind schedules for the rotor speed, blade pitch and aerodynamic torque. To exemplify the possible use of such wind schedules in the design of control laws, model-based controllers are formulated in this paper that are capable of performing power curve tracking throughout all wind speeds, in contrast with commonly adopted approaches that use switching controllers to cover the various operating regimes of the machine. The proposed regulation policies and control laws are demonstrated in a high fidelity simulation environment for a representative 3 MW machine.

Structural capacity and the 20 MW wind turbine

This article addresses a generic and fundamental question: given the specification for a structure, how can one assess or even express its capacity without first designing it? The question is motivated by the consideration of how wind turbines (offshore wind turbines in particular) may scale up further. An underlying premise is that the wind-turbine designs that have evolved from 50 kW to 5 MW may warrant re-examination to check whether scaling effects demand new approaches at larger scales. Since several in the industry are already setting sights on 8 MW and even 10 MW machines as extrapolations of current designs, this article considers the development of a 20 MW (232 m diameter) machine. The conventional three-bladed arrangement on a slender tower is examined for notional 5 and 20 MW machines for similar rated wind conditions. Initially, the article considers the structural capacity requirements for conventional designs at both 5 and 20 MW and how these can be reduced substantially – using a floating framework in place of a tower and by deploying cable bracing on the rotor. When tower bending and the non-productive blade bending moments have thus been addressed, the turbine torque itself emerges as a major cause of cost. The turbine torque directly influences the costs of blades, pitch and yaw bearings, direct-drive generators (if present), gearbox (if present), and main shaft, and it is confirmed that in all cases, the total amounts of material required are at least proportional to the turbine torque for conventional designs. An alternative wind-turbine configuration is examined. In this, gravity is co-opted as an ally rather than an enemy, and power conversion takes place entirely within the rotor by allowing gravity to move bodies within the rotor. The concept is illustrated for a 20 MW machine and shown to be entirely workable in principle. A strong feature of this proposed new machine configuration is that the costs are almost exclusively structural and can be quantified in terms of the uniform currency of structural capacity. The scalability of this concept becomes clear by observing that the mean velocities of the moveable masses relative to the main rotor structure are virtually independent of machine scale.

Mightiest Wind

This article explores the increasing use of wind turbines for generating power. It also discusses that changing the economics of wind power can make it more practical for deep-ocean turbines to harness strong, steady offshore winds. Engineers around the globe are focusing on creating conventional motors that can improve performance of wind turbines. Companies have found that a direct-drive generator built with superconducting windings would produce twice as much power per volume as a conventional generator, with a small parasitic loss due to cryogenic cooling. The prospect of producing more power per tower, which would be the net effect of using 10 MW turbines, might enable more offshore wind projects to become economically feasible. Sinovel, a Chinese generator company, is already planning to build 5 MW machines using existing technology. Once 10 MW machines become available, it is conceivable that they would quickly adopt them for offshore installations. It would be a step toward clearing the coal-fed brown haze that envelopes much of East Asia.

Fatigue loads on wind turbines of different control strategies operating in complex terrain

The impact of complex terrain wind conditions on the loading of wind turbines is examined using state-of-art computational means. The analysis is two folded. The first part considers variations of all the wind defining parameters: the turbulence intensities of the wind inflow, the corresponding length scales, the coherence decay factors, the vertical and lateral mean shears, the yaw misalignment and the mean velocity at the hub height. Running several wind turbines it is found that the main driving fatigue mechanism is, as expected, the turbulence intensity. Small length scales and strong three-dimensionality of the inflow, which are characteristic of complex terrains, are secondary factors of increase of the fatigue loads but still in some cases significant. The significance of yaw misalignment is confirmed. In the second part of the work different design concepts are compared in terms of fatigue loads. Four designs at the class of 0.5 MW machines are considered: a classical stall and a classical pitch regulated machine, a “structurally soft” stall machine and a fixed pitch variable speed machine. As regards the significance of the control strategy, it is found that there are no merits for any of them. The “soft” design however seems to have reduced loads.

Stressing of turbine-generator-exciter shafts by variable-frequency currents superimposed on DC currents in asynchronous HVDC links and following disturbances at converter stations

Ripple currents on the DC side of both HVDC synchronous and asynchronous links together with cleared HVDC and AC system disturbances can excite in some circumstances onerous torsional vibrations in large steam generator shafts. The problem has assumed importance in recent months on account of the HVDC link between Scotland and Northern Ireland going ahead, on account of the proposed Eire/Wales link, and because AC/DC/AC couplers are to be installed extensively to interconnect the East and West European grid systems. This paper discusses and analyses excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts in close proximity to HVDC power converter substations by: (i) variable-frequency ripple currents superimposed on the DC currents in asynchronous links; and (ii) disturbances at bi-polar converter stations. Detailed simulation of the HVDC converter and generator is necessary for precise assessments of shaft torsional response following HVDC converter station faults. 500 MW, 660 MW, 1000 MW and 1300 MW machines are considered in the analyses that are made. >

Investigations and developments in the field of superconductive turbogenerators

Investigations and developments in the field of superconductive electrical machines have been given high priority in the USSR. Having constructed and tested small capacity generators of 5, 18 and 1200 kW, work was carried out on 20 and 300 MVA superconductive turbogenerators. At the present time developmental studies and investigations are in progress concerning the structural elements of a future 1200 MW machine and the simulation of its normal and transient processes. These studies are part of the development of a new class of large electrical machines with high efficiency, smaller masses and higher reliability, compared to the conventional machines.

High performance disk alternators

The basic concept, characteristics, and current status of flywheel-disk alternators are examined. The alternator should generate 15 kW/Kg continuously and about 100 kW/kg for one second pulses. The machine can operate at very high voltages a frequencies and requires no output brushes, and both its excitation and output currents can be controlled electronically. The disk alternators contain over 50% useful conductor in the form of petal-shaped layers to form a flywheel disk capable of withstanding rotation at maximum speed and deceleration at maximum rate. Particular attention is given to three machines under development: a single-disk machine for the 9 MJ antiarmor system; a counterrotating, electrically driven, second-generation machine; and a continuous-duty, 20 MW machine.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>