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Electromechanics of Vertical Axis Wind Turbines
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.ORCID iD: 0000-0001-5467-4527
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wind power is an established mean of clean energy production and the modern horizontal axis wind turbine has become a common sight. The need for maintenance is high and future wind turbines may need to be improved to enable more remote and offshore locations. Vertical axis wind turbines have possible benefits, such as higher reliability, less noise and lower centre of gravity. This thesis focuses on electromechanical interaction in the straight bladed Darrieus rotor (H-rotor) concept studied at Uppsala University.

One of the challenges with vertical axis technology is the oscillating aerodynamic forces. A force measurement setup has been implemented to capture the forces on a three-bladed 12 kW open site prototype. The normal force showed good agreement with simulations. An aerodynamic torque could be estimated from the system. The total electrical torque in the generator was determined from electrical measurements. Both torque estimations lacked the expected aerodynamic ripple at three times per revolution. The even torque detected is an important result and more studies are required to confirm and understand it.

The force measurement was also used to study the loads on the turbine in parked conditions. It was discovered that there is a strong dependence on wind direction and that there is a positive torque on the turbine at stand still. The results can assist to determine the best parking strategies for an H-rotor turbine.

The studied concept also features diode rectification of the voltage from the permanent magnet synchronous generator. Diodes are considered a cheap and robust solution for rectification at the drawback of inducing ripple in the torque and output voltage. The propagation of the torque ripple was measured on the prototype and studied with simulations and analytical expressions. One key conclusion was that the mechanical driveline of the turbine is an effective filter of the diode induced torque ripple.

A critical speed controller was implemented on the prototype. The controller was based on optimal torque control and according to the experiments and the simulations it was able to avoid a rotational speed span. Finally, the optimal torque control was evaluated for multiple turbines with diode rectification to a common DC-link. The setup can potentially reduce the overall complexity of wind farms. The simulations suggest that stability of the system can be obtained by controlling the DC-link load as a semi constant voltage.

The thesis is based on nine papers of which six are treated in the thesis summary.

 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 81
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1581
Keywords [en]
Wind power, Diode rectification, Control, Measurement, Simulation, Electromechanical coupling
Keywords [sv]
vindkraft, diodlikriktning, reglering, mätning, simulering
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Energy Systems
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-331844ISBN: 978-91-513-0117-4 (print)OAI: oai:DiVA.org:uu-331844DiVA, id: diva2:1150387
Public defence
2017-12-08, Å2005, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
StandUpStandUp for WindAvailable from: 2017-11-14 Created: 2017-10-18 Last updated: 2018-03-07
List of papers
1. Evaluation of a Blade Force Measurement System for a Vertical Axis Wind Turbine Using Load Cells
Open this publication in new window or tab >>Evaluation of a Blade Force Measurement System for a Vertical Axis Wind Turbine Using Load Cells
Show others...
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 6, p. 5973-5996Article in journal (Refereed) Published
Abstract [en]

Unique blade force measurements on an open site straight-bladed vertical axis wind turbine have been performed. This paper presents a method for measuring the tangential and normal forces on a 12-kW vertical axis wind turbine prototype with a three-bladed H-rotor. Four single-axis load cells were installed in-between the hub and the support arms on one of the blades. The experimental setup, the measurement principle, together with the necessary control and measurement system are described. The maximum errors of the forces and accompanying weather data that can be obtained with the system are carefully estimated. Measured forces from the four load cells are presented, as well as the normal and tangential forces derived from them and a comparison with theoretical data. The measured torque and bending moment are also provided. The influence of the load cells on the turbine dynamics has also been evaluated. For the aerodynamic normal force, the system provides periodic data in agreement with simulations. Unexpected mechanical oscillations are present in the tangential force, introduced by the turbine dynamics. The measurement errors are of an acceptable size and often depend on the measured variable. Equations are presented for the calculation of measurement errors.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260148 (URN)10.3390/en8065973 (DOI)000357489700067 ()
Funder
StandUpStandUp for Wind
Available from: 2015-08-17 Created: 2015-08-17 Last updated: 2017-12-04
2. Frequency analysis of tangential force measurements on a vertical axis wind turbine
Open this publication in new window or tab >>Frequency analysis of tangential force measurements on a vertical axis wind turbine
2016 (English)Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents experimental results of the torque ripple obtained from a three bladed 12 kW experimental H-rotor prototype. The measurements are performed by means of load cells installed on the base of the struts and by electrical measurements on the generator. The resulting torques are analysed in terms of frequency spectrum and order spectrum (synchronized with rotation). The measurements are compared to aerodynamic simulations of the turbine. The expected large torque ripple at three times the rotational speed (3 p) is only weakly represented at the hub and in the generator. This suggests that the system is filtering the ripple and/or that the simulations are overestimating the 3 p component. The torque ripple loads on the drive train are therefore lower than anticipated. Even if highly attenuated, most of the low frequencies correlating to aerodynamics are still represented in the generator electrical torque. Given a certain baseline, this opens for possible online monitoring of unbalances in the turbine by electrical measurements.

Keywords
wind power, vertical axis wind turbine, forces
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-306558 (URN)10.1088/1742-6596/753/4/042016 (DOI)
Conference
The 6th international conference on “The Science of Making Torque from Wind”, Munich, October 5-7, 2016.
Funder
StandUpStandUp for Wind
Available from: 2016-10-28 Created: 2016-10-28 Last updated: 2017-11-28
3. Mechanical torque ripple from a passive diode rectifier in a 12 kW vertical axis wind turbine
Open this publication in new window or tab >>Mechanical torque ripple from a passive diode rectifier in a 12 kW vertical axis wind turbine
2017 (English)In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 32, no 1, p. 164-171Article in journal (Refereed) Published
Abstract [en]

The influence of passive rectification on the mechanical torque of a permanent magnet generator for a directly driven vertical axis wind turbine has been studied. Passive diode rectification introduce electromagnetic torque ripple from the generator. The conversion of electromagnetic torque ripple into mechanical torque ripple and rotational speed ripple has been modeled, analytically evaluated, and simulated. The simulations have been compared to measurements on an open site 12 kW prototype. A parameter study with the model illustrates the impact of shaft torsional spring constant, generator rotor inertia, generator inductance, and dc-link capacitance. The results show that the shaft and generator rotor can be an effective filter of electromagnetic torque ripple from diode rectification. The measured mechanical torque ripple amplitude on the prototype is less than +/- 0.9% of nominal turbine torque. The measurements compare well with the simulations.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-272363 (URN)10.1109/TEC.2016.2626783 (DOI)000396130300016 ()
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2017-10-18Bibliographically approved
4. Critical Speed Control for a Fixed Blade Variable Speed Wind Turbine
Open this publication in new window or tab >>Critical Speed Control for a Fixed Blade Variable Speed Wind Turbine
2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 11, article id 1699Article in journal (Refereed) Published
Abstract [en]

A critical speed controller for avoiding a certain rotational speed is presented. The controller is useful for variable speed wind turbines with a natural frequency in the operating range. The controller has been simulated, implemented and tested on an open site 12 kW vertical axis wind turbine prototype. The controller is based on an adaptation of the optimum torque control. Two lookup tables and a simple state machine provide the control logic of the controller. The controller requires low computational resources, and no wind speed measurement is needed. The results suggest that the controller is a feasible method for critical speed control. The skipping behavior can be adjusted using only two parameters. While tested on a vertical axis wind turbine, it may be used on any variable speed turbine with the control of generator power.

Keywords
vertical axis wind turbine, variable speed, control, optimal torque, critical speed, speed exclusion zone, natural frequencies, eigenfrequencies
National Category
Energy Systems
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-331782 (URN)10.3390/en10111699 (DOI)000417046500018 ()
Funder
StandUpStandUp for Wind
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2018-03-09Bibliographically approved
5. Multiple Vertical Axis Wind Turbines with Passive Rectification to a Common DC-link
Open this publication in new window or tab >>Multiple Vertical Axis Wind Turbines with Passive Rectification to a Common DC-link
2018 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 127, p. 1101-1110Article in journal (Refereed) Published
Abstract [en]

Wind turbines are commonly placed in wind farms, usually operating as separate units. Possible benefits could be found by allowing turbines to share a common DC-link. Diode rectifiers are a robust and cost effective way to rectify variable speed wind turbines, with loss of direct control of the generator. This paper studies the electromechanical interactions between four passively rectified vertical axis wind turbines connected to a common DC-link. Two different load approaches for the DC-link are compared using simulations in terms of performance and stability: a power source and a voltage source. The optimal torque (or optimal power) control is implemented for the two loads approaches. In addition, three-phase and dual stator winding (six-phase) generators are compared. The results show that all suggested solutions work with similar performance. However, the power load requires a large DC-link capacitance to achieve stability. More generatorphases improve the system with passive rectification in most cases. The simulations suggest that the common DC-link systems are expected to have a few percent lower energy capture due to the lack of individual turbine control. On the other hand, there is a significant reduction in peak power and a potential for smoother output power.

National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-331784 (URN)10.1016/j.renene.2018.05.013 (DOI)000437077300099 ()
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2018-09-20Bibliographically approved
6. Force measurements on a VAWT blade in parked conditions
Open this publication in new window or tab >>Force measurements on a VAWT blade in parked conditions
2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 12, article id 1954Article in journal (Refereed) Published
Abstract [en]

The forces on a turbine at extreme wind conditions when the turbine is parked is one of the most important design cases for the survivability of a turbine. In this work, the forces on a blade and its support arms have been measured on a 12 kW straight-bladed vertical axis wind turbine at an open site. Two cases are tested: one during electrical braking of the turbine, which allows it to rotate slowly, and one with the turbine mechanically fixed with the leading edge of the blade facing the main wind direction. The force variations with respect to wind direction are investigated, and it is seen that significant variations in forces depend on the wind direction. The measurements show that for the fixed case, when subjected to the same wind speed, the forces are lower when the blade faces the wind direction. The results also show that due to the lower forces at this particular wind direction, the average forces for the fixed blade are notably lower. Hence, it is possible to reduce the forces on a turbine blade, simply by taking the dominating wind direction into account when the turbine is parked. The measurements also show that a positive torque is generated from the blade for most wind directions, which causes the turbine to rotate in the electrically-braked case. These rotations will cause increased fatigue loads on the turbine blade.

National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-331818 (URN)10.3390/en10121954 (DOI)000423156900025 ()
Funder
StandUp for WindStandUp
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2018-03-23Bibliographically approved
7. Measurements of the Aerodynamic Normal Forces on a 12-kW Straight-Bladed Vertical Axis Wind Turbine
Open this publication in new window or tab >>Measurements of the Aerodynamic Normal Forces on a 12-kW Straight-Bladed Vertical Axis Wind Turbine
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 8Article in journal (Refereed) Published
Abstract [en]

The knowledge of unsteady forces is necessary when designing vertical axis wind turbines (VAWTs). Measurement data for turbines operating at an open site are still very limited. The data obtained from wind tunnels or towing tanks can be used, but have limited applicability when designing large-scale VAWTs. This study presents experimental data on the normal forces of a 12-kW straight-bladed VAWT operated at an open site north of Uppsala, Sweden. The normal forces are measured with four single-axis load cells. The data are obtained for a wide range of tip speed ratios: from 1.7 to 4.6. The behavior of the normal forces is analyzed. The presented data can be used in validations of aerodynamic models and the mechanical design for VAWTs.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260569 (URN)10.3390/en8088482 (DOI)000360586600052 ()
Available from: 2015-08-20 Created: 2015-08-20 Last updated: 2017-12-04Bibliographically approved
8. Permanent Magnet Working Point Ripple in Synchronous Generators
Open this publication in new window or tab >>Permanent Magnet Working Point Ripple in Synchronous Generators
2017 (English)In: The Journal of Engineering, ISSN 1872-3284, E-ISSN 2051-3305, p. 1-8Article in journal (Refereed) Accepted
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-303518 (URN)10.1049/joe.2016.0309 (DOI)000398521800001 ()
Funder
Swedish Research Council, 2010-3950Carl Tryggers foundation , 15:152
Available from: 2016-10-19 Created: 2016-09-20 Last updated: 2017-10-18
9. Designing, simulations and experiments of a passive permanent magnet bearing
Open this publication in new window or tab >>Designing, simulations and experiments of a passive permanent magnet bearing
Show others...
2016 (English)In: International journal of applied electromagnetics and mechanics, ISSN 1383-5416, E-ISSN 1875-8800, Vol. 51, no 2, p. 131-149Article in journal (Refereed) Published
Abstract [en]

This paper presents simulations and experimental results for two types of Passive Permanent Magnet Bearings. The bearing system under investigation consists of two permanent magnet rings opposing to each other in two different configurations. The influence of parameters, such as thickness and radius of permanent magnets, in the force is presented through FEM calculations. Two setups of passive magnetic bearings have been built. Static measurements of radial and axial forces are reported and results compared with simulations. Also, dynamic tests show the behavior of the bearing and the magnitudes of force in the foothold. The results are presented to show trends in the parameters, so the conclusions are applicable for other sizes and applications. As an example, the application as a top bearing for a 12 kW vertical axis wind turbine is considered.

Keywords
Permanent magnetic bearing, finite element method, vertical axis bearing
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-299628 (URN)10.3233/JAE-150162 (DOI)000377871000004 ()
Funder
Swedish Research CouncilStandUp
Available from: 2016-07-25 Created: 2016-07-25 Last updated: 2017-11-28Bibliographically approved

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