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A Generator Perspective on Vertical Axis Wind Turbines
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The wind energy conversion system considered in this thesis is based on a vertical axis wind turbine with a cable wound direct drive PM generator. Diode rectifiers are used to connect several such units to a single DC-bus and a single inverter controls the power flow from the DC-bus to a utility grid. This work considers the described system from a generator perspective i.e. the turbine is primarily seen as a torque and the inverter is seen as a controlled load.

A 12 kW VAWT prototype with a single turbine has been constructed within the project. The power coefficient of this turbine has been measured when the turbine is operated at various tip speed ratios. This measurement determines both how much energy the turbine can convert in a given wind and at what speed the turbine should be operated in order to maximise the energy capture. The turbine torque variation during the revolution of the turbine has also been studied.

A PM generator prototype has been constructed in order to study power loss in the stator core at low electrical frequencies. Heat exchange between the stator and the air-gap between the stator and the rotor has been studied. Heat exchange between the stator and the air-gap is increased by turbulence caused by the rotor. The generator was also used in a demonstration of a DC-grid where two diode rectified PM generators supplied power to a single DC load.  An initial study of an inverter suitable for grid connection of the 12 kW PM generator has been performed.

Several turbine control strategies are evaluated in simulations. The control strategies only require the parameter "turbine speed" to determine the optimal system load.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 80 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1034
Keyword [en]
VAWT, PM generator, Wind power, Stator core loss
National Category
Energy Systems
Identifiers
URN: urn:nbn:se:uu:diva-197855ISBN: 978-91-554-8642-6 (print)OAI: oai:DiVA.org:uu-197855DiVA: diva2:614518
Public defence
2013-05-22, Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2013-04-25 Created: 2013-04-05 Last updated: 2013-08-30Bibliographically approved
List of papers
1. Offshore underwater substation for wave energy converter arrays
Open this publication in new window or tab >>Offshore underwater substation for wave energy converter arrays
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2010 (English)In: IET Renewable Power Generation, ISSN 1752-1416, Vol. 4, no 6, 602-612 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the design, construction, deployment and operation of an offshore underwater substation is discussed. The seabed placed substation interconnects three linear generator wave energy converters (WECs) at the Swedish Lysekil wave energy research site. The power from the WECs fluctuates because of their direct-driven topology. The generator voltage has varying electrical frequency and amplitude. To reduce the fluctuations, the individual voltages of the WECs are rectified and the power is added on a common DC-bus in the substation. The voltage is inverted, transformed and power is transmitted to an on-shore resistive load. The substation was retrieved on two occasions since the deployment in the spring of 2009. The functionality of the substation is validated by comparing voltage and current wave forms from Simulink with measured results from laboratory experiments. In addition, a sample of results from real offshore operation is illustrated and discussed. With a proportional-integral-derivative (PID)-regulator in the inverter control, the small fluctuations in the DC-bus voltage could be minimised. However, this would reduce the energy storage capability of the DC-link smoothing capacitors.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-112954 (URN)10.1049/iet-rpg.2009.0180 (DOI)000284511300013 ()
Available from: 2010-01-22 Created: 2010-01-22 Last updated: 2016-04-18Bibliographically approved
2. Power coefficient measurement on a 12 kW straight bladed vertical axis wind turbine
Open this publication in new window or tab >>Power coefficient measurement on a 12 kW straight bladed vertical axis wind turbine
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2011 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 36, no 11, 3050-3053 p.Article in journal (Refereed) Published
Abstract [en]

A 12 kW vertical axis H-rotor type wind turbine has been designed and constructed at Uppsala University. A measurement campaign has been performed to collect data to calculate the power coefficient using the method of bins. The measurement was performed at different constant rotational speeds on the turbine during varying wind speeds to observe the power coefficients dependence on tip speed ratio. The power coefficient peaked at 0.29 for a tip speed ratio equal to 3.3.

Keyword
VAWT, H-rotor, Experimental, Measured, Cp
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-156579 (URN)10.1016/j.renene.2011.03.031 (DOI)000292441400035 ()
Available from: 2011-08-07 Created: 2011-08-04 Last updated: 2017-12-08Bibliographically approved
3. Adapting a VAWT with PM generator to telecom applications
Open this publication in new window or tab >>Adapting a VAWT with PM generator to telecom applications
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2010 (English)In: European Wind Energy Conference & Exhibition, Warsaw, Poland, 2010Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Warsaw, Poland: , 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-142522 (URN)
Conference
European Wind Energy Conference & Exhibition
Available from: 2011-01-14 Created: 2011-01-14 Last updated: 2017-01-25Bibliographically approved
4. No-load core loss prediction of PM generator at low electrical frequency
Open this publication in new window or tab >>No-load core loss prediction of PM generator at low electrical frequency
2012 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 43, 389-392 p.Article in journal (Refereed) Published
Abstract [en]

A method for measurement of frequency dependent electromagnetic core loss of a permanent magnet generator is presented. Core loss of a PM generator is measured at electrical frequencies ranging from 4 to 14 Hz. Core loss in the same interval is simulated using the finite element method and frequency domain loss separation. The specific loss is both extrapolated from specific loss at 50 Hz and measured directly at 4, 8, 12 and 16 Hz. Core loss simulations based on extrapolated specific loss are 38–53% smaller than measured loss. Core loss simulations based on specific loss measured at 4, 8, 12 and 16 Hz are 19–23% smaller than measured loss.

Keyword
Core loss Variable speed PM generator Cable wound generator Loss separation
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-190099 (URN)10.1016/j.renene.2011.12.002 (DOI)000301311500042 ()
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved
5. Robust VAWT control system evaluation by coupled aerodynamic and electrical simulations
Open this publication in new window or tab >>Robust VAWT control system evaluation by coupled aerodynamic and electrical simulations
2013 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 59, 193-201 p.Article in journal (Refereed) Published
Abstract [en]

A control system for a vertical axis wind turbine is presented.The control strategy is to determine the electric load solely from the rotational velocity and the characteristics of the turbine, thus measurement of the incoming wind velocity is not required.The control system is evaluated with an aerodynamic vortex model coupled with an electrical model.Three different sets of control system parameters are tested, due to the trade off between high power absorption and achieving a fast system with high stability.The control systems are tested against a reference strategy where the wind speed is known.The simulations show that the three control systems provide a similar power absorption as the reference case.For dynamic cases, with fast changes in wind speed, the fast control strategies are beneficial.All control strategies are stable throughout the simulations when proper power absorption characteristics of the turbine are used. It is also shown that if peak power absorption is estimated at a too low tip speed ratio, the control strategies may inadvertently stop the turbine.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Wind power, VAWT, Darrieus Turbine, Control system, Numerical simulation, Vortex method
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-183738 (URN)10.1016/j.renene.2013.03.038 (DOI)000320827300024 ()
Available from: 2012-11-01 Created: 2012-11-01 Last updated: 2017-12-07Bibliographically approved
6. Aerodynamic and electrical evaluation of a VAWT farm control system with passive rectifiers and mutual DC-bus
Open this publication in new window or tab >>Aerodynamic and electrical evaluation of a VAWT farm control system with passive rectifiers and mutual DC-bus
2013 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 60, 284-292 p.Article in journal (Refereed) Published
Abstract [en]

A wind farm with a simple electrical topology with passive rectifiers and a single inverter (mutual topology) is compared to a more complex topology where each turbine has a separate inverter (separate topology).In both cases, the turbines are controlled electrically by varying the extracted power with the rotational velocity as control signal.These two electrical topologies are evaluated with respect to absorbed power for a farm of four turbines placed either on a line or in a square formation.The evaluation is done with an aerodynamic vortex model coupled with an electrical system model.Simulations predict that individual control is beneficial for aerodynamically independent turbines if flow velocities differ significantly between turbines. If the differences in flow velocities are caused by one turbine operating in the wake of another, the deviations in power output between the topologies are less prominent.The mutual topology even deliver more power than the separate topology when one turbine is in the wake of another turbine if the wind speed changes rapidly.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Wind power, Wind farm, VAWT, Vortex model, Control system
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-183739 (URN)10.1016/j.renene.2013.05.028 (DOI)000323628600031 ()
Available from: 2012-11-01 Created: 2012-11-01 Last updated: 2017-12-07Bibliographically approved
7. Torque ripple of a straight-bladed Darrieus turbine with PM generator estimated from diode rectifier output current.
Open this publication in new window or tab >>Torque ripple of a straight-bladed Darrieus turbine with PM generator estimated from diode rectifier output current.
(English)Manuscript (preprint) (Other academic)
National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-197853 (URN)
Available from: 2013-04-05 Created: 2013-04-05 Last updated: 2013-08-30
8. Stator cooling by axial flow through air-gap of high torque PM generator.
Open this publication in new window or tab >>Stator cooling by axial flow through air-gap of high torque PM generator.
(English)Manuscript (preprint) (Other academic)
National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-197854 (URN)
Available from: 2013-04-05 Created: 2013-04-05 Last updated: 2013-08-30
9. Laboratory verification of system for grid connection of a 12 kW variable speed wind turbine with a permanent magnet synchronous generator
Open this publication in new window or tab >>Laboratory verification of system for grid connection of a 12 kW variable speed wind turbine with a permanent magnet synchronous generator
2012 (English)In: EWEA 2012 Annual Event, Copenhagen, Denmark, 2012, 2012Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

In this paper the first laboratory tests of the gridconnection system, connected to a resistiveload, for a vertical axis wind turbine (VAWT)with a permanent magnet generator arepresented. The system is based on a taptransformertopology with a voltage sourceinverter and an LCL-filter. The use of a taptransformer topology eliminates the need for aDC-DC converter to handle the variations inDC voltage. The harmonic content of thecurrents from experiments and simulationsperformed in Simulink using different taps onthe transformer are presented. The simulatedcurrents, fed to the resistive load, have a totalharmonic distortion (THD) of 0.5% to 0.9% forthe different taps. The experimental systemhas a current THD ranging from 1.8% to 2.8%.The difference is expected to be due tounbalances, delays and dead times in theexperimental set-up as the major THDcontribution is from harmonic orders below 11.The results show that an LCL filter can bedesigned to meet the demands on powerquality for grid connection of the system withall the taps of the tap transformer inaccordance with IEEE 519-1992.

Keyword
VAWT, Tap transformer, LCL-filter, voltage source inverter
National Category
Energy Systems Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-182434 (URN)
Conference
EWEA 2012 Annual Event, Copenhagen, Denmark, 2012
Available from: 2012-10-10 Created: 2012-10-10 Last updated: 2016-10-06

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