| 1. |
- Apelfröjd, Senad, et al.
(författare)
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Laboratory verification of system for grid connection of a 12 kW variable speed wind turbine with a permanent magnet synchronous generator
- 2012
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Ingår i: EWEA 2012 Annual Event, Copenhagen, Denmark, 2012.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Bülow, Fredrik, 1980-
(författare)
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A Generator Perspective on Vertical Axis Wind Turbines
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
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| 4. |
- Bülow, Fredrik
(författare)
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Extreme Load Conditions for Wind Powered Direct Drive PM Generators
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wind energy is a rapidly growing energy source with vast potential. Vertical axis wind turbines could be more cost-effective than the three bladed horizontal axis turbines that dominate today. A concept based on the straight bladed Darrieus turbine together with a PMgenerator operating in direct drive is studied at Uppsala University; this thesis is a part of that project. This thesis contains a study of stator core loss at low electrical frequencies. The purpose of the study was to test the reliability of core loss simulations where the specific loss at low frequency is extrapolated using specific loss at 50Hz. The simulated loss was compared to the measured loss. Passive rectifiers can be used to connect all wind energy converters in a wind farm to a mutual DC load. This principle has been demonstrated using two PM generators with very different properties. This thesis also contains two studies of a full vertical axis wind turbine prototype. The first study concerns the power coefficient of the turbine. The power coefficient was measured at several wind speeds and turbine rotational speeds. The power coefficient attains its maximum value of 29% when the tip speed ratio is 3.3. In a second study, the armature currents of the generator were used to map the harmonic content of the turbine torque. The third harmonic of the turbine torque was found to be the strongest harmonic. This thesis also presents a wind energy converter that has been tailored for a telecom tower. Several aspects of the design are unique.
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| 5. |
- Bülow, Fredrik, et al.
(författare)
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No-load core loss prediction of PM generator at low electrical frequency
- 2012
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Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 43, s. 389-392
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
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| 7. |
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| 8. |
- Goude, Anders, et al.
(författare)
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Aerodynamic and electrical evaluation of a VAWT farm control system with passive rectifiers and mutual DC-bus
- 2013
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Ingår i: Renewable energy. - : Elsevier. - 0960-1481 .- 1879-0682. ; 60, s. 284-292
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Goude, Anders, et al.
(författare)
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Robust VAWT control system evaluation by coupled aerodynamic and electrical simulations
- 2013
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Ingår i: Renewable energy. - : Elsevier. - 0960-1481 .- 1879-0682. ; 59, s. 193-201
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Holm, Niels R, et al.
(författare)
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OCT or Angiography Guidance for PCI in Complex Bifurcation Lesions.
- 2023
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Ingår i: The New England journal of medicine. - 1533-4406. ; 389:16, s. 1477-1487
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Tidskriftsartikel (refereegranskat)abstract
- Imaging-guided percutaneous coronary intervention (PCI) is associated with better clinical outcomes than angiography-guided PCI. Whether routine optical coherence tomography (OCT) guidance in PCI of lesions involving coronary-artery branch points (bifurcations) improves clinical outcomes as compared with angiographic guidance is uncertain.We conducted a multicenter, randomized, open-label trial at 38 centers in Europe. Patients with a clinical indication for PCI and a complex bifurcation lesion identified by means of coronary angiography were randomly assigned in a 1:1 ratio to OCT-guided PCI or angiography-guided PCI. The primary end point was a composite of major adverse cardiac events (MACE), defined as death from a cardiac cause, target-lesion myocardial infarction, or ischemia-driven target-lesion revascularization at a median follow-up of 2 years.We assigned 1201 patients to OCT-guided PCI (600 patients) or angiography-guided PCI (601 patients). A total of 111 patients (18.5%) in the OCT-guided PCI group and 116 (19.3%) in the angiography-guided PCI group had a bifurcation lesion involving the left main coronary artery. At 2 years, a primary end-point event had occurred in 59 patients (10.1%) in the OCT-guided PCI group and in 83 patients (14.1%) in the angiography-guided PCI group (hazard ratio, 0.70; 95% confidence interval, 0.50 to 0.98; P=0.035). Procedure-related complications occurred in 41 patients (6.8%) in the OCT-guided PCI group and 34 patients (5.7%) in the angiography-guided PCI group.Among patients with complex coronary-artery bifurcation lesions, OCT-guided PCI was associated with a lower incidence of MACE at 2 years than angiography-guided PCI. (Funded by Abbott Vascular and others; OCTOBER ClinicalTrials.gov number, NCT03171311.).
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