| 1. |
- Apelfrojd, Senad, et al.
(author)
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A Back-to-Back 2L-3L Grid Integration of a Marine Current Energy Converter
- 2015
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In: Energies. - : MDPI AG. - 1996-1073. ; 8:2, s. 808-820
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Journal article (peer-reviewed)abstract
- The paper proposes a back-to-back 2L-3L grid connection topology for a marine current energy converter. A prototype marine current energy converter has been deployed by a research group at Uppsala University. The concept behind the prototype revolves around a fixed pitch vertical axis turbine directly connected to a permanent magnet synchronous generator (PMSG). The proposed grid connection system utilizes a well known and proven two level voltage source converter generator-side combined with a three-level cascaded H-bridge (CHB) multilevel converter grid-side. The multilevel converter brings benefits in terms of efficiency, power quality and DC-link utilization. The system is here presented for a single marine current energy converter but can easily be scaled up for clusters of marine current energy converters. Control schemes for both grid-side and generator-side voltage source converters are presented. The start-up, steady state and dynamic performance of the marine current energy converter are investigated and simulation results are presented in this paper.
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| 2. |
- Carpman, Nicole
(author)
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Marine Current Resource Assessment : Measurements and Characterization
- 2015
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Licentiate thesis (other academic/artistic)abstract
- The increasing interest in converting energy from renewable resources into electricity has led to an increase in research covering the field of marine current energy, mainly concerning tidal currents and in-stream tidal turbines. Tides have the advantage of being predictable decades ahead. However, the tidal resource is intermittent and experiences local variations that affect the power output from a conversion system. The variability is mainly due to four aspects: the tidal regime, the tidal cycle, bathymetry at the site and weather effects. Each potential site is unique, the velocity flow field at tidal sites is highly influenced by local bathymetry and turbulence. Hence, characterizing the resource requires careful investigations and providing high quality velocity data from measurement surveys is of great importance. In this thesis, measurements of flow velocities have been performed at three kinds of sites.A tidal site has been investigated for its resource potential in one of all of the numerous fjords in Norway. Measurements have been performed to map the spatial and temporal variability of the resource. Results show that currents in the order of 2 m/s are present in the center of the channel. Furthermore, the flow is highly bi-directional between ebb and flood flows. The site thus have potential for in-stream energy conversion. A model is proposed that predicts peak current speed from information on tidal range at the site. A corresponding model can be set up and implemented at other similar sites affected by tides, i.e. fjord inlets connecting the ocean to a fjord or a basin.A river site serves as an experimental site for a marine current energy converter that has been designed at Uppsala University and deployed in Dalälven, Söderfors. The flow rate at the site is regulated by an upstream hydrokinetic power plant nearby, making the site suitable for experiments on the performance of the vertical axis turbine in its natural environment. The turbine has been run in uniform flow and measurements have been performed to characterize the extent of the wake.An ocean current site was a target of investigation for its potential for providing utilizable renewable energy. A measurement campaign was conducted, mapping the flow both spatially and temporally. However, the site was shown to not be suitable for energy conversion using present technique.
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| 3. |
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| 4. |
- Dolguntseva, Irina, et al.
(author)
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Contour-Based Dead-Time Harmonic Analysis in a Three-Level Neutral-Point-Clamped Inverter
- 2015
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In: IEEE Transactions on Industrial Electronics. - : IEEE. - 0278-0046 .- 1557-9948. ; 62:1, s. 203-210
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Journal article (peer-reviewed)abstract
- The term dead time refers to a prime safety factor for most power electronic converter topologies, and it is included either in the control software or in the gate/base driver hardware, depending on the application as well as the control requirements. In this paper, the authors present a comprehensive numerical analysis of dead-time effects on the output voltage of a three-level neutral-pointclamped (NPC) inverter. To incorporate the dead-time effect in the output voltage, 3-D models of three-level carrier pulse width modulation (PWM) methods are modified for two dead-time implementations. Closed-form expressions of inverter phase voltage harmonics for phase opposition disposition (POD) PWM are derived based on the double Fourier series approach and modified contour plots. The harmonic spectra from numerical evaluations, simulations, and experiments for natural sampling (NS), symmetrical regular sampling (SRS), and asymmetrical regular sampling (ARS) are compared to validate the mathematical models. In addition, the fundamental voltage with respect to the dead time and the load phase angle is presented based on analytical results and simulation.
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| 5. |
- Ekström, Rickard, et al.
(author)
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Electrical damping of linear generators for wave energy converters : A review
- 2015
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In: Renewable & sustainable energy reviews. - : Elsevier BV. - 1364-0321 .- 1879-0690. ; 42, s. 116-128
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Research review (peer-reviewed)abstract
- The electrical damping of point-absorber wave energy converters is crucial to optimize the power output. Many circuit topologies have been proposed, but the possible increase in power absorption must be weighed against parameters such as cost, reliability and control system complexity. In this paper, the known electrical damping circuits are categorized, described and compared. The hydrodynamic damping of the buoy is covered, and how a linear generator can be used as a power take-off unit to apply a damping force. A qualitative comparison of the circuits is presented in the end. A more complex and costly power electronics system may be viable for wave energy converters (WECs) of large-scale power rating. However, for farm operation with small-scale WECs, a simpler and passive damping may be more suitable.
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| 6. |
- Elamalayil Soman, Deepak, et al.
(author)
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DC-Link Stress Analysis for the Grid Connection of Point Absorber Type Wave Energy Converters
- 2015
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In: 2015 International Conference On Clean Electrical Power (ICCEP). - 9781479987047 ; , s. 61-66
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Conference paper (peer-reviewed)abstract
- Highly random nature of input power from wave energy converters (WEC), especially from direct-driven point absorbers, demands customized power electronic converters for grid connection. In this paper, analysis and comparison of the DC-link stresses in the converter systems for two cases - a single and three collective units, of wave energy converters is given. The AC/DC/AC converter system includes a conventional uncontrolled three phase rectifier, a DC/DC converter to boost the DC-link voltage and an inverter with RL load. The system has been studied under two different controller actions for the DC/DC converter: with constant boost factor and with constant DC-link voltage. A Proportional Integral controller has been used to regulate the voltage in the latter case. Matlab/Simulink based system simulation has been done to compare the DC-link stress. The analysis shows the comparison in DC-link stresses and the requirements of the system for different cases, proving the advantages and the importance of having customized active power conversion methods for minimizing the DC-link stresses.
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| 7. |
- Elamalayil Soman, Deepak, et al.
(author)
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Development of Power Electronics Based Test Platform for Characterization and Testing of Magnetocaloric Materials
- 2015
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In: Advances in Electrical Engineering. - : Hindawi Publishing Corporation. - 2356-6655 .- 2314-7636. ; 2015:670624, s. 7-
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Journal article (peer-reviewed)abstract
- Magnetocaloric effects of various materials are getting more and more interesting for the future, as they can significantly contribute towards improving the efficiency of many energy intensive applications such as refrigeration, heating, and air conditioning. Accurate characterization of magnetocaloric effects, exhibited by various materials, is an important process for further studies and development of the suitable magnetocaloric heating and cooling solutions. The conventional test facilities have plenty of limitations, as they focus only on the thermodynamic side and use magnetic machines with moving bed of magnetocaloric material or magnet. In this work an entirely new approach for characterization of the magnetocaloric materials is presented, with the main focus on a flexible and efficient power electronic based excitation and a completely static test platform. It can generate a periodically varying magnetic field using superposition of an ac and a dc magnetic field. The scale down prototype uses a customized single phase H-bridge inverter with essential protections and an electromagnet load as actuator. The preliminary simulation and experimental results show good agreement and support the usage of the power electronic test platform for characterizing magnetocaloric materials.
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| 8. |
- Engström, Jens, et al.
(author)
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Properties of the Energy Transport for Plane-Parallel Polychromatic Surface Gravity Waves in Waters of Arbitrary Depth
- 2015
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In: IEEE Journal of Oceanic Engineering. - 0364-9059 .- 1558-1691. ; 40:2, s. 408-416
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Journal article (peer-reviewed)abstract
- It is well known that the energy transport of ocean waves propagates with the group velocity and that the energy decreases exponentially with depth. Expanding this theory, we will derive expressions for the energy transport as a function of depth and the total instantaneous transport's development over time for waves in waters of finite depth. Solutions to the Laplace equation are found for plane-parallel polychromatic waves with linearized boundary conditions. A time series of wave elevation collected at Uppsala University's wave energy research test site is chosen to present the results. Solutions for waters of both infinite and arbitrary depths are presented and compared. The solutions are convolution-type integrals with the wave elevation where we have found efficient ways to calculate the kernels. The difference in group velocity between finite depth and infinite depth and its impact on the energy transport is clearly seen in the results. The use of the deep-water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. We further show that the total instantaneous energy transport can actually have a direction that is opposite to the direction of the waves as observed from a reference frame fixed to the seabed.
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| 9. |
- Forslund, Johan, 1985-, et al.
(author)
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Experimental Results of a DC Bus Voltage Level Control for a Load-Controlled Marine Current Energy Converter
- 2015
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In: Energies. - : MDPI AG. - 1996-1073. ; 8:5, s. 4572-4586
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Journal article (peer-reviewed)abstract
- This paper investigates three load control methods for a marine current energy converter using a vertical axis current turbine (VACT) mounted on a permanent magnet synchronous generator (PMSG). The three cases are; a fixed AC load, a fixed pulse width modulated (PWM) DC load and DC bus voltage control of a DC load. Experimental results show that the DC bus voltage control reduces the variations of rotational speed by a factor of 3.5 at the cost of slightly increased losses in the generator and transmission lines. For all three cases, the tip speed ratio can be kept close to the expected . The power coefficient is estimated to be 0.36 at ; however, for all three cases, the average extracted power was about \%. A maximum power point tracking (MPPT) system, with or without water velocity measurement, could increase the average extracted power.
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| 10. |
- Forslund, Johan, 1985-
(author)
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Experimental Results of a Load-Controlled Vertical Axis Marine Current Energy Converter
- 2015
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Licentiate thesis (other academic/artistic)abstract
- This thesis investigates the load control of a marine current energy converter using a vertical axis turbine mounted on a permanent magnet synchronous generator. The purpose of this thesis is to show the work done in the so far relatively uncharted territory of control systems for hydro kinetic energy conversion. The work is in its early stage and is meant to serve as a guide forfuture development of the control system.An experimental power station has been deployed and the first results are presented.A comparison between three load control methods has been made; a fixedAC load, a fixed pulse width modulated DC load and a DC bus voltage control of a DC load. Experimental results show that the DC bus voltage control reduces the variation of rotational speed with a factor of 3.5. For all three cases, the tip speed ratio of the turbine can be kept close to the expected optimal tip speed ratio. However, for all three cases the average extracted power was significantly lower than the average power available in the turbine times the estimated maximum power coefficient. A maximum power point tracking system, with or without water velocity measurement, should increase the average extracted power.A simulation model has been validated using experimental data. The simulated system consists of the electrical system and a hydrodynamic vortex model for the turbine. Experiments of no load operation were conducted to calibrate the drag losses of the turbine. Simulations were able to predict the behaviour in a step response for a change in tip speed ratio when the turbine was operated close to optimal tip speed ratio. The start position of the turbine was varied in the simulation to view the influence on the step response from a changed turbine position relative to the direction of the water flow.
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