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Offshore Marine Substation for Grid-Connection of Wave Power Farms: An Experimental Approach
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wave power is a renewable energy source with great potential, which is why there are more than a hundred ongoing wave power projects around the world. At the Division of Electricity, Uppsala University, a point-absorber type wave energy converter (WEC) has been proposed and developed. The WEC consists of a linear synchronous generator placed on the seabed, connected to a buoy floating on the surface. Power is absorbed by heave motion of the buoy, and converted into electric energy by the generator.

The point-absorber WEC must be physically much smaller than the wavelength of the incoming waves, and can therefore not be scaled to very high power levels. Instead, the total power output is boosted by increasing the number of WECs, connecting them in wave power farms. To transfer the electric energy to the grid, an intermediate marine substation is proposed, where an AC/DC/AC conversion step is performed.

Within this PhD-work, a full-scale offshore marine substation has been designed, constructed and experimentally evaluated. The substation is rated for grid-connection of seven WECs to the local 1kV-grid, and is placed on the seabed 3km off the coast at a depth of 25m. Various aspects of the substation design have been considered, including the mechanical and electrical systems, the WEC electrical interface, offshore operations and the automatic grid connection control system. A tap change circuit and different multilevel topologies have also been proposed.

This dissertation has an experimental approach, validating a major part of the work with lab results. The final substation electrical circuit has been tested at rated grid voltage with a fluctuating input power source. The efficiency has been measured and the implemented functions are verified. Offshore operations have been successfully carried out and offshore wave farm data is expected in the nearby future.  

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 93 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1166
Keyword [en]
Wave power, Offshore Marine Substation, Grid-Connection, Electrical Systems, Voltage-Source Inverter, On-load Tap Change, Cascaded H-bridge Multilevel Inverter
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-229191ISBN: 978-91-554-9004-1 (print)OAI: oai:DiVA.org:uu-229191DiVA: diva2:738384
Public defence
2014-10-03, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-09-09 Created: 2014-08-05 Last updated: 2015-01-22
List of papers
1. Evaluating Constant DC-Link Operation of Wave Energy Converter
Open this publication in new window or tab >>Evaluating Constant DC-Link Operation of Wave Energy Converter
Show others...
2014 (English)In: Journal of Dynamic Systems Measurement, and Control, ISSN 0022-0434, E-ISSN 1528-9028, Vol. 136, no 1, 014501- p.Article in journal (Refereed) Published
Abstract [en]

A wave energy converter (WEC) based on a linear generator and a point-absorbing buoy has been developed at Uppsala University. Interconnecting an array of WECs in parallel requires a point of common coupling, such as a common dc-bus. The dc voltage level seen by the generator is directly linked to the electromagnetic damping of the generator. A lower dc-level results in a higher damping factor and is important for increased absorption of the wave power. The drawback is increased losses in generator windings and cable resistance. There will be an optimal dc-level for maximum power output. This is a function of not only generator and buoy characteristics, but the current sea state. Experimental results of the full-scale system have been carried out, and used as validation of a simulation model of the system. The model is then used to evaluate how the dc-level seen by the generator influence the power output. The results indicate that higher dc-levels should be used at higher sea states, and power output may vary by up to a factor five depending on which dc-level is chosen.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-213447 (URN)10.1115/1.4024789 (DOI)000327497500019 ()
Available from: 2014-01-04 Created: 2013-12-23 Last updated: 2017-12-06Bibliographically approved
2. Electrical Declutching of Wave Energy Converter with Integrated Asymmetric H-bridge CHB grid connection
Open this publication in new window or tab >>Electrical Declutching of Wave Energy Converter with Integrated Asymmetric H-bridge CHB grid connection
(English)In: IET Renewable Power Generation, ISSN 1752-1416, E-ISSN 1752-1424Article in journal (Refereed) Submitted
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-229196 (URN)
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
3. Electrical damping of linear generator WECs - a review
Open this publication in new window or tab >>Electrical damping of linear generator WECs - a review
(English)Article in journal (Refereed) Submitted
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-229193 (URN)
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2014-12-02Bibliographically approved
4. Optimal Constant DC Link Voltage Operation of aWave Energy Converter
Open this publication in new window or tab >>Optimal Constant DC Link Voltage Operation of aWave Energy Converter
2013 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 6, no 4, 1993-2006 p.Article in journal (Refereed) Published
Abstract [en]

This article proposes a simple and reliable damping strategy for wave powerfarm operation of small-scale point-absorber converters. The strategy is based on passiverectification onto a constant DC-link, making it very suitable for grid integration of the farm.A complete model of the system has been developed in Matlab Simulink, and uses real sitedata as input. The optimal constant DC-voltage is evaluated as a function of the significantwave height and energy period of the waves. The total energy output of the WEC is derivedfor one year of experimental site data. The energy output is compared for two cases, onewhere the optimal DC-voltage is determined and held constant at half-hour basis throughoutthe year, and one where a selected value of the DC-voltage is kept constant throughout theyear regardless of sea state.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212669 (URN)10.3390/en6041993 (DOI)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2017-12-06
5. Grid Connection of Wave Power Farm Using an N-Level Cascaded H-Bridge Multilevel Inverter
Open this publication in new window or tab >>Grid Connection of Wave Power Farm Using an N-Level Cascaded H-Bridge Multilevel Inverter
2013 (English)In: Journal of Electrical and Computer Engineering, Vol. 2013, 9- p.Article in journal (Refereed) Published
Abstract [en]

An N-level cascaded H-bridge multilevel inverter is proposed for grid connection of large wave power farms. The point-absorber wave energy converters are individually rectified and used as isolated DC-sources. The variable power characteristics of the wave energy converters are discussed, and a method of mitigating this issue is demonstrated. The complete power control system is given in detail and has been experimentally verified for a single-phase setup of the 9-level inverter. Theoretical expressions of the power sharing between multilevel cells are derived and show good correspondence with the experimental results.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212664 (URN)10.1155/2013/562548 (DOI)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2014-12-02
6. Fast solid-state tap-change using two current-controlled voltage-source inverters
Open this publication in new window or tab >>Fast solid-state tap-change using two current-controlled voltage-source inverters
2014 (English)In: IET Power Electronics, ISSN 1755-4535, E-ISSN 1755-4543, Vol. 7, no 10, 2610-2617 p.Article in journal (Refereed) Published
Abstract [en]

A new topology for on-load tap change using two current-controlled voltage-source inverters is demonstrated. With this topology, no additional tap change circuitry is required. This configuration can be useful when optimising the power extraction from renewable energy sources, and may replace a DC/DC converter. It is possible to run the inverters in parallel for higher current capacity, or run them in series across the transformer to improve the harmonic output. The topology can be extended to any number of taps, adding one mechanical contactor per tap. The procedure for the tap change, including the control implementations to keep active and reactive grid power flows constant throughout the tap change, is described. Experimental results are presented to show the viability of the proposed topology. Tap changes are conducted on all phases simultaneously, and individually at each phase current zero-crossing. Variations in grid power are investigated for the two strategies. Also, a dead-time in the tap change is introduced to evaluate how control system delays will appear as grid power dips.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-229192 (URN)10.1049/iet-pel.2013.0939 (DOI)000343973800018 ()
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
7. Measurement system design and implementation for grid-connected marine substation
Open this publication in new window or tab >>Measurement system design and implementation for grid-connected marine substation
2013 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 55, 338-346 p.Article in journal (Refereed) Published
Abstract [en]

A marine substation for a grid-connected wave power plant has been designed and constructed at Uppsala University. The measurement system for the substation has been developed from basics, and the procedure is described in this article. Subsea electrical installations set high constraints on the control and data acquisition systems used, and traditional GSM networks and GPS time synchronization are difficult and inefficient. These circumstances exclude many well-proven methods to be utilized. A compactRIO-based system, with integrated real-time controller and Field-Programmable Gate Array chip, is chosen as most suitable for this task. The system is designed to meet the special requirements encountered in this unusual application, including large data sampling, grid connection control and protection systems. The system is communicating to shore via a subsea copper cable and single-pair high-speed digital subscriber line modems. The design was successful and full-fills all the system requirements. The aim of the article is to provide future researchers with a good design and implementation procedure for setting up large measurement systems.

Keyword
Measurement system design, Offshore substation, compactRIO, Wave power, Grid-connection
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-198886 (URN)10.1016/j.renene.2012.12.028 (DOI)000316535900038 ()
Available from: 2013-05-02 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
8. Experimental Verifications of Offshore Marine Substation for Grid-Connection of Wave Energy Farm
Open this publication in new window or tab >>Experimental Verifications of Offshore Marine Substation for Grid-Connection of Wave Energy Farm
2013 (English)Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Istanbul, Turkey: , 2013
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212686 (URN)
Conference
3rd International Conference on Electric Power and Energy Conversion Systems (EPECS)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2016-10-06
9. Marine substation design for grid-connection of a research wave power plant on the Swedish West coast
Open this publication in new window or tab >>Marine substation design for grid-connection of a research wave power plant on the Swedish West coast
2013 (English)Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Aalborg, Denmark: , 2013
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212687 (URN)
Conference
10th European Wave and Tidal Conference (EWTEC)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2016-11-24
10. Control of offshore marine substation for grid-connection of a wave power farm
Open this publication in new window or tab >>Control of offshore marine substation for grid-connection of a wave power farm
2014 (English)In: International Journal of Marine Energy, ISSN 2214-1669, Vol. 5, 24-37 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Elsevier, 2014
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-229188 (URN)10.1016/j.ijome.2014.04.001 (DOI)
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2014-12-02Bibliographically approved
11. Lower order grid current harmonics for a voltage-source inverter connected to a distorted grid
Open this publication in new window or tab >>Lower order grid current harmonics for a voltage-source inverter connected to a distorted grid
2014 (English)In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 106, 226-231 p.Article in journal (Refereed) Published
Abstract [en]

As a result of lower order grid voltage harmonics, the grid current injected by a voltage-source inverter will also be partially distorted. In large-scale applications, active harmonic filters or notch filters are used to reduce the grid current distortion. For small-scale units, this may not be economically viable. In this article, two different grid phase tracking methods are evaluated with respect to the grid current distortion. The first method uses the zero-crossing detection (ZCD) method together with a look-up table (LUT), to generate a perfectly sinusoidal voltage synchronized with the grid. The second method uses a single-phase phase-locked loop (PLL). This method will reflect the grid harmonics in the inverter output, resulting in either cancellation or superposition of the harmonics of the grid current. A theoretical expression for the grid current as a function of the grid voltage harmonics is derived. Individual grid current harmonics as well as the total harmonic distortion (THD) are experimentally evaluated for both ZCD and PLL, and compared with theory. Results are presented for different power flows into the grid and compared with grid codes.

Keyword
Grid current distortion, PLL, ZCD, VSI, THD, Grid codes
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-211756 (URN)10.1016/j.epsr.2013.08.018 (DOI)000326558100023 ()
Available from: 2013-12-03 Created: 2013-12-02 Last updated: 2017-12-06Bibliographically approved
12. FPGA Control Implementation of a Grid-Connected Current-Controlled Voltage-Source Inverter
Open this publication in new window or tab >>FPGA Control Implementation of a Grid-Connected Current-Controlled Voltage-Source Inverter
2013 (English)In: Journal of Control Science and Engineering, Vol. 2013, 10- p.Article in journal (Refereed) Published
Abstract [en]

The full control system of a grid-connected current-controlled voltage-source inverter (CC-VSI) has been designed and implemented on a field-programmable gate array (FPGA). Various control functions and implementation methods are described and discussed. The practical viability of the system is evaluated in an experimental setup, where a VSI supplies 30 kW into the local grid at 400 V. A phase-locked loop (PLL) is used for grid phase tracking and evaluated for simulated abnormal grid conditions. Power factor is kept at unity, and the implemented control system is stressed with step responses in the supplied active power. A moving-average filter is implemented to reduce the effects of noise and harmonics on the current control loops. A coupling between active and reactive power flow is observed for the step responses but may be ignored in this context. The proposed system is fully comparable with more conventional microprocessor-based control systems.

National Category
Natural Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212642 (URN)10.1155/2013/713293 (DOI)
Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2014-12-02
13. Transformer Magnetization Losses Using a Nonfiltered Voltage-Source Inverter
Open this publication in new window or tab >>Transformer Magnetization Losses Using a Nonfiltered Voltage-Source Inverter
2013 (English)In: Advances in Power Electronics, Vol. 2013, 7- p.Article in journal (Refereed) Published
Abstract [en]

Results from the magnetization of an 80 kVA power transformer, using a directly coupled nonfiltered three-phase voltage-source inverter (VSI), are presented. The major benefits of this topology are reduction in switching filter size as well as filter losses. Drawbacks include higher stress on the transformer windings and higher transformer magnetization losses. In this paper, the total magnetization losses are presented for different voltage levels. The transformer has been magnetized with the rated frequency of 50 Hz at various voltage levels. The saturation characteristics as well as the magnetizing resistance are derived as functions of the voltage. These are used as inputs for the simulations. The magnetization losses have been experimentally measured and simulated for three different DC levels. Results from the simulations show good agreement with the experimental results. As expected, the pulsed voltage waveforms generate larger magnetization losses than the corresponding 50 Hz case. The losses are strongly dependent on the DC level.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212665 (URN)10.1155/2013/261959 (DOI)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2016-10-06
14. Transformer Magnetizing Inrush Currents Using a Directly Coupled Voltage-Source Inverter
Open this publication in new window or tab >>Transformer Magnetizing Inrush Currents Using a Directly Coupled Voltage-Source Inverter
2013 (English)In: ISRN Electronics, Vol. 2013, 8- p.Article in journal (Refereed) Published
Abstract [en]

The connection of a power transformer to the grid is associated with magnetizing inrush currents that may result in power quality issues as well as faulty relay tripping. In distributed generation, the transformer may instead be premagnetized from the source to avoid this. In this paper, a VSI is directly coupled to a transformer. Three different strategies of premagnetization are implemented into the control system, and the inrush currents are measured for various values of the remanent flux in the core. The results show good reduction in the peak magnetizing inrush currents without using any external circuitry.

National Category
Natural Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212663 (URN)10.1155/2013/361643 (DOI)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2016-10-06

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