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Lower order grid current harmonics for a voltage-source inverter connected to a distorted grid
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
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.

Place, publisher, year, edition, pages
2014. Vol. 106, 226-231 p.
Keyword [en]
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: urn:nbn:se:uu:diva-211756DOI: 10.1016/j.epsr.2013.08.018ISI: 000326558100023OAI: oai:DiVA.org:uu-211756DiVA: diva2:669140
Available from: 2013-12-03 Created: 2013-12-02 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Offshore Marine Substation for Grid-Connection of Wave Power Farms: An Experimental Approach
Open this publication in new window or tab >>Offshore Marine Substation for Grid-Connection of Wave Power Farms: An Experimental Approach
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
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:nbn:se:uu:diva-229191 (URN)978-91-554-9004-1 (ISBN)
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

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Ekström, RickardLeijon, Mats

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