Experimental Verification of a Back-to-Back 2L-3L Grid Connection System for a Marine Current Energy Converter

• Rivers, tides and other ocean currents are renewable energy sources with great potential across the globe.A research group at Uppsala University is working on converting the power in rivers and ocean tidesusing a vertical axis turbine with a directly driven permanent magnet generator. The concept in focususes an omnidirectional, fixed pitch vertical axis turbine directly connected to a permanent magnetgenerator. Few moving parts and an overall low mechanical complexity is the main idea behind theconcept. A first full prototype was deployed in 2013 in the river Dalälven in the town of Söderfors. Thework presented here is a step towards grid connection of the marine current energy converter prototype.A back-to-back 2L-3L grid connection topology has been proposed. The system is adapted to the scaledprototype but is intended for larger turbines. The proposed grid connection system utilizes a three-levelcascaded H-Bridge voltage source converter (3L-CHBVSC) on the grid side together with a well knowtwo-level voltage source converter on the generator side. The use of a multilevel converter brings severaladvantages such as higher efficiency, more sinusoidal voltages and currents and smaller grid filtersamong other advantages. The proposed grid connection system has been constructed in the laboratory.The work presented here aims to evaluate the system in the laboratory. A synchronous generator with avery similar design to the on-site generator is used for the tests. The generator, driven by an inductionmotor, is connected to the system and power is successfully transferred to the grid. The conducted testsare used to verify the functionality of the system before installation at the research site.

Nyckelord

marine current energy converter

grid connection

three-level cascaded H-bridge

Engineering Science with specialization in Science of Electricity

Teknisk fysik med inriktning mot elektricitetslära

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