Low Frequency AC transmission as an alternative to VSC-HVDC for grid interconnection of offshore wind

Low Frequency AC transmission (16.7Hz) has recently gained notable interest in literature as an alternative to the existing VSC-HVDC transmission solution for integrating offshore wind parks. Its key advantages being the substantial gain in the range of transmissible power over traditional 50 Hz methods and the removal of the offshore converter compared to VSC-HVDC. This paper presents a system level techno-economic comparison of conventional VSC-HVDC to LFAC and focuses on the design and operation issues of the onshore step-up conversion to the grid frequency using a thyristor based cycloconverter. This analysis also includes aspects such as power quality and the ability to provide system services. The study finds that LFAC transmission with a BtB VSC converter onshore has greater power losses when compared to the cycloconverter. However, the BtB VSC solution is seen as a more realistic selection option for offshore developers considering LFAC, given lower filtering requirements and greater reliability with respect to grid integration.

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