energies Review Challenges in All-DC Offshore Wind Power Plants Alessandra Follo 1,2, * , Oscar Saborío-Romano 1 , Elisabetta Tedeschi 2,3 and Nicolaos A. Cutululis 1   Citation: Follo, A.; Saborío-Romano, O.; Tedeschi, E.; Cutululis, N.A. Challenges in All-DC Offshore Wind Power Plants. Energies 2021, 14, 6057. https://doi.org/10.3390/en14196057 Academic Editor: Miguel-Angel Tarancon Received: 6 May 2021 Accepted: 13 September 2021 Published: 23 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Wind Energy, Technical University of Denmark, 4000 Roskilde, Denmark; osro@dtu.dk (O.S.-R.); niac@dtu.dk (N.A.C.) 2 Department of Electric Power Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway; elisabetta.tedeschi@ntnu.no 3 Department of Industrial Engineering, University of Trento, 38123 Povo, Italy * Correspondence: afollo@dtu.dk Abstract: As the size and distance from shore of new offshore wind power plants (OWPPs) increase, connection to shore using high-voltage (HV) direct-current (DC) technology becomes more cost- effective. Currently, every offshore wind power plant has a collection system based on medium- voltage alternating-current technology. Such systems rely on heavy and bulky low-frequency (i.e., 50 or 60 Hz) transformers: a drawback offshore, where equipment weight and space are restricted. Consequently, there is growing interest in medium-voltage direct-current collection systems, in which low-frequency transformers are replaced with DC/DC converters equipped with lighter and smaller medium-frequency transformers. However, the deployment of all-DC OWPPs still faces several challenges. Based on a very comprehensive and critical literature review, three of them are identified and discussed in this paper. The first challenge is the technological gap at component level. In this work, the DC/DC converter topologies most suitable for application to OWPPs are described and compared. The second challenge is the controllability of DC collection systems. Parallel, series and hybrid DC collection system layouts are presented and discussed. The third challenge is the compliance of all-DC OWPPs with current requirements for their connection to the onshore grids. The three challenges are discussed to highlight current research gaps and potential future directions. Keywords: offshore wind power plant; DC collection system; DC/DC converter 1. Introduction With the growing demand for electricity produced from renewable sources [1] such as wind, solar, wave and tidal, offshore wind will play a key role. The global installed capacity of offshore wind energy is forecast to almost triple between 2019 and 2024 [2,3]. Due to superior wind conditions and lower space restrictions away from cost lines, there is a clear trend towards the installation of larger, better performing wind turbines (WTs) and the deployment of larger wind power plants (WPPs) [3,4]. To date, most offshore wind power plants (OWPPs) rely on high-voltage (HV) alternating- current (AC) transmission systems. For large-scale OWPPs located beyond some tens of km from shore, HVAC transmission systems may require midpoint reactive compensation plat- forms, due to the high charging currents of the transmission cables [5–7]. In contrast, HV direct-current (DC) technology enables a more efficient transmission of power over long dis- tances: with lower cable losses and dispensing with charging currents (and corresponding reactive power compensation) [6–8]. Currently, all OWPPs rely on medium-voltage (MV) AC collection systems. In such mature, widely used technology, low-frequency (i.e., 50 or 60 Hz) transformers facilitate the stepping up or down of the collection system voltage. DC collection systems offer several advantages over their MVAC counterparts. As pre- viously mentioned for HVDC transmission, DC cables have lower losses and do not need reactive power compensation. Another advantage is the replacement of heavy and bulky low-frequency transformers with DC/DC converters equipped with lighter and smaller Energies 2021, 14, 6057. https://doi.org/10.3390/en14196057 https://www.mdpi.com/journal/energies