The Alternating Evolution of DC Power Transmission [Historical]

After more than one century of evolution of alternating current (ac) and thanks to the competitive costs of alternators and transformers, today a great majority of electricity is produced, transmitted, and distributed worldwide by means of ac polyphase systems [1], [2]. Nevertheless, about 200 high-voltage direct current (HVDC) systems are spread over the world to provide services where they are more convenient than ac systems. Moreover, the number of direct current (dc) systems is increasing, primarily due to the expansion of intermitted renewable energy sources and the inherent need to provide them with storage capabilities and, secondly, because of the enhanced competitiveness of solid-state converters. These two aspects can merge to form visionary concepts such as the energy hub, which fosters medium-voltage direct current transmission.