Multiport DC–DC Autotransformer for Interconnecting Multiple High-Voltage DC Systems at Low Cost

This paper proposes a multiport dc-dc autotransformer (multiport dc auto) that is used to interconnect multiple HVDC systems with different voltage levels. The multiport dc auto is able to reduce 50-80% the converter cost compared with conventional dc-ac-dc technology. Different from the conventional dc-ac-dc technology using magnetic coupling at the ac sides of the converters, there is direct electrical connection between the interconnected dc systems in the multiport dc auto. Such direct electrical interconnection significantly reduces the used power converters in the multiport dc auto. Taking interconnecting a ±250, ±320, and ±400 kV dc system with the rated exporting/importing dc power at each of the dc system being 500, 1000, and 1500 MW as an example, the conventional multiport dc-ac-dc technology requires a total of 3000 MW power converters while only 775 MW power converter is required in the multiport dc-dc autotransformer. The required power converter in the multiport dc-dc autotransformer is only 26% of the converter used in the conventional multiport dc-ac-dc technology. Cost and operating power loss is therefore significantly reduced.