Analysis and Design of Tripole Converter System for AC Distribution Lines Upgraded to MVDC

Upgrading of existing AC distribution lines to MVDC by using tripole converter is an alternative way to increase the transmitted power with lower investment compared to building a new AC line. Conventional tripole conversion system consists of a bipolar DC system and a polarity reversible VSC DC system as third pole, which brings high cost and large footprint due to the additional third pole converters and transformers. In this paper, a three-phase-four-legs converter based tripole conversion system is introduced for AC distribution line upgrading to MVDC. Compared with conventional tripole converter, the proposed solution employs sharing resistors and bypass circuits which further increase the power transmission capacity. Operation principle of the proposed tripole system is presented. Drawback of existing 3-phase-four-legs based tripole conversion system is analyzed. Finally, additional power losses brought by the sharing resistors and increased power capacity with the proposed tripole system are calculated.