Power Routing Strategy for an Offshore-Onshore Bipolar VSC-HVDC Interconnector

Abstract

Distantly-located offshore energy hubs need to be connected to the shore via High Voltage Direct Current (HVDC) links to allow for an efficient bulk power exchange. A bipolar configuration of the HVDC link is suitable for a point-to-point connection, as it provides redundancy, and, therefore, a larger reliability, e.g. half of the rated transfer capacity can still be transferred via one of the poles in case of a fault occurring on the other pole. Nevertheless, a control strategy of the converters that can effectively enable a situation-dependent power routing between the two poles constitutes a research challenge. In this paper, two control strategies are proposed for the offshore Modular Multi-level Converters (MMCs) of a bipolar HVDC link connecting a 2 GW offshore hub to the shore. The strategies, based on DC current and DC voltage measurements, respectively, enable to track and adjust the amount of power flowing through each pole of the link. Real-time digital simulations show that both strategies can effectively route the power exchanges through the bipolar HVDC link, e.g. operation under balanced or unbalanced conditions. The strategy based on DC current seems more suitable to manage the dynamic performance of the HVDC link.