Side-by-side connection of LCC-HVDC links to form a DC grid

HVDC is suitable for bulk-power and long-distance transmission. Most of the existing HVDC systems are point-to-point LCC-HVDCs. By interconnecting existing LCC links can achieve more economical benefits and higher flexibility of power transfer. However, due to different voltage levels and control modes of these links, it is impossible to interconnect them together directly through dc cables. The side-by-side connection of LCC links using DC transformers at a geographic crossing point or close proximity between the LCC links is proposed. Such connection is to achieve the benefits of dc grid operation without incurring vast amount of costs by constructing new DC transmission lines but making full use of existing LCC lines. Two LCC links tied by a DC transformer are tested as a case study. The DC transformer based on full-bridge sub-module is designed for coping with power reversal, temporary DC and AC faults and reduced voltage operation. And its control system is designed to match the operation of LCC links.