Optimal power flow in multi-terminal HVDC networks with embedded high-power DC-DC converters for voltage matching and flexible DC transmission

High-power DC-DC converters are integrated into multi-terminal high-voltage DC (MTDC) networks, typically for voltage matching or flexible DC transmission. This study addresses droop-controlled radial and mesh MTDC networks with transmission loss minimisation considering the impact of integrating a high-power DC-DC converter. DC voltage droop control can achieve minimum power loss in radial MTDC transmission network by controlling the power-sharing among the receiving nodes inversely proportional to their transmission line resistances. Updating the droop characteristics by an optimisation algorithm guarantees optimal power flow (OPF) during renewable source power generation fluctuation. Meanwhile, the mesh network DC voltage references can be updated by the OPF for transmission loss minimisation. This study presents the impact of incorporating a high-power DC-DC converter at different system access lines of a radial MTDC network while targeting efficient power transfer and DC transmission loss minimisation. The effect of introducing a high-power DC-DC converter on the droop characteristic settings during steady-state operation in a radial MTDC network is investigated. Moreover, the impact of a high-power DC-DC converter in mesh MTDC network with OPF operation is explored. To validate the presented concepts, the CIGRE B4 MTDC network is used as a benchmark system with a metaheuristic optimisation technique.