DC/dc Converters for Interconnecting Independent HVDC Systems into Multiterminal DC Grids

Future multi-terminal HVDC (MTDC) grids will be based on Voltage Source Converter (VSC) technology. However, existing and planned VSC HVDC connections for offshore wind farms and cross-border interconnections are mainly point-to-point links which are operated at different voltages and can have different grounding schemes. Thus, DC/DC converters will become necessary for interconnection of independent HVDC transmission schemes into future meshed MTDC grids. This paper starts with a brief review of topologies for DC/DC converters in HVDC applications with particular attention directed towards interconnection of HVDC systems with different grounding schemes. The control and operation of a front-to-front transformer-coupled AC/DC/AC solution is analyzed as an example of a topology that will be suitable when galvanic separation is required. The investigated topology is demonstrated by time-domain simulations in Matlab/Simulink when interconnecting a monopolar HVDC link with a meshed bipolar MTDC configuration defined in the Cigré DC grid test system.