Performance of a DC/AC/DC VSC system to interconnect HVDC systems

Interconnection of High Voltage Direct Current (HVDC) links into networks is key to a future HVDC grid spanning large regions. Various factors including the evolution of cable technology will lead to a variety of DC voltages and a need to provide a DC/DC step up/down at high power ratings (beyond 500 MW). This paper analyses a High Voltage (HV) DC/DC converter utilising an intermediate AC link between two modular multi-level converters (MMC) based on the Alternate Arm Converter (AAC) topology. System operation is demonstrated using a detailed Simulink simulation. Power loss estimates derived from the simulation are used to illustrate the dependence of efficiency on design choices and operating point. Furthermore, the system is shown to block a DC-side fault and prevent its affect propagation through the system.