A DC voltage control strategy for MMC MTDC grids incorporating multiple master stations

One of the responses to the increasing penetration of renewable energies along with the growth in electricity demand is the interest in the construction of multi-terminal DC grids (MTDC), as mean to connect onshore AC systems with remotely located offshore wind energy sources. This paper investigates the current methodologies for MTDC control stating their limitations for severe contingency conditions and proposes a control strategy able to ensure grid reliability and voltage stability in the case of a sudden outage of the converter station controlled in Vdc-Q. This strategy, acting on the Vdc reference signal, allows the presence of multiple master converters in the DC grid. A five-terminal DC scheme, which connects onshore AC systems with offshore wind farms, has been developed in RT-Lab Simulator. Each sub-station (terminal) is equipped with a modular multilevel converter (MMC). The converters are either Vdc-Q controlled (master stations) or are P-Q (with droop control) or Vac-frequency regulated. The dynamic response of the system is observed after a converter outage to assess the validity of the proposed DC voltage control strategy.