Intelligent DC-DC Converter Based Substations Enable Breakerless MVDC Grids

Medium-voltage dc (MVDC) grids are discussed for various high-power applications being a superior solution over the well-established ac technology. Besides advantages in improved controllability, system efficiency, and economical feasibility, protection strategies of dc grids are more challenging than of ac grids. Since dc (fault) currents have no natural zero crossing, the turn-off of short-circuit currents demands specially designed dc circuit breakers. Due to the changing decentralized structure of power generation and power consumption, as well as the increasing amount of storage systems, future application scenarios and grid topologies necessitate the breakerless implementation of multi-terminal dc grids. This paper presents the control strategy of dc-grid-connected converters during grid faults and the contribution of the three-phase Dual-Active Bridge (DAB3) dc-dc converter to limit short-circuit currents in future dc grids. This approach allows isolating grid faults with high-speed mechanical disconnectors, hence the implementation of breakerless MVDC grids. Simulation results are provided to validate the proposed control strategy.