AC/DC fault handling and expanded DC power flow expression in hybrid multi-converter DC grids

Hybrid multi-converter dc grids are extended dc grids formed by combinations of different converter topologies such as the line-commutated converter (LCC), modular multilevel converter (MMC), alternate arm converter (AAC) and other various topologies. The stable and secure operation of such systems in steady-stage and transient conditions is critical. This paper investigates the ac and dc fault-ride-through (FRT) capability and the expanded expression of dc power flow in a hybrid multi-converter dc grid consisting of LCCs, MMCs and AACs. Fault handling schemes of single converters are combined, extended and coordinated to provide satisfactory transient response of the hybrid dc grid under both ac and dc faults. This paper proposes an expanded expression of dc power flow considering the initial power flow determination and the power flow after converter outages under mixed P/V and I/V droop control. The expanded dc power flow expression can be used in LCC and modular VSC-based hybrid multi-converter dc grids to derive the initial dc power flow and assess the system static security after converter outages. Simulation results for steady-state and transient operation based on a detailed equivalent model verify the accuracy of the proposed dc power flow expression, and validate the fault handling capability of the hybrid dc grid.