Analysis and Modeling of a Hybrid MMC Coordinated With a Circuit Breaker Under DC Short-Circuit Fault Conditions

In recent years, hybrid modular multilevel converters (H-MMCs), which employ both half-bridge submodules (HB-SMs) and full-bridge submodules (FB-SMs) in each arm, have gained significant attention in high-voltage DC systems. This paper thoroughly investigates the behavior of an H-MMC with an arbitrary ratio of FB-SMs per arm, coordinated with a DC circuit breaker (CB), during a pole-to-pole short-circuit (SC) fault. Upon detecting an SC fault, the CB is activated, and the H-MMC is temporarily blocked to reduce the peak fault current. The behavior of the H-MMC during this short interval is highly non-linear, and this paper aims to provide a precise model to analyze the H-MMC's behavior and calculate arm currents, DC fault current, and voltage increase in the FB-SMs. The model accounts for all non-linear states during the blocking state, including arm current decay and the effect of arm inductors on current commutation, which becomes critical when fast CBs or short DC lines are considered in HVDC systems. A new index is also introduced to compare the power switch requirements for various MMC and CB combinations as a function of the FB-SM ratio and CB interruption times. Finally, the analytical model and mathematical equations are validated through simulations and experimental testing on a scaled-down prototype.