DC Fault Management Strategies in Hybrid MMC-based HVdc Systems

The hybrid half-bridge (HB) and full-bridge (FB) modular multilevel converter (MMC) is recognized for its capability to handle dc faults in high-voltage dc (HVdc) systems. During a dc fault, various fault management strategies are employed in the hybrid MMC to reduce the dc-side voltage and the fault current. These strategies have a significant impact on the performance of the MMC, particularly on the submodule (SM) capacitor voltages. This paper investigates different fault management strategies employed in hybrid MMC-based HVdc systems. The fault management strategies considered in this paper are based on the dc fault current management achieved through active control and blocking SMs. In the active control mode, the dc side voltage of the MMC is managed in a way to reduce the fault current after fault detection, while keep the control on the MMC. In another strategy, all the power switches are turned off temporarily, leading to the injection of counter-emf voltage by FB-SMs and current reduction. However, it will lose the control temporarily on the MMC current and voltages. In this paper, different aspects of these two techniques are compared, and the validity of the comparison is proved through simulation in Matlab/Simulink environment and experiments on a scaled-down laboratory prototype.