Hybrid cascaded modular multilevel converter with DC fault ride-through capability for HVDC transmission system

A new hybrid cascaded modular multilevel converter for high-voltage dc (HVDC) transmission system is presented. The half-bridge (HB) cells are used on the main power stage and the cascade full-bridge (FB) cells are connected to its ac terminals. The main power stage generates the fundamental voltages with quite low switching frequency, resulting relatively low losses. The cascaded FB cells only attenuate the harmonics generated by the main power stage, without contribution to the power transfer. Thus, the energy storage requirement of the cascaded FB cells is low and the capacitance of FB cells is reduced significantly. Due to the dc fault reverse blocking capability of the cascaded FB cells, the proposed topology can ride-through the pole-to-pole dc fault. In addition the soft restart is achieved after the fault eliminates, without exposing the system to significant inrush current. Besides, the average-value model of the proposed topology is derived, based on which the control strategy is presented. The results show the feasibility of the proposed converter.