Fault current analysis and active current limiting strategy for hybrid cascaded LCC/MMC HVDC system.

• The equivalent discharging circuit of the hybrid cascaded LCC/MMC inverter is simplified into a first-order RL circuit by assuming the firing-advance angle is constant after the fault. • The mathematical fault current expressions of the hybrid cascaded LCC/MMC high voltage direct current system are analyzed. • By changing the inserted sequence of submodules, an active fault current limiting strategy based on the submodule sorting algorithm is proposed. • The proposed active current limiting strategy can cooperate with other current limiting strategies to further suppress the fault current. The hybrid cascaded LCC/MMC high voltage direct current (HC-HVDC) system is a prospective topology that can avoid the commutation failure problem of LCC-HVDC while ensuring bulk power transmission capability. However, the complex topology results in different DC fault characteristics at the receiving end, which brings challenges to fault detection and current limiting technology. As a result, it is of great theoretical importance to investigate the fault characteristics of the HC-HVDC system and propose the corresponding fault current limiting strategy. Firstly, this paper simplifies the equivalent discharging circuit of the hybrid cascaded LCC/MMC inverter into a first-order RL circuit by assuming the firing-advance angle is constant after fault, and proposes the fault current expressions by Laplace transform. Secondly, by changing the insertion sequence of submodules, an active fault current limiting strategy based on the submodule sorting algorithm (SMS-FCL) is proposed, which can accomplish fault current suppression, reactive power and DC voltage support, and energy loss reduction simultaneously. Moreover, the proposed SMS-FCL can cooperate with other FCLs to further suppress the fault current, and the cooperation prerequisites are analyzed. Finally, an LCC-LCC/MMC HVDC system is built in PSCAD/EMTDC to validate the accuracy of the proposed fault current expressions and the effectiveness of the current limiting strategy. [ABSTRACT FROM AUTHOR]