Impacts of three MMC-HVDC configurations on AC system stability under DC line faults

This paper analyzes the dc line fault transient stability characteristics of AC/DC power systems based on three different modular multilevel converter based high power direct current (MMC-HVDC) configurations. The first configuration is half bridge sub-module based MMC (H-MMC) HVDC configuration, which clears dc line faults by tripping the ac circuit breakers. Voltage source converters (VSC) with dc line fault clearance ability constitute the second configuration. A line-commutated convener (LCC) and MMC hybrid HVDC configuration with dc line fault clearance ability, named LCC-diode-MMC (LCC-D-MMC), is the third configuration. The detailed processes of clearing dc line faults in three MMC-HVDC configurations are analyzed. The equal area criterion is utilized to analyze the dc line fault transient processes. Besides, to evaluate the power system transient stability characteristics in three MMC-HVDC configurations, an index, called critical ac transmitted power (Pac_critical), is proposed. Under the same dc line to ground fault, transient stability characteristics of three test systems based on the three different MMC-HVDC configurations are compared. Excellent performances of the LCC-D-MMC HVDC system under the dc line fault are demonstrated through comparison of Pac_critical in the three test systems. Finally, the study is extended to a modified New England 39 bus system, and the simulation results also tally with the theoretical analysis.