Y-connected modular multilevel converter (Y-MMC) is a new topology of direct AC/AC power conversion, with competitive application prospects in Fractional Frequency Transmission System (FFTS). To improve the uninterrupted operation capability of Y-MMC, the control strategy considering unbalanced grid fault is proposed in this paper. This paper proposes the mathematical model of Y-MMC under asymmetric grid conditions according to the symmetrical component method and superposition principle of linear systems. Based on the proposed mathematical model, positive and negative sequence current inner loop controller, and outer loop controller to suppress the negative sequence current are specially designed for the stable operation of Y-MMC under asymmetric grid conditions. The effectiveness of the proposed control strategy has been verified by the simulation results obtained from a 10 kV, 10 MW Y-MMC system built in MATLAB /Simulink.

negative sequence current,symmetrical component method,Y-connected modular multilevel converter,fractional frequency transmission system,unbalanced grid fault