Y-connected modular multilevel converter(Y-MMC) is a new circuit topology of direct AC/AC power conversion, with competitive application prospects in fractional frequency offshore wind power system. Different from the previous converter topology, Y-MMC realizes the frequency conversion by single-phase voltage cancellation. Simultaneously, the voltages between main bridges and auxiliary bridges can be determined by the reactive power distribution coefficients. Considering the frequency leakage problem, this paper proposes the new mathematical model of Y-MMC and builds corresponding control strategy. Then, the state equation of the system and its small-signal models are established, the small distribution stability is analyzed. Furthermore, to improve the stability and dynamic performance of the closed-loop control system, the parameters of controller are optimized based on the particle swarm optimization algorithm. Finally, the effectiveness and superiority of the proposed control strategies are verified by simulation results.

Frequency leakage; Parameters optimization; Particle swarm optimization algorithm; Small signal stability; YMMC converter