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.　©　2019　IEEE.