Why ac generator connected in star connection?
That configuration allows you to connect single phase loads from any one, two or all of the three phases to the common of all three phases, called the neutral. We have mobile generators that connect it that way for 480Y/277V 3-phase 4-wire service, but at the flip of a mutipole switch, reconfigures it in Wye for 240/120V 3-phase 4-wire service. In this latter case, the midpoint of one of the secondary windings is connected to ground (earthed), and becomes the neutral for 120V single-phase loads that connect from one or two of the phases to neutral, but not from the third phase which is at an elevated voltage of 208V.
