D = delta connected winding
Y = wye connected winding
Capital letter = HV winding
Lower case letter = LV winding
Number = equivalent phase shift between primary and secondary, based on a clock dial (5 = five o'clock, or 150° lagging, 11 = eleven o'clock, or 30° leading)
Just remember, phase shift depends on phase rotation.
Dy11 vector group for phase rotation 1-2-3 becomes Dy1 for phase rotation of 1-3-2
see: thread238-11381
Hope it helps
RCC