Grounded Wye:
a- Solidly grounded (Effective & Very Effective)
b- Impedance Grounded.
1.Resistance Grounded(Low & High).
2.Reactance grounded (Low & High).
3.Inductance & Resistance (resonant).
Grounded Delta:
a- Corner grounded.
b- Center-tap grounded.
c- Derived neutral grounded w/zig-zag transf that may be used as high resistance grounded system.
d- Special connections......
Please notice that "ungrounded" systems are couple to ground thru distributed capacitance of cables, equipment, etc.
One of the most detailed winding connections you may find in the following references:
1.- Transmission & Dist. Reference Book-Table 5; Westinhouse/ABB.
2.- IEEE/ANSI 141, 142 & 241.
For a simplified overview on transformer and connection please the enclosed web site.
PS: Power tranformer also require to ground the tank regaless the winding connections. Also if surge arrester is provided as part of the transfomer scope must be grounded as required per code and standard eng. practice.