This post contains Schematics for Wye Connected Voltage Boosting Transformers.
The Transformers used are normal Isolated Power Transformers, with Split Primary and Secondary coils.
The Primaries are setup for "Lower Input Voltage" [such as "120 VAC" from a "120 / 240 VAC" rating].
To make these Transformers "Voltage Bucking" (Voltage Reducing), simply swap the Input / Output connections.
Schematics set #1; 120/208 VAC in, 132/229 VAC out 3 phase 4 wire Wye:
Pictorial Diagram
1 Line Diagram
Schematics set #2; 120/208 VAC in, 144/249 VAC out, 3 phase 4 wire Wye:
Pictorial Diagram
1 Line Diagram