In a Wye secondary, as you know you have three voltages out of phase by 120 deg.If you measure to the neutral from any phase you get 120 volts. If you measure phase to phase you get 208 volts. Now if you remove one of the three transformers; you still get 120 volts line to neutral on two phases. You can still read 208 volts across the two lines that are left. if you reverse the secondary connection on one of the transformers, you move the phase angle through an angle of 180 deg. The phase displacement on a wye circuit is 120 deg. if you rotate that line 180 degrees (By reversing the connections) you end up with 300 degrees. The difference between 300 deg. and 60 deg. is whether you measure the inside angle or the outside angle. Now you still measure 120 volts from line to neutral from either line, but from line to line where you previously read 208 volts, you now read 120 volts because the phase angle has been changed. You now read 120 volts line to line as well as line to neutral. You now have a corner grounded 120 Volt Open*Delta*bank.There is not much call for 120 Volts*Delta*three phase so change the internal transformer connections back to 120/240 volts. Now you have a corner grounded 240 Volt Three Wire Open*Delta. Much more usable. Do you have single phase loads?Move the ground connection from the corner to the center tap of one of the transformers and from there run a neutral to the service head. You now have a four wire Open*Delta*with 120/240 Single Phase as well as 240 Volt three phase.