Single phase single bushing transformer primary current return path explanation

members i real your help in understanding these transformer (as seen in the attached picture), 
1. In these type of transformers, what is the return path for the primary winding current, because i understand that they are connected to only one phase, that is either phase A, phase B or phase C of the Medim Voltage (MV) supply

I work for an electricity distributor (not on that side though) and I have seen transformers like this on our network.  In Australia distribution lines are very long and we sometimes use what is called SWER (Single Wire Earth Return).  As the name implies, the earth is the return leg.  This requires good earth connections at each end of the line.  I suspect this is what is in your picture, a SWER transformer.  But I'll run it past some of our power guys next week and confirm.PS just looking around on the net, electric railways use very similar technology.

i understand SWER, but this is not it, in this single phase single bushing transformer, both live and neutral wires from the secondary of the transformer goes to the consumer, what i fail to understand is how does the primary current completes its path, as one end of the primary winding is fed to one phase conductor of the MV supply and the other end is connected the transformer casing which is then grounded

The return path is the neutral wire, which is grounded, so does not need a bushing. Look up three-phase four-wire connections: phase A, phase B, phase C, neutral.