How does circuit completes when physical ground(earth) is used as neutral?
But in this case, there is only one wire coming from sub-station and another is just physicaly grounded, how the circuit even completes? Where is the closed loop?


In my experience, any time a question is asked with a 'just' word used in it, that is where the problem lies. 'Just' tends to be used in the sense of 'with no consequence', or 'this is irrelevant information'.

'and another is just physically grounded'

Re-write that as 'and another is physically grounded'. It is connected to ground. It makes an electrical connection to ground, so that current flows into and out of that connection, with little voltage drop. 

The ground connection works to hold the neutral wire at around 0v, approximately the same 0v as your nearby neighbours on different phases, and the same 0v as the generating company, or at least their nearest three phase grounded centre tap transformer. Although a current flow between different earth points will generate a potential difference, when the ground rods are long enough, and the water table is high enough, that voltage difference tends to be low. It's low enough that most of the voltage drop from the phases appears on your loads, rather than on your ground connection.