As a very very rough back of the envelope calculation, I'd expect a DC resistance measured L-N on the secondary side to be about 0.22 milliohm, about 1/1000 of what you measured.
I estimate this as follows: based on DOE efficiency standards, I expect the transformer efficiency to be 99.5%, so at full load I expect 18kW of losses. I assume that these losses are 1/3 primary coils, 1/3 secondary coils, and 1/3 core (like I said, very very rough). So about 6kW of losses to resistance in the primary coil. Since this is a 3 phase system I assign 2 kW per coil.
Transformer full load current is about 3000A, so I just use I^2 * R = P with 3000^2 * R = 2000. Since you are measuring the transformer on the wye side this gives the L-N resistance.
Given all of my estimates, I would not be surprised by any value between 0.1 and 2 milliohms. 200 milliohms (the original measurement) is IMHO simply not possible.
IMHO the 0.2 ohm measurement is simply equipment limitations. A typical multimeter is simply not useful below 10 ohms; 0.2 ohms is probably some combination of lead and contact resistance.
To properly measure less than 1 ohm you need an instrument like this:
https://www.tequipment.net/Extech/380560/Milli-Ohmmeter/
or
You will note that these instruments use high current (10A test current) and 'kelvin probe' test leads, where 1 pair of leads injects the current and the other pair of leads measures the voltage across the resistance. This way the test current is not flowing through the measurement leads, and the resistance of the leads gets eliminated from the measurement.