Transformer Connections

It is pretty obvious that you are using 3-phase power. The "200A maximum socket rating" has no bearing on the number of transformers required.

The short answer is yes - you get more "Flow" out of three transformers than you do two. Here is a brief description of a possible set of configurations. (There are others but these are the most common). Note also that there other voltages produced using these configurations as well. For simplification I will use normall "small commercial" level voltages.

In those instances where there are three transformers, there are several different configurations possible:

The two most likely are

3Ø 4-wire grounded WYE - This would give you voltages like 208 phase to phase or 120V Phase to neutral on all three legs.

The reason for calling this a WYE configuration is that one leg of each transformer is connected to a single point and this point is (usually) grounded. It defines the Neutral of the system. The other side of each transformer becomes one of the the three phase legs. If you were to draw this you would make a "Y" shape, ergo a "WYE" configuration. Each unconnected end of the WYE would define a phase leg with 208V between each and 120V across each transformer to neutral.

or

3Ø 4-wire center tapped DELTA - This would give you voltages like 240V phase to phase or 120V Phase to Neutral on Two of the three legs to Neutral. (Both 120V legs and the neutral are all located on only one of the transformers).

The three transformers are connected end to end, so that if you drew them they would form a triangle or, as from calculus forming (after a fashion) the DELTA symbol, ergo the name. Each point of the triangle would define a phase leg with 240V across each transformer.

Now if you recall, the WYE configuration has its leg to leg "phase" voltage taken between two transformers but the DELTA has its leg to leg "phase" voltage taken across each transformer.

The two transformer configuration is called an "OPEN DELTA" and the two transformers are tied together at one end only. The other two ends are left unconnected. If you take that DELTA triangle from before and removed one side leaving the other two in the same place (like a V on its side) you would have drawn an "OPEN DELTA". Each "point" still defines a phase leg and the voltage is the same as a regular DELTA.

If one of the two transformers is "center tapped" you can even get some 120V power. The connection is similar to the WYE in that you are not "looking" across a transformer but "between" two others for that phase to phase leg.

To address the "Flow " issue

The capacity of each of these transformer banks is equal to the sum of their individual ratings.

If you were to use identical transformers for the 3 and 2 unit configurations, the 2 unit setup ( OPEN DELTA) would have 2/3 the capacity in available power.

Also, if the load on an OPEN DELTA is not pretty well balanced you can get some pretty wierd voltages on the different legs. The standard application was for three phase motor loads or induction heaters with realtively small 120 or 24 single phase connections. Mostly used in industrial facilities.