The attached (LTspice and jpeg)is what I mean by the horror currents of delta 3 phase…as you see, the phases of the star that get used to make the delta are slightly different (139Vpk vs 141vpk)…a tiny difference, and yet look at the “Horror over current of delta” that result from this.
But no one in real life has zero impedance 3 isolated voltage sources. They have one voltage source, a magnet spinning inside a 3 phase winding. Any minor 1% variation in voltage due to loads being different, just passes through to the load without causing circulating currents (not talking about harmonics). The voltage variations pass through the transformer btw.
Also no one in real life parallels two different 3 phase ac systems without regard to balancing. You can parallel large 3 phase generators btw. The intrinsic impedance of a large 3 phase synchronous generator is around 20 to 30%. A 2% difference in voltage only causes at most a 10% difference in current.
Most of the time, its a top down generator to transformer distribution to load.
I have a rotary phase converter on my lathe that nominally produces 240v 3 phase, dropping to 240-215-210v under heavy load.. if i were to put a A star delta transformer on the output, i would not get 240-216-210 underload at the lathe. I would get a 30% phase shift and the voltages under load would be different, however the percentage of imbalance would be precisely the same. Neglecting the resistance of the transformer, i would have the same starting torque.. If i put a scott t transformer after my star delta transformer, i would have the same voltages as i do at the beginning, at my two phase rpc, made from a 5 ton scroll compressor. There would be no circulating current in the delta secondary either.
Transformers store no energy, neglecting the 1% leakage inductance for sub 200kva transformers. (Leakage inductance increases as the transformer gets larger. Gigawatt transformers have about a 20% impedance due to leakage inductance, but they are 99.8% efficient)