180920-0751 EDT
LarryFine:
We need a circuit diagram for the question.
Suppose three separate transformers connected delta on the primary, we open one supply line at the transformers. Now we have one transformer, A, with full supply voltage to its primary. The other two primaries are connected in series across the powered primary, A. The voltage division will depend upon the reflected impedances from the secondaries of transformers B and C. If B and C are unloaded, then the division will be around 1/2 assuming all the transformers are essentially the same.
One does not loose voltage to one bushing. This is because there is a series connection of two impedances. The lost supply line just doesn't have normal voltage and phasing.
Secondary voltage is a ratio from the primary voltage for a tightly coupled transformer.
In my neighborhood we have a delta source, only 3 wires on the pole, no primary neutral. Only path back to substation is via earth conduction and water pipes. It is probably a wye at the substation or other grounding means. Beyond me one supply wire was downed. It happened that this wire was one of my supply wires, but I still had full constant voltage. Then Edison decided to cut that leg between me and the substation. Now my voltage started to swing between about 10 and 90 V. There were a number of customers, possibly up to 10, between me and the substation. Assume these customers were distributed uniformly between A-B, B-C, and C-A. Assume A is the open line, then B-C always has power and we are not concerned with its load. Customers on A-B and C-A have refrigerators, and these have motors with thermal over-temperature protection. These loads will cycle on and off causing a variation in load impedance, and therefore in voltage.
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