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As you connect the secondary of the 700kVA one phase to one primary phase of the 500kVA transformer. Ground the neutral of the 700kVA solidly and ignore it from now on. The primary of the 500kVA has no neutral connection. From the voltages you list I believe that the secondary of the 500kVA is also Wye. So ground it and take the neutral to any of the single phase loads. The breaker on the secondary should be sized to protect the conductors for it to the 500kVA transformer. The 500kVA should be protected by a breaker rated 115% of the rated current of the transformer. I do not have my calculator handy, but I believe that 500kVA at 230V is 1,260A. 115% of that is approx. 1500A You will need to look at the size of the conductors connecting these two to determine the breaker size at the secondary of the 700kVA transformer.