3-leg core type vs 4-leg shell type vs 3 single phase bank

[pwrtran]

A Y/YG transformer, Y side connects to a source and Y grounded side connects to lines. There is no zero sequence current return path as the Y side is ungrounded. However, for a core type transformer the residual leakage flux through the tank wall cannot be ignored. The air gaps have very high reluctance and because at any time the flux cannot make a sudden change, therefore for a given flux at a time the higher the reluctance the higher current is required, and thus the lower the Z0. For a shell type transformer Y/YG, the zero sequence current is trapped in the core steel that has very low reluctance than the air gap, on the other side it has very high Z0 compared to core type Y/YG transformer. What about a 3-single phase connected Y/YG bank?

 

[prc]

The bank made up of single phase units will act similar to shell type 3 phase unit as zero sequence flux has the return magnetic path in the single phase core.