An industrial customer is currently fed via a 2000kVA 13860V delta-600 GrdY 347V. The medium term plan is to replace the 13860V system by a 24940GrdY system, and install a new 2500kVA transformer connected GrdY-GrdY
What would be the drawbacks to remove the present transformer (delta primary) and feed him until then, ie a few years, with a 2000kVA 13860V Y - 600 GrdY 347V. (Note the transformer H0 would be insulated and left floating; only X0 would be connected to the ground grid).
Will there really be no zero sequence currents flowing in the LV grounded neutral? The Primary will be an ungrounded wye, but the secondary will be solidly grounded. With no zero sequence current, I am increasingly worried that this may lead to ferroresonnance if one of the primary phases loses voltage.
The delta winding meant that the transformer is an effective source of zero sequence current for the LV system. With this removed and the H0 bushing isolated, you will no longer have this.
The main problem that you would have is that in the event of a ground fault on one phase on the 600V side, the L-N voltage on that phase will collapse to zero, meaning that no fault current will flow, while on the other two phases, the L-N voltage will rise to 600V (assuming the transformer isn’t saturating). The reason for this is that the ungrounded wye point on the high-voltage side is not tied to anything, and so it will “shift” so that its potential is equal to that of the potential of the high-voltage side of the faulted phase.
Presumably, your existing circuit protection on the 600V is based around detecting overcurrent to trip in the event of an earth fault. This won’t function any more.
If you want to muddle through a more technical explanation, read “The Whys of Wyes”, which is an excellent paper produced by GE on the subject of transformer wye connections:
Above is a snippet.