But secondary current imbalances cause currents in the primary that are "self-canceling" if X-0 is allowed to float.
"The primary feed is 3 phase 4 wire 480/277. The neutral lands on the XO and has a bonding jumper strap to the enclosure, which is tied in to ground rods."
Read post 21 again. A wye-delta should be wired like a delta-delta.
My limited understanding is unbalanced currents in the secondary will be coupled into the primary and be on the neutral, disconnecting the neutral helps cancel?
You do not want XO bonded on the primary side, if the load is unbalanced, it will create excessive heating of the transformer. I’ve replaced several that have failed due to that. (Assuming the poco is supplying a grounded wye on their side.)
Downstream is just a step-down xfrm. The 2300 is just to mitigate voltage drop, if that helps
Ungrounded systems still need EGC's, bonding jumpers, and grounding electrode systems. You don't want differences in potential between non current carrying components, and when a ground fault does occur, your system essentially becomes a grounded system with the fault point being the point where it is grounded. A second fault will not be much different than a fault of an ungrounded conductor on a grounded system and will result in high fault currents and operation of protection devices.
I’ve got audio of one of those such wired transformers screaming. (208 to 480 back fed) It was recently connected, customer complained about being so noisy. My guy wired it the same way the old one was. (It was replaced because it was overheating and noisy) when I seen the photo of the connections, I knew what was wrong. Had tech remove the primary bond to XO, tech turned it back on, he had to check the output to see if it was still working because it was so quiet! Transformer was no longer overheating.
Well, both. There should be no wire on x-0 at all.
XO bond on the primary, no neutral required on input. Transformer frame is still connected to the EGC. Any fault to ground on the primary side will still active the OCP because the input source is grounded/bonded.
Yes. Think of the voltages applied at the wye primary as three equal length arrows (i.e., vectors) that are lined up tail-to-tail and spaced 120° apart. On the delta secondary the arrows will line up head-to-tail in an equilateral triangle (equal sides at a 60° angle to each other). And so if you disconnect any two windings of the delta from each other, there will be zero volts across the open connection because the arrows (voltage vectors) line up perfectly with each other. Therefore zero current will flow through the delta windings if the delta is closed. However, if you try to apply unequal L-N voltages on the primary then the open-circuit voltages on the delta secondary windings will not add up to zero, and so if you close the delta high currents will flow. Floating the X0 on the primary allows the Xn-to-X0 voltages to adjust such that the vectors on the delta secondary form a closed triangle (except not equilateral), and therefore minimal currents are drawn.