Wye-Delta transformer bank

[synchro]

Could the voltage rise be related to power factor correction capacitors that were still connected when the 3-phase load was removed? If the voltage rise was enough to start causing a transformer core to saturate, then the transformer might overheat after several hours under this condition.
Just tossing out some ideas...

 

[kwired]

I'm failing to see how this will provide stable output volts if the H2 terminals are not connected to primary neutral with a multiple core bank.

Seems even more likely to not be stable if the three units are not same size, as that would make each primary winding impedance different.

A significant three phase load will help keep things balanced to some degree though.

 

[jim dungar]

Back home, I have a bulletin with formulas for creating 240/120V 4 wire delta banks. one of the examples shows how a closed delta bank can become overloaded simply by removing half of the single phase load, causing a complete imbalance. This is part of the reason most of these systems are built using an open delta configuration. It is possible something like this was involved in your incident.