Open Delta Transformer Bank

[wareagle]

I had a question put to me and wanted to see if you would
comment on my conclusion. The transformers were connected primary gnd wye and sec 240 volt delta with the lighting pot(Ltr)secondary center grounded. The utility had a cutout blow on the primary which cause a loss of power to the Ltr. At that moment the customer served from the bank experienced damage to a number of 120 volt appliances connected to the Ltr secondary along with the expected damage to a 3 phase A/C unit. Looking at the secondary wiring dwg of these two transformers, LTr secondary is still connected to the secondary of the other transformer. Looking at this circuit it appears to be the same as a buck/boost transformer. If so this would explain the damage to the 120 volts units. Your comments would be appreciated.

 

[waross]

This is one for our old answer, "It depends".
The transformers should not act as a boost. However, the teaser transformer may backfeed the lighting transformer through the windings of the A/C. If the unit is turning it will regenerate three phase. If one winding burned open and the open delta winding shorted it could backfeed full voltage to the lighting transformer.
If I were on site I would be looking at the size of the load relative to the A/C unit the sizes of the transformers and the type of damage.
I would expect under voltage damage.
If I thought there was over voltage damage then I would suspect a voltage surge as the possible cause of the original outage.
Hope this helps.
respectfully