Your breaker seems way too small.
50A @ 480 for a 37.5 kVA delta to wye dry type transformer would be a common primary CB feeding a 125A Main secondary breaker at 208Y120.
You now are reversing this....
Just loading up the coil...which is going to look like a dead short in the first cylce... will take a 50A into trip territory.
Up size your breaker...Your primary now ought to be the big one: 125A. Your Secondary needs to be OCPD at 50A.
Also check for any possible damage to the secondary (480) wiring load side of the transformer.
