Causes for 3 phase 480VAC power surges

[stockman1]

I have a 13.8KV station service feeding a stepdown transformer to 480VAC for a distribution panel. Off of one breaker, the 480VAC feeds into another isolation transformer (480/480); this power goes out to a floating structure.

Question:
Recently, we have noticed the voltage swinging as high as 515-520VAC and have already burned up a few of our welders. What is causing this swing and how do I prevent or minimize these surges?

 

[davidbeach]

Voltage what to where? Single phase or three phase? Grounding?

 

[davidbeach]

Also, transformer connections, all levels?