Hally this mostly depends upon the type of welding taking place and the gases you are using (if you are), it would be prudent to involve your electricity provider, make them aware of your intentions and work out the best solution between you.

Hally, one thing we may be overlooking here is that the substation may be owned by your company, in this case providing you carry out your risk assessment and provide suitable access in case of emergency i believe there should be no other major concerns, i work in the fabrication industry and i can inform you that we weld within a couple of meters of substations & switchrooms, our electrical providers & HV maintenance company have never raised concerns during their inspections. Best of luck.