The blast illustrated the challenges of restoring a power grid that was already crumbling before it was devastated by the Category 4 hurricane.
Officials said the fire was quickly extinguished and that the explosion knocked two other substations offline.
Carlos Monroig, an administrator for the press office of the Electric Power Authority, told CBS News that the cause of the explosion was due to mechanical failure.
"We are trying to restore that as quickly as possible," the company said.
Monroig told CBS News that power should be restored by Monday night at the latest.