An explosion and fire at an electric substation threw much of northern Puerto Rico into darkness late Sunday in a setback for the U.S. territory’s efforts to fully restore power more than five months after Hurricane Maria started the longest blackout in U.S. history. The island’s Electric Power Authority said several municipalities were without power, including parts of the capital, San Juan, but they were optimistic it could be restored within a day as they worked to repair a substation that controls voltage. The blast illustrated the challenges of restoring a power grid that was already crumbling before it was devastated by the Category 4 hurricane. In many cases, power workers are repairing equipment that should have long been replaced but remained online due to the power authority’s yearslong financial crisis.