Crews have been working for months to restore the electrical grid.
The cause of the blast remains unknown, but no injuries were reported..
It was not immediately known what caused the fire, although officials said the explosion knocked two other substations offline and caused a total loss of 400 megawatts worth of generation.
San Juan Mayor Carmen Yulin Cruz tweeted an image of a solar light following the blackout. As of around 9:15 p.m., Cruz tweeted that it appeared the fire had been nearly totally extinguished, and there were reports that parts of San Juan were getting electricity back.
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SBI which reported its Q3 earnings on Friday was the top Sensex loser. "Attraction is back into pharma stocks", he added. While the midcap index closed 217 points higher at 16,852 level, the small cap index rose 290 points to 18,463 points.
Around 9 p.m., Cruz stated that areas of San Juan - such as Placita and Santurce - have been getting their power turned back on.
The blackout largely affected the northern part of Puerto Rico, just months after Hurricane Maria cut power to the entire island. In late January, PREPA issued a statement saying more than 450,000 customers remained without electricity.
Puerto Rico Gov. Ricardo Rosselló recently announced that he planned to privatize the state-owned power company, which is $9 billion in debt and relying on infrastructure almost three times older than the industry average. "Of all the realities (hurricanes) Irma and Maria confronted us with, without a doubt the most evident is that Puerto Rico's energy system does not work".
Board president Jose Carrion said Monday at a Puerto Rico investment summit that the power company has been bankrupted by administrators and abused by corruption, and that privatization is needed even when power is fully restored.