Updates with Luma statement in paragraphs 4-6

By Scott DiSavino and Laila Kearney

April 17 (Reuters) - Nearly half of the homes and businesses in Puerto Rico that receive electricity from the commonwealth's main utility were still without power on Thursday, a day after a widespread blackout struck the island, Luma Energy said in a statement.

Hospitals, airports and prisons were among the structures still out.

Puerto Rico's electrical system has continued to falter since two powerful hurricanes devastated Puerto Rico in 2017, decimating the grid and killing nearly 3,000 people, according to official estimates. On New Year's Eve last year an underground power line was the most recent failure that led to island-wide blackouts.

As of 6 p.m. EST on Thursday, 844,973 Luma customers, or 57.6% of its total, had power restored.

"We continue on track with our initial projection of  restoring service to at least 90% of customers in the next 48 hours, conditions permitting and generation is available," the company said in a statement.

It also added that some customers who have had service restored may experience temporary system outages throughout the day, and possibly tomorrow.

This time, a power line is also believed to have triggered outages.

Following overnight aerial patrols with helicopters, Luma said a preliminary analysis suggested a number of factors had caused the power outage.

It cited a protection system failure as the trigger, followed by the presence of vegetation on a transmission line between Cambalache and Manatí in the north of the island.

"As part of our response efforts, we are investigating the cause of this incident, including what role and effect the long-recognized impact of the fragility of the system had on this island-wide outage," Luma said.

Luma, which started operating the Puerto Rico power grid in 2021, is a joint venture between units of Canadian energy firm ATCO ACOx.TO and U.S. construction and engineering firm Quanta Services PWR.N.

(Reporting by Scott DiSavino and Laila Kearney in New York and Noel John and Anmol Choubey in Bengaluru; Editing by Ros Russell, Barbara Lewis and Sandra Maler)

((Laila.kearney@thomsonreuters.com))