MANILA (2nd UPDATE) - Electricity has been restored in all parts of Mindanao after a massive power breakdown Thursday morning left millions in the dark.
The National Grid Corporation of the Philippines (NGCP), in a statement, said power transmission service was brought back to normal just past noon.
"All NGCP substations connected to backbone lines in Mindanao were connected to the grid as of 12:18PM," it said.
"NGCP is still determining the cause and will issue a statement when investigation is completed," it added.
The power cuts began before dawn and affected heavily populated areas in Mindanao, home to a quarter of the country's nearly 100 million population.
Power outage was reported in Zamboanga City, Zamboanga del Norte, General Santos City, Davao City, Davao Norte, Davao del Sur, Davao Oriental, Cagayan de Oro, Cotabato, and Compostela Valley.
The NGCP shifted its operations to "island mode" to restore power in the affected areas.
The NGCP said at least 12 of Mindanao's key cities and provinces -- including major trading hubs -- were affected, although limited power was restored in some parts a few hours later.
At around of 9:30 a.m., NGCP said transmission service was restored in Zamboanga City through the Western Mindanao Power Corp.; General Santos City through Southern Philippines Power Corp.; and Davao City through Pulangi IV Hydroelectric Power Plant.
Power has also been restored in Cagayan de Oro, Pagadian City, parts of Misamis Oriental, Misamis Occidental, Lanao del Sur, Lanao del Norte, and some parts of Northern Mindanao, according to the NGCP.
NGCP spokesperson Cynthia Alabanza said it is still investigating the cause and extent of the power interruption.
"At this time, we have no information on the actual cause because the interruption affected the entire grid of Mindanao... Our focus is still on restoration. When we have finished giving back power to Mindanao grid, then we will focus on the investigation to find out what was the cause of the disturbance," she told ANC.
Alabanza said it is unusual for the entire Mindanao grid to be affected, making it difficult to speculate on the cause.
"While a grid disturbance happens, it could be anything from supply or transmission. It could be anything minor or major, it's very difficult to speculate at this point," she said.
Chronic power shortages
Mindanao, the nation's main southern island which relies mostly on hydroelectricity, has been grappling with chronic power shortages for years.
Energy Secretary Jericho Petilla said the outage was likely caused by a tripped transmission line.
An electricity shortage last year forced the NGCP to ration off supply in Mindanao, resulting in up to 12 hour daily blackouts which damaged the local economy as factories slowed production.
The power cuts on Thursday hit islanders just as they were getting ready for work.
"I had to keep using my lighter because the shops were still closed and we didn't have candles," said market vendor and mother-of-two Mary Villasenor from the southern Davao City.
"The kids had to go to school with wrinkled clothes because we couldn't plug in the flat iron," she said. - reports from Roxanne Arevalo, ABS-CBN News Iligan; Lerio Bompat, ABS-CBN News Central Mindanao; and Joel Ramirez, ABS-CBN News Northern Mindanao; ANC; Agence France-Presse