The Department of Energy (DOE) is investigating the Mindanao-wide power outage at 1 a.m. Sunday, starting with the state-run Agus-Pulangi hydroelectric power complex in Lanao del Sur province.
Investigators are tracing the events from the "first tripping" at the Agus 6 and Agus 7 areas, Energy Secretary Carlos Jericho Petilla said in a text message.
Whether the outage was caused by power plants shutting down on their own or by an "external event" will be determined in the investigation, he said.
The privately run 210-megawatt (MW) Mindanao coal-fired power plant is operating, Steag State Power Inc. spokesperson Jerome Soldevilla said in a text message. The plant is the largest connected to the Mindanao grid.
Transmission services were restored at 7:50 a.m., National Grid Corp. of the Philippines (NGCP) said earlier.
NGCP spokesperson Cynthia Alabanza said in a text message that all transmission substations and plants connected to backbone lines in Mindanao were "synchronized to the grid as of 7:50 a.m." That means all are running and connected to the main transmission lines.
Household-level electricity supply now depends on the power distribution firms and electricity cooperatives.
They are now "normalizing" operations, Alabanza said.
"NGCP is still determining the cause and will issue a statement when investigation is completed," she said.
The last Mindanao-wide power outage was on Feb. 27, 2014. Initial reports then said the interruption was caused by the tripping of several power plants in Mindanao that started with the state-owned Agus 1 in Marawi City, Lanao del Sur.
It was later found that the 210-MW Mindanao coal-fired power plant had an "unwanted loss" of generation output (to 160 MW from 210 MW) just as there was a surge in electricity demand.
The antiquated Agus 1 hydroelectric power plant was unable to handle the combination of events and tripped, according to investigators.
Sunday's power outage took place although Mindanao's peak demand for the day was 1,249 megawatts against the prevailing system capacity of 1,350 megawatts, NGCP said on its website.
Five hours after power was restored in the affected areas, including Davao City and the four Davao provinces, Compostela Valley, the whole of Central Mindanao, the Agusan and Surigao provinces, Western and Northern Mindanao, the NGCP said it was still investigating.
Only the islands of Siargao and Basilan were not affected as power providers there use generators for their consumers.
Although Alabanza said that all transmission substations and plants had been reconnected to backbone lines in Mindanao and were synchronized to the grid as of 7:50 a.m., consumers in such areas as Davao del Sur said power was fully restored past 8 a.m.
In Davao City, power has yet to be fully restored, according to an advisory issued by Davao Light and Power Co.
In North Cotabato province, the outage failed to dampen the Easter celebrations.
The Catholic faithful in the Diocese of Kidapawan City had to light candles though and used megaphones for the traditional "salubong."
Bambi Capulong, NGCP communications officer for Mindanao, said teams of linemen were immediately dispatched to investigate the cause of the power interruption.
"We are still determining the cause and extent of the problem. Root cause is still under investigation," Capulong said in a text message to the Inquirer.
"NGCP is working to restore power services to affected customers in Mindanao," Capulong added.
In recent weeks, NGCP said the power situation in Mindanao was worsening with the decreasing water level at the hydropower plants in Lanao and Bukidnon, which account for the bulk of the island's power supply because of the dry spell being experienced since early this year.