Several areas in Negros and Panay islands on Tuesday, Oct. 5, experienced lengthy brownouts after power plants in Negros Oriental and Iloilo “tripped”, the National Grid Corporation of the Philippines (NGCP) said.

The brownouts started at around 5:24 a.m. in many areas serviced by different distribution utilities or electric cooperatives, that are dependent on NGCP for their power supply.

NGCP’s first advisory said the power interruptions were caused by Under Frequency Relay activation, meaning the automatic shedding of load when a low supply of power threatens the power system stability.

Michelle Visera, NGCP’s corporate communications officer for Negros Island, in an interview with the Philippine News Agency, said those affected by the power interruptions were areas serviced by the Negros Oriental Electric Cooperative I (NORECO I), Negros Oriental Electric Cooperative II (NORECO II), Central Negros Electric Cooperative, Inc. (CENECO), Northern Negros Electric Cooperative, Inc. (NONECO), and Negros Occidental Electric Cooperative (NOCECO).

She could not speak for Panay Island as it is out of her coverage area but said some parts there were also experiencing power outages.

Visera said the NGCP had to impose manual load shedding to “protect the integrity of their system” while the power plants were still working to fix their problems so they could return to the grid.

By manual load shedding, the NGCP supplies a certain amount of power according to availability from the plants to the different electric cooperatives which then decide how to distribute this and to whom, she said.

She declined to name the power plants that had tripped but NGCP is now investigating the causes behind the tripping.

Many parts of Dumaguete and Negros Oriental, meanwhile, experienced rotating brownouts starting around close to noontime Tuesday.

NORECO II information officer Sharon Araula said they had to also impose rotating power interruptions in their service areas after some of its feeders were already energized.

“Because of the limited power capacity of NGCP, unenergized are Feeder 9 of Bagacay Substation and Feeders 1 and 3 of Pulantubig Substation,” she said.

Feeder 11 of Dauin Substation, Feeders 4, 5, and 6 of Pulantubig Substation, and Siaton Substation have already been energized, restoring power in many areas in its area of coverage, she added.

It was not immediately known how soon power would stabilize and be restored to full capacity in both islands, Visera said.