Some Luzon residents experienced brownout on Monday after the power grid was placed under "red alert" due to unplanned power plant outages, transmission line tripping, and derating of electricity generation of some power facilities.

In an advisory, the National Grid Corporation of the Philippines announced a "red alert" originally from 1:00 p.m. to 4:00 p.m. but was initially extended until 8:00 p.m., which means affected customers may experience rotational brownouts. Before that, a "yellow alert" was placed from 9 a.m to 1:00 p.m. 

However, in the late afternoon, the power transmission operator said the red alert was lifted by 5 p.m., a few hours ahead of its original schedule.

NGCP reported that at least seven electric generating plants were simultaneously forced outages leading to a loss of more than 3,400 megawatts in the power system.

The generating facilities on unscheduled shutdowns included Unit 1 of GNPower Dinginin plant, Calaca-2 plant, Masinloc 2 and 3, Sual 1 and 2 plants, and Quezon Power plant.

The outages were also perpetrated by the derated capacity of the other power plants, which resulted in 226MW additional capacity lost in the Luzon grid.

Pangilinan-backed Manila Electric Company (Meralco), the country's largest power distributor, assured that interruptible load program participants are ready to switch on their generating units to ease the capacity in the main grid anytime.

Last Sunday, the thin power supply in the grid already resulted in brownouts following the tripping of the 500-kilovolt Nagsaag-Bolo lines 1 and 2 of NGCP.

In a separate statement on Monday, Energy Secretary Raphael Perpetuo Lotilla assured that there are no fuel supply constraints despite thin power reserves. 

"We assure the public that the DoE is verifying the cause of the forced outages this morning as there appear to be no fuel supply constraints. A team from the DOE will conduct actual physical spot checks and validate the condition of the transmission lines and the affected power plants," he said. When reserves fall below ideal levels, a yellow alert is placed over the grid. On the other hand, when a supply-demand balance worsens, a red alert, which points to severe power deficiency that may lead to rotating power interruptions, is declared.