An unplanned shutdown of two coal power plants triggered a yellow alert across Luzon on Wednesday — the third time this month a power alert was raised due to supply constraints.

In a notice, the National Grid Corporation of the Philippines (NGCP), the country’s sole transmission system operator, raised yellow alerts on the main island from 2 to 4 p.m. and from 5 to 10 p.m.

NGCP said the alert was prompted by the tripping of Unit 2 of the Sual Power Plant in Pangasinan and the GNPower Dinginin plant in Bataan due to boiler tube leaks.

Apart from this, seven other plants were on forced outage from January to May, and 10 other plants from June to July.

Meanwhile, five plants are running on derated capacities. In total, the outages slashed 3,017.9 megawatts (MW) from the grid.

NGCP data yesterday showed available capacity in Luzon was 12,969 MW, while peak demand was likely to hit 11,768 MW, leaving only 1,201 MW of a supply margin.

The Visayas and Mindanao grids, on the other hand, maintained normal conditions.

Last 17 and 18 July, the Luzon grid also entered a yellow alert status caused by the tripping of the gas-fired power plant, among others.

A yellow alert is issued when the operating margin is insufficient to meet the transmission grid’s regulating and contingency requirements.

When this status is in place, all power plants must run normally, otherwise a red alert may be raised.