What Caused the Luzon Wide Blackout at the Height of Typhoon “Milenyo”

Anyone might ask why the typhoon “Milenyo” that hit the southern part of Luzon and Metro Manila on Sep. 28, 2006 can cause a Luzon wide blackout since only a portion of the island was ruined by the typhoon.

Though, there were transmission lines, generating power plants, and loads that are not affected by the typhoon, specially, at the areas of northern Luzon, there is one thing in common among grid users in the entire area that can trigger a system wide blackout, the system frequency.

The system frequency of the most transmission grid is 60 Hz. According to the Grid Code,
“The System Operator (TRANSCO) shall maintain the fundamental frequency within the limits of 59.7 Hz and 60.3 Hz during normal condition”.

Allowing the system frequency to exceed this limit can trigger the Automatic Load Dropping (ALD) devices that disconnect a generator or a load to the Grid. However, Grid Code said
“that in case the system frequency momentarily rises to 62.4 Hz or falls to 57.6 Hz, all generating units shall remain in synchronism with the Grid for at least 5 seconds to allow the system operator to under take measures to correct the problem”.

Take note that, a decrease in frequency indicates a shortage of generation, and an increase in frequency indicates an over generation. An increase and decrease of frequency outside of the allowable limit is very destructive for both the generating plants and loads.

What was happened during the passage of the typhoon “Milenyo” in the southern part of Luzon and Bicol area is that it wrecked the transmission lines, disallowing the transport of generated power from the South to Manila, which is the load center. Thus, causing a generation shortage and the system frequency to fall outside the limit for a normal condition. However, as the typhoon passed through Manila, the fallen transmission and distribution wires of Transco and MERALCO caused the tripping of the lines, thus reducing the demand drastically. What suddenly followed was over generation and a surge of system frequency to more than the 62.4 Hz triggering automatic load dropping and isolation of generation plants to the Grid, then, a Luzon wide black out.

One Hour and 15 minutes later, the system restoration was started. People in Santiago City in Isabela was the one who have their lights on first.