Hawaiian Electric Co. officials said all 291,000 O'ahu customers lost power after yesterday's earthquakes because an automated system shut down the grid to prevent severe damage.

HECO said the earthquakes, one with a magnitude of 6.6, caused two generators on O'ahu to shut down. The two — one at Kahe Point and the other at Aloha Tower Downtown — accounted for about 12 percent of the generating capacity of the system at the time.

Once those generators stopped producing, HECO's automated system started shutting off power to customers, so demand would not overwhelm the 11 generators that were still working.

HECO could not say last night why the automated system needed to shut down power to the entire island 18 minutes after the two generators went off line, instead of just shutting off just 12 percent of the island.

Once the power was off, it took HECO more than 12 hours to restore electricity to most of O'ahu.

"We appreciate everyone's patience, and we know it is frustrating. We're doing it carefully and slowly because of the need to ensure that we do not create outages that are longer term," said HECO spokeswoman Lynne Unemori.

"That is why we have to do it very slowly and methodically. You don't want to create imbalances and end up with problems that are longer term," said Unemori.

HECO said it had to bring neighborhoods back incrementally to avoid overloading their circuits and causing a more prolonged blackout.

As of 10 last night, about 46 percent of O'ahu customers were still without power, while electricity had been restored to most of the Neighbor Island areas hit by outages.

T.C. Cheng, a University of Southern California electrical engineering professor with expertise in power systems, said he wasn't surprised that O'ahu's power was down for most of the day while service was restored more quickly on the other islands.

Cheng noted that New York experienced a citywide outage in the 1960s, and it took five days to get that system back online. The overall technology hasn't changed much since, said Cheng.

"The bigger (the system), the worse it could be," said Cheng, who noted that Hawai'i, unlike California and other Mainland states, cannot tap nearby states for their power needs.

Although the utility was hoping to have all power restored by last night, there could be pockets of the island without electricity for longer periods, and the utility would have to check those areas to find out why, HECO officials said.

The utility also had to manually disconnect circuits of customers hooked to the large transmission lines to prevent damage as the system was brought back online, HECO officials said.

Neighborhoods closer to the main power plants at Waiau, Kahe Point and Aloha Tower were restored first because of their proximity to those plants, Unemori said.

The company then began restoring electricity to more distant neighborhoods.

Yesterday's power outage knocked out street lights and shut down grocery stores, retailers and gasoline pumps. Heavy call volume also clogged O'ahu's wireless networkings, making it difficult to place cell phone calls.

In explaining why O'ahu customers had to wait longer than Neighbor Islanders to get power restored, Unemori and company spokesman Jose Dizon said O'ahu's system relies on larger, more complicated generating units that take longer to return to normal operations than the smaller, diesel-powered ones on the other islands.

If generators are brought back online too quickly and something happens, the utility would have to start the process all over again, they said.

"It could cause damage that can take weeks or months to fix," said Dizon.

According to Unemori, the power failure occurred when earthquakes knocked generators off line at HECO's Downtown and Kahe power plants.

HECO's computerized system immediately began to shut down customers. The imbalance caused by the downed generators prompted HECO's computer system to began a process of automatically shutting off customers to try to match demand with the remaining generating capacity.

When the shutting off of customers failed to adequately reduce demand, the system started automatically shutting off other generators to prevent damage to them. That process was repeated several times until all of O'ahu was without power at 7:27 a.m.

"This is a situation where the system worked as designed," Unemori said.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com and Rob Perez at rperez@honoluluadvertiser.com.