As Hawaiian Electric Co. officials explained how they brought up power for O'ahu neighborhoods, several lawmakers called for an investigation into Sunday's blackout, which affected 291,000 homes and businesses.

HECO did not play favorites when it decided which neighborhoods to turn on first, company spokeswoman Lynne Unemori said yesterday.

"We know someone is always going to come (online) at the end, and that's a frustrating position to be in," Unemori said.

The decision to power up a neighborhood is not based on where powerful politicians or HECO employees live, as some residents posting comments on honoluluadvertiser.com have suggested.

HECO gave priority to hospitals, airports, reservoirs, sewage plants and refineries, Unemori said. The company also restored power earlier to areas such as Waikiki, where the potential economic impact from an outage is greater, she said.

A team of 15 to 18 top HECO executives and managers made the decisions based on input from Civil Defense and public safety officials, Unemori said.

Sunday's blackout, triggered by two earthquakes with magnitudes of 6.7 and 6.0, knocked out power on O'ahu for most residents for at least 12 hours. Most homes and businesses had power restored by 10 p.m. Sunday. As of last night, there were 14 customers still without power.

State lawmakers yesterday called for the Public Utilities Commission to investigate the power failure.

State Sen. Kalani English, D-6th (E. Maui, Moloka'i, Lana'i), and Sen. Fred Hemmings, R-25th (Kailua, Waimanalo, Hawai'i Kai), said they want to know why the power system failed and how to lessen the impact of future outages.

"We have to take this as a major wake-up call for the people of Hawai'i, for our government and our businesses, and to make sure that when this happens ... we are better prepared," said English, who is chairman of the Senate Committee on Energy, Environment and International Affairs.

The PUC expects to question HECO officials in the next few days.

Hawaiian Electric officials said the shutdown was necessary to prevent severe damage to its power grid and to avoid a more prolonged blackout.

HECO said Sunday's earthquakes caused two generators on O'ahu to shut down, causing a chain reaction that led to the outage. The two generators — 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.

"The system operated to protect itself, and the consequences (of not shutting down) could have resulted in a more prolonged outage that no one would want," said Unemori.

Consumers were skeptical of HECO's explanation for its power outage.

Waikiki resident Edward Conklin wondered why the loss of about 12 percent of HECO's generating capacity on O'ahu could bring down the entire system at a time when demand is typically low.

"Sunday morning has to be one of the lowest points in the week for electricity demand on O'ahu," said Conklin, a retired engineer.

"At this particular time, the system should have had plenty of what's called 'spinning reserve' (generators on but not running at capacity) and easily able to withstand a 12 percent loss," said Conklin.

Sunday's power outage on O'ahu resembled large Mainland blackouts in which a minor problem multiplies into a major power problem.

So called "cascading" blackouts can be triggered by a fallen tree or, as in the case of Sunday's outage, an earthquake-induced shutdown of just two of 13 power generators. While rare, such systemwide electricity shutdowns can happen, according to experts.

Just why the earthquakes, which did not cause any significant structural damage, caused such a major blackout is still unclear. However, the blackouts likely were exacerbated by the state's geographically isolated power grid.

Power outages typically follow large earthquakes, said Dennis Murphy, a spokesman for GE Energy in Atlanta, Ga., which sells power generation and management systems.

"If there's a large enough tremor, (power generators) shut down. That's by design," Murphy said. "You're dealing with something that's the size of a bus that runs with the precision of a wristwatch."

On the Mainland, a similar systemwide shutdown may not have had as significant an impact because electricity can be brought in from other states, Murphy said. That's not an option in Hawai'i.

Sunday's outages resembled cascading blackouts that have occurred sporadically on the Mainland, said James Bushnell, research director for the University of California Energy Institute in Berkeley, Calif.

Cascading blackouts are caused when an electrical grid is unable to handle an unexpected large imbalance between available power and electrical demand.

They often result when sudden network problems cannot be immediately isolated or contained, causing a successive series of power generators to automatically shut down to prevent permanent damage.

"A generator has to take itself out to protect itself," Bushnell said. "Once a power plant shuts down, it can take 12 hours to restart because they have to go through a thorough checklist. If that happens, in some cases it can take several days to restore power."

Among the major cascading power outages in recent years was a major blackout in the Northeastern United States in 2003 that knocked out power to 50 million U.S. and Canadian citizens. Closer to home, an O'ahu-wide blackout in 1991 was caused when a fallen tree took out a power line.

Utility grids are designed with automated systems capable of adjusting to withstand such incidents, Bushnell said. However, as Sunday's blackout illustrates, these systems don't always work as planned.

"It doesn't always work, and it could be quite expensive to make the network so resistant that you never have these kinds of blackouts every five or 10 years," Bushnell said.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com and Sean Hao at shao@honoluluadvertiser.com.