Hawaiian Electric Co. executives defended their response to Sunday's earthquakes at a public hearing yesterday, saying the islandwide blackout on O'ahu was unavoidable.

The company provided its most detailed explanation to date of the events that led to the power outage, including a minute by minute chronology of their attempts to prevent the blackout.

Workers thought they were on top of the situation, but were not aware of the severity of the earthquakes' impact. By the time they were, the systemwide outage could not be avoided, HECO said.

"Valiant efforts were made to save the system from complete shutdown," said Tom Joaquin, HECO senior vice president of operations, at the state Public Utilities Commission briefing.

HECO officials said they did the best they could but will investigate further to see if they can improve on Sunday's response.

HECO said it has hired an Idaho-based consulting firm, Power Engineers, to investigate the outage that left 291,000 customers on O'ahu in the dark for hours.

"All of us at HECO apologize and empathize for the inconvenience the Oct. 15 outage caused," said Joaquin, the top HECO executive at the hearing. "We know it was frustrating and sincerely appreciate your understanding."

RESPONDED AS TRAINED

When the first earthquake hit the Big Island at 7:07 a.m., the shaking set off alarms at HECO generators and caused operators to shut down three generators, hoping to prevent damage to the power plants.

"Operator feels building trembling, sees and hears many alarms," HECO wrote in its presentation to the PUC. "Operator responds as trained to initiate a trip (shutdown) of the unit."

Joaquin said the operators "had a potentially serious, serious situation, so they tripped the units appropriately and in accordance with their training."

Ten more generators were shut off automatically following the shaking.

As generators were going down, HECO automatically cut power to some customers to balance the system, a process known as load shedding. If they can cut off customers fast enough, they can keep the active generators from taking on too much stress and shutting down.

NOT ENOUGH POWER

After the first three generators shut down, about 13,000 customers were shed and the system appeared to stabilize at 7:11 a.m.

But then at 7:14 five more generators were shut down either automatically or manually within 24 seconds of each other.

As the five generators went offline, the automated system shed more than 100,000 households, or about 37 percent of HECO's customers, but it was too late.

With only five generators still in operation, down from 13 generators at the time of the first quake, there wasn't enough power and the system crashed.

HECO said it will answer questions from the public at a briefing at the state Capitol on Monday at 2 p.m.

HECO'S RESPONSE TO EARTHQUAKE

7:07:49 — A 6.7-magnitude earthquake hits off the coast of the Big Island (origin time at epicenter).

7:08:23 — Quake felt on O'ahu.

7:08:58 — Operator of a 90-megawatt generator at HECO's Kahe Point plant hears alarms go off and feels the building shake. The operator shuts off the generator.

7:09:08 — Operator at a 55-megawatt generator at HECO's Aloha Tower plant feels the building tremble and also hears alarms go off. He begins to take the system offline.

7:11:01 — HECO's automated system cuts off power to 13,100 customers in wake of the loss of the two generators. The practice, known as "load shedding," stabilizes the system for the time being.

7:12:45 — A 142-megawatt generator at Kahe Point automatically trips off to protect the generator from being damaged.

7:14:02 — HECO's automated system cuts off power to 32,200 more customers.

7:14:04 — A 90-megawatt generator owned by Kalaeloa Partners at Campbell Industrial Park trips off.

7:14:13 — The automated system removes power to 29,700 more customers.

7:14:21 — A 46-megawatt H-Power unit at Campbell shuts down.

7:14:23 — The automated system cuts off electricity to 59,000 more customers.

7:14:23 — A 180-megawatt unit, also at Campbell, goes offline.

7:14:28 — A 142-megawatt generator at Kahe Point shuts down.

7:17:28 to 7:18:30 — A 90-megawatt Kahe generator stops operating while two 90-megawatt and two 60-megawatt generators at HECO's Waiau power plant shut down.

7:14:50 — HECO technicians take 4,700 customers offline manually.

7:27:20 — A 90-megawatt Kahe unit, the last operating generator on the island, shuts off, resulting in an Islandwide blackout.

Source: Hawaiian Electric Co.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com.