O'ahu's power grid yesterday suffered its third major electric generator failure since late last year, leaving 37,000 customers across the Island without power for up to four hours.

The incident, in which three generators failed at the same time that five other generators were down for routine maintenance, spurred another call for electricity conservation by Hawaiian Electric Co. in an effort to prevent worse outages.

Energy demand on O'ahu has risen steadily for years but HECO's capacity to produce power hasn't changed since 1992. The generating system is feeling the strain — and that means more unscheduled, emergency maintenance.

"(Generating) units have been working harder, they are getting older," HECO spokesman Jose Dizon said.

Dizon said the "generating margin" — the surplus capacity that keeps power supplied when some generators are not working — is shrinking.

"It's a problem and it's going to get worse," he said. "We can expect to see more forced outages until we can install a new generating unit," he added, referring to a planned $130 million plant to be built at Campbell Industrial Park by 2009.

Three times since November, HECO issued urgent calls for conservation during generator problems. The first two times, customers cut back and blackouts were averted, but not yesterday.

The problem began about noon, when a generating unit at Campbell Industrial Park went down. The plant is operated by Kalaeloa Partners, which sells electricity to Hawaiian Electric Co.

Then about 2 p.m., two generators at HECO's Waiau plant went off line and forced the utility to begin shutting off power to various locations on the island, Dizon said. If power wasn't manually reduced, there could have been more serious problems, including an islandwide outage, Dizon said.

HECO is still investigating why the generators went down.

By 2:15 p.m., nearly 37,000 out of HECO's 291,000 customers were without electricity. The affected communities included Hawai'i Kai, Waimanalo, Kahala, Manoa, Kalihi, Iwilei, Mapunapuna, Pearl City, Waipahu, Kunia, 'Ewa Beach and Makakilo.

Power was restored to all customers by 6:09 p.m.

But that wasn't soon enough for the Highway Inn in Waipahu, which closed yesterday afternoon and missed a usually busy dinner crowd. Power there went out at about 2:15 p.m. and restaurant owners had no idea when it would come back on.

"We tried to call Hawaiian Electric Co. and I was on hold for at least 20 minutes. We had to close because we didn't know when the power was going to come on," said Kinu Toguchi, Highway Inn operations manager. "It was almost three hours until we came back on, but by then we had decided to shut down for the day."

Toguchi said she won't know until today how much food may have spoiled during the outage. Highway Inn runs an adjacent poke shop, where seafood needs to be refrigerated.

Toguchi said the Hawaiian food restaurant remained opened during lunch, but she said the heat inside was "pretty uncomfortable."

IT'S HAPPENED BEFORE

This was the third time since November last year that generator problems led to an urgent call for conservation.

In January, HECO asked customers to cut back after a generator was taken off-line for emergency repairs. Two other generators already were down for scheduled maintenance and a turbine at an independent power plant that sells energy to HECO also was down, cutting energy production from that source.

Customers responded and blackouts were averted.

In November 2005, the utility urged customers to use less power for three straight days when five of its 19 generating units were out of service or operating at reduced capacity.

Officials have said increased demand strains the system and requires more maintenance — including emergency maintenance.

While short-term conservation measures can prevent blackouts, power company officials and environmentalists have said O'ahu must increase its ability to generate electricity — either with a new fossil fuel plant or through alternative energy sources.

Annual energy demand has increased by an average of 2 percent each year on O'ahu for several years and usage during peak periods — between 5 and 9 p.m. weekdays — has increased even more dramatically, power company officials have said.

Yesterday's outage left dark offices and stores, along with downed traffic signals in affected areas, causing backlogs along major thoroughfares across the island.

Dozens of police officers were sent to intersections to direct traffic. But the spreading out of resources did not affect police service, said Capt. Frank Fujii, HPD spokesman.

"Traffic safety will be our primary concern, but we will never allow it to get to the point where public safety is jeopardized," Fujii said. He said district commanders had the ability of extending officers' shifts or calling in officers.

"There were no emergencies that we couldn't handle," he said.

Warren Yamamoto, a traffic engineer with the city's Traffic Control Center, said this was the worst outage he has experienced in nearly 20 years.

"It's been awhile since we've had something this widespread," Yamamoto said. He added that it was difficult to determine how bad traffic was because traffic cameras also were not working during the outage.

CAUSE UNKNOWN

Dizon said HECO is investigating why the units went off line. He said HECO did not believe it had anything to do with yesterday's muggy weather and increased use of electricity.

"We were anticipating higher usage because it was hotter," Dizon said. "Under normal circumstances we had enough power to meet everyone's power requirements and to have that backup."

On Wednesday, HECO generated about 1,170 megawatts during the peak afternoon demand and projected 1,200 megawatts during the same period yesterday. He said March and April generally pull the lowest demand for electricity with September, October and November the highest.

But Dizon said HECO did not anticipate three generators shutting down at about the same time. It didn't help that one unit at the Kahe plant and three others at Waiau were down for routine maintenance.

Once the units went off line, HECO had to manually "shed load" to stabilize the system, Dizon said. He did not have the exact numbers, but he said if 200 megawatts of power were lost, engineers would have had to reduce the same amount of power to customers to avoid major problems and possible damage to the electrical system.

"The operators at the dispatch center determined they needed this many megawatts removed from the system right away. In this case there was no hierarchy on how this was done," Dizon said.

Dizon said at no time did HECO implement rolling blackouts yesterday.

LIVES DISRUPTED

By about 4 p.m., one generating unit was brought back on-line and power was slowly restored to about 17,000 customers in Iwilei, Mapunapuna, Waipahu and parts of Pearl City, 'Ewa Beach and Makakilo. About two hours later, most people had electricity again, Dizon said.

The midday outage disrupted lives across the Island.

Steve Lane, a private investigator who lives in Manoa, said he returned home to find the power out and "the neighbors are all outside saying, 'Where are we going to go for dinner?' We might have sandwiches on the lawn.' "

Kalaeloa Partners provides 208 megawatts of power to HECO, Dizon said. HECO's 19 units have a capacity of about 1,700 megawatts, which he said includes reserve electricity.