Hawaiian Electric Co.'s investigation into the cause of Friday's islandwide blackout focused on lightning strikes to four transmission lines that may have triggered an automatic shutdown of the company's three major power plants.

HECO's 294,000 O'ahu customers were without power beginning shortly after 6 p.m. Friday. All but 125 customers had been restored by 9 p.m. yesterday.

The islandwide blackout — the company's second in a little more than two years — left many residents wondering why HECO was again unable to contain a power outage to a limited area of the island.

HECO spokeswoman Lynne Unemori said O'ahu's generating units are all connected to the same grid, and when four major transmission lines are knocked out, thousands of customers are left without power. That means the power generated now exceeds demand and causes a dangerous imbalance.

If allowed to continue, the imbalance could cause permanent damage to HECO's network as well as to appliances and electronic goods at the homes of customers who are still online.

"Because of that imbalance, because of that instability, the generating units tripped offline in an effort to keep the system stable," said HECO spokesman Darren Pai. "Unfortunately, the system did shut down."

The power imbalance prompted HECO to cut down on the power it generates to balance the system.

However, the company was not able to reduce power fast enough to resolve the imbalance and maintain power in some areas.

"It is not easily done," said Stan Johnson, manager of situation awareness at the North American Electric Reliability Council. Princeton, N.J.-based NERC is a quasi-regulatory body that develops and enforces reliability standards for the nation's utility industry.

"It's like losing your balance when you're walking on a balancing beam and trying to regain your balance."

HECO yesterday wasn't ready to say lightning was the cause of the blackout.

"We don't have any clear indication that there may have been any type of direct strikes by lightning," Pai said.

Honolulu Police officials told the city's Department of Emergency Management Friday night that they were able to determine that several of the transmission lines had been hit by lightning.

Pai said, "We do know that it is possible that if there was some type of near-miss that could have caused those lines to trip."

HECO officials were able to inspect the affected power lines by helicopter yesterday, and found that the lines were serviceable and could be used to restore power.

The inspection also found damage to a guy wire, or supporting wire, for the Waiau to Ko'olau transmission line, said Unemori.

The company also found a "black spot" on another line, which is prompting further investigation, she said.

The four 138-kilovolt transmission lines involved in the outage are the largest in HECO's grid.

Three of them carry electricity from the Kahe Power Plant toward Halawa and Pearl City, and one connects the Waiau Power Plant in Pearl City to the Ko'olau region.

According to HECO, the Waiau-Ko'olau transmission line tripped at around 6:13 p.m. on Friday. A transmission line connected to the Kahe Power Plant went down 19 minutes later, followed by the shutdowns of two more Kahe lines within four minutes.

At 6:38 p.m., two Waiau units went down followed by the shutdowns of two more units at Kahe and one Waiau generator seven minutes later.

By 6:55, all but one HECO generator, the Kahe 4 turbine, were offline, resulting in the loss of power to more than 90 percent of HECO's customers.