Lightning hit the O'ahu power grid at least five times in a span of 20 minutes on Dec. 26, leading to an islandwide blackout, Hawaiian Electric Co. officials said yesterday.

HECO officials said one of the strikes was so powerful that it not only knocked out a major transmission line but also caused a severe drop in voltage.

The drop in voltage led to the shutting down of seven power generators at five different power plants nearly simultaneously.

"What happened is a set of severe and unusual lightning strikes hit our system, caused short circuits on a transmission system and caused instability in the system," said Robbie Alm, HECO executive vice president. "That instability caused a series of generation plant shutdowns and the eventual islandwide shutdown."

Yesterday, HECO officials provided the local media with the first in-depth look at the cause of the Dec. 26 outage, which left most of the company's 294,000 customers in the dark for more than 12 hours, disrupted traffic throughout O'ahu and caused retailers to shut down early during one of the busiest shopping days of the year.

The outage drew international attention in part because President-elect Barack Obama and his family were vacationing on the island at the time, and their beachfront home was left without power.

PRELIMINARY FINDINGS

HECO officials said their findings are preliminary and that they will hire Mainland consulting firms to conduct a more in-depth study. The state Public Utilities Commission and state Consumer Advocate Catherine Awakuni are also likely to conduct separate investigations.

State lawmakers will hold hearings next week.

One energy expert expressed concern that a lightning storm could knock out an entire system.

Typically, a system should be designed with enough protections such as shield wires to protect transmission lines and lightning arrestors to protect substations and transformers should lightning hit, said Gordon Weil, a Maine-based energy consultant and author of "Blackout," which described some of the causes of the North America power outage in 2003.

Weil said that the electrical utilities in the Midwest and Florida, where lightning strikes are prevalent, are equipped with such protective devices.

"You shouldn't have a total collapse" as a result of lightning, Weil said. "I'm not prepared to say it's impossible but it's very unusual."

HECO has said that its systems are protected by lightning arrestors and shield wires, but the type of lightning that hit on Dec. 26 and the frequency of the strikes on the company's system was unusual.

HIT TWICE BY LIGHTNING

According to Alm, HECO technicians found that at least four of the company's major transmission lines experienced short-circuits as a result of lightning strikes, and that one of the lines was hit twice.

The affected lines included a 138 kilovolt line connecting HECO's two main power plants at Kahe Point and Waiau in Pearl City, which was hit by a bolt of lightning around 6:35 p.m., according to Alm.

Normally, lightning hits one or two of the three wires that make up that transmission line, which results in a drop-off of power lasting a few seconds. But in this case, the lightning hit all three wires, Alm said.

DROP IN VOLTAGE

The strike not only caused the Waiau-Kahe transmission line to go down but the corresponding drop in voltage in the lines triggered safety mechanisms at the Waiau and Kahe power plants to begin shutting down generators.

The downed transmission line also triggered safety mechanisms at power plants operated by independent producers AES Hawaii Inc., HPower and Kalaeloa Partners LP, which tripped within minutes of the Waiau and Kahe shutdowns, Alm said.

Stan Johnson, manager of situation awareness at Princeton, N.J.-based North American Electric Reliability Council, said it's highly unusual for lightning to strike all three wires that make up a transmission line.

But when it does, it can create a tremendous amount of instability to the grid and the power generation system, said Johnson, whose organization is a quasi-regulatory body that develops and enforces reliability standards for the nation's utility industry.

"It looks like Mother Nature threw some strong lightning your way," he said.

AIR-TO-GROUND STRIKES

When asked why HECO couldn't isolate, or "island" its Waiau and Kahe plants to save the rest of its electrical system, Alm said the problem was not isolated to those two power plants. The affect of the trip on the Waiau to Kahe Power Plant transmission line caused trips to the independent power producers within minutes, he said.

"When those seven units fail, it doesn't matter what else you do after that," Alm said.

Alm added that the type of lightning that struck on Dec. 26 was extremely rare for Hawai'i. Typically, the lightning strikes occur high in the atmosphere and travel from cloud to cloud, he said.

But this time, the lightning traveled from the air to the ground, he said. That kind of air-to-ground lightning is more common on the Mainland, Alm said.

One HECO technician told the company he witnessed two bolts of lightning joining in mid-air into a massive bolt that struck the ground somewhere near Pearl City.

"I'm 57 years old and I've lived pretty much my entire life here, and I can never remember air-to-ground lightning like that," Alm said.