Chugach Electric Association has determined the point on the grid where a major power outage began Sunday evening, but cannot yet determine what triggered the sequence of events that left tens of thousands of customers from Homer to Fairbanks without power.

The trouble began at 9:09 p.m. Sunday when a 7-foot long string of porcelain insulators shattered on a transmission structure near the exit for Fort Richardson along the Glenn Highway. The insulator string holds one of three wires of a 230,000-volt transmission line away from the tower. When the device broke, the line suffered a phase-to-ground fault. The short circuit de-energized two key segments of the transmission grid and created an imbalance on the electric system between supply and demand.

At that point, system protection designed to keep the grid up and running kicked in and tripped a combination of generators and substations off line. Power dispatchers took additional steps intended to stabilize the system and by 9:25 p.m. the situation was under control. At that point the normally unified Railbelt power grid was electrically separated into three pieces, generators were off line at power plants at Bradley Lake, Anchorage, Eklutna and Beluga, and a significant number of customers of five utilities were without power.

Utility workers began putting the system back together a piece at a time, restoring power to customers as generators were returned to service, breakers closed and lines re-energized. By about 1 a.m. Monday service had been restored to most customers.

Directed by the electronic data collected during events like these, a Chugach crew found the broken insulator string at the transmission tower along the Glenn Highway early Monday morning and replaced it later that day.

By Tuesday, engineers had completed an initial review of the physical evidence and electronic record of the outage, reconstructed the sequence of events and determined that the broken insulator string was the origin of the problem.

Still to be determined is exactly what caused the insulator string to break. There was bad weather across a swath of Southcentral Alaska at the time and some have suggested a lightning strike could have caused the insulator failure. While the National Oceanic and Atmospheric Administration had no record of lightning in the Anchorage area at that time, NOAA could not rule out the possibility. Mechanical failure or another problem may also be responsible.

Chugach will send the broken insulator to a national testing laboratory for analysis.

In the meantime, the broken insulator has been replaced and the electrical system reconfigured to once again create a unified Railbelt power grid.

Chugach is the largest electric utility in Alaska, providing power for Alaskans throughout the Railbelt through retail, wholesale and economy energy sales.