PG&E investigates an all-day Monterey Peninsula outage, and grid vulnerability.

The exact cause of the major outage is still under investigation, but what we know is this: A high-voltage line just outside the Dynegy power plant in Moss Landing came down, the result of a damaged transmission tower. The tower was brand new; it was installed in September and came into service Oct. 13.

This outage is unusual for a few reasons. First, almost all the power outages to homes and businesses are caused by problems with the distribution system, like trees or wind taking our lines – not transmission problems.

“I don’t think you’re likely to ever see this happen again,” says Chris Wadhams, PG&E’s electric planning distribution supervisor for the Central Coast. “Transmission towers don’t [fail]; it’s not something that happens.”

Another thing that makes this outage unusual is that homes and businesses felt the effects of a transmission problem at all. Contingencies are built in to the system, meaning power is usually always flowing from two directions – say, the Moss Landing power plant and a power plant in the Central Valley. If PG&E switches one source for another, customers generally don’t know, Wadhams says: “You may get a blink. It’s almost invisible to the customer.”

To power the Peninsula back up, power was restored circuit by circuit, explaining the scattershot fashion in which the lights came back on Oct. 18, spokesperson Mayra Tostado says.

Each substation delivers power through dozens of circuits, which each serve about 3,000 to 4,000 customers.

“PG&E focuses restoration on the largest number of customers as quickly as possible, and critical facilities like hospitals, water pumping stations and fire and police departments,” Tostado says.

PG&E is bracing for an El Niño winter with beefed-up planning on the distribution side. “PG&E has been getting ready – for the past 18 years,” according to an Oct. 14 press release just days before the Moss Landing outage. (Storms and flooding in the winter of 1997-98 contributed to outages affecting more than a million customers in Central and Northern California.)

Since then, PG&E created an entire emergency preparation department, and built two new distribution control centers that guide the flow of power through some 140,000 circuit miles of distribution lines.

But when the problem happens in a transmission line, no amount of distribution preparedness can guarantee power will be back up and running quickly.

“If a big transmission line goes out, it’s just like shutting down a major highway,” says Steven Greenlee, spokesman for California Independent System Operator, effectively the middleman between power plants and utilities, making sure power gets safely delivered to PG&E. “It’s one big machine. Sometimes it just breaks.”