Nearly three-quarters of the 74 transmission line outages were caused by fallen trees during a snowstorm that hit the Northeast in October 2011 and shut off the lights for more than 3.2 million homes and businesses, concludes a report released jointly by the Federal Energy Regulatory Commission (FERC) and the North American Electric Reliability Corp. (NERC).

The report follows an inquiry focusing on what caused outages on transmission lines and 44 transmission substations during the storm. It also sought to determine whether particular companies violated FERC-approved reliability standards or other applicable rules.

Of the transmission facilities affected, 24 were bulk power system elements and were necessary for operating the interconnected transmission network. But less than 5% of customer outages at the peak of the storm on Oct. 29 and Oct. 30 were caused by problems along the transmission system, the entities said. The report also says that most of the damage and customer outages were due to impacts to the distribution system (which is generally overseen by state or local entities).

Owners of transmission lines that are operated at voltages of 200 kV and above are currently required by a federal law, passed after the August 2003 Northeastern blackout, to develop transmission vegetation management programs. Those programs must include a schedule for vegetation inspections and specific vegetation clearance distances.

Many of the trees, which were healthy, were located outside utility rights-of-way. They fell onto the lines after being uprooted by the weight of the snow, compounded by the soft, wet ground. The snowstorm blanketed the Northeast with up to two and a half feet of heavy snow, breaking snowfall amounts of all previous October records throughout the Mid-Atlantic and New England regions.

To reduce the adverse impacts of future weather events on the transmission system, FERC and NERC recommend utilities take targeted steps to address off-right-of-way danger trees and employ best practices in managing vegetation on full rights of way.

Sources: POWERnews, NERC, FERC