Damage to transmission system played minor role in 2011 Northeast blackout

Damage to the power grid caused by the late October 2011 Nor’easter was concentrated on the region’s distribution system; damage to the area’s transmission system contributed to a small percentage of the outages, according to the joint FERC/NERC report on the storm issued May 31.

The report says the storm that dumped record amounts of snow on the Northeast region of the United States left more than 3.2 million homes and businesses without power. However, the report says less than 5% of customer outages were attributable to damage to the bulk power system.

The report said 74 transmission lines of 100-kV or higher and 44 transmission substations experienced sustained outages of 10 minutes or more. Of those facilities, 23 lines and one substation are considered elements of the bulk power system.

At the storm’s peak, transmission line outages were responsible for approximately 130,000 customer outages, according to the report. Most of those outages lasted for less than two days, and none lasted for more than five days. Across the region, thousands of customers were without power for more than a week, and some for as long as 11 days.

Nearly three-quarters of all of the transmission outages were caused when trees weighed down with heavy, wet snow fell onto transmission lines.

Unlike the vast majority of NERC reliability standards, which apply to all bulk power system facilities (generally, those operated at or above 100-kV), NERC’s vegetation management standard applies only to bulk power system “transmission lines operated at 200-kV and above and to any lower voltage lines designated by [a NERC regional entity] as critical to the reliability of the electric system,” the report said.

That means that the standard does not apply to lines operated at voltages under 200-kV in the Northeast Power Coordinating Council (NPCC) region unless NPCC has designated those lines as “critical” under the standard. NPCC did not designate any lines with voltages under 200-kV as critical.

In fact, NERC’s vegetation management standard applied to only one transmission facility forced out of service due to vegetation contact — a 345-kV transmission line in Connecticut — and that line outage did not cause any loss of service to customers. However, the standard’s applicability to the October event is limited because it does not specifically address off-right-of-way vegetation management, according to the report.

The report makes six recommendations to help reduce the adverse impacts of future, similar weather events on the transmission system.

Because off-right-of-way tree fall-ins accounted for about half of the storm’s transmission line outages and nearly 75% of all confirmed vegetation-caused outages, the report recommends that utilities analyze their transmission systems and identify danger trees that could have an impact on critical transmission lines — particularly those species of trees that have a tendency to fail.

The report also recommends that utilities employ best practices in managing vegetation on full rights-of-way.

Noting that “preventing fall-ins from both inside and outside the right-of-way is easier if utilities consider vegetation management needs when siting new transmission lines,” the report recommends that utilities carefully assess vegetation and growth rates in the area of planned lines in order to establish the appropriate right-of-way width when siting new transmission lines.

Utilities should evaluate and enhance their storm preparedness and response plans, the report recommends.

To ensure that regulators have sufficient information to allow them to make informed policy decisions about vegetation-caused outages, the report recommends that all tree contact-caused bulk power system facility outages be reported to NERC, including those that are not required to be reported under the NERC standard.

Finally, the report recommends that utilities improve the content of required disturbance reports, providing “thorough, descriptive, high-quality information in the initial reports as it becomes available to them.” Utilities should also ensure their responses are comprehensive, providing all the relevant information in their possession, when filing their final disturbance reports.