Linemen assemble cascade towers to prevent domino effect during storms.
An ice storm ravaged central Nebraska five years ago on Christmas Eve. Then, as line crews were restoring power, another ice storm took out the central portion of Nebraska Public Power District's (NPPD) electric system on New Year's Eve. Rain continued to fall, and the raindrops froze as they hit any surface. It rained for 30 hours straight, and with the temperature at 31°F, it created slick road conditions and wreaked havoc on the utility's infrastructure.
Anything from 115 kV to 345 kV was damaged beyond repair. Steel towers measuring 119 ft crumpled up like tin cans, wood poles were lying on the ground and downed trees were everywhere. NPPD first cleared the highways and railroad tracks. The utility even had to dispatch one of its workers to Grand Island, Nebraska, to help a truck driver, whose vehicle was wrapped up in conductor.
After the wave of initial public concerns, the utility then assessed the damage to the system. To quickly discover the extent of the damage, NPPD hired Hawkeye Helicopter to fly NPPD's field managers over the site. The governor of Nebraska also dispatched the National Guard to assist the utility with a helicopter.
NPPD then determined what it could fix within the next week, and the utility's emergency response team started putting together a plan for reconstruction.
Designing a Cascade Containment Structure
NPPD could have easily lost a 60-mile section of line in that ice storm. The weight of the ice, combined with the strength of the wind, can cause conductors to gallop, putting great strain on structures. Then, once one structure breaks, they often all crash down like dominos.
Fortunately, NPPD had installed cascade mitigation structures with assistance from the Federal Emergency Management Agency (FEMA) to reduce the length of line that was vulnerable to cascade failure.
Fifteen years ago, the utility first started installing these structures every 7 miles to 9 miles on long sections of tangent line in its system. The utility first thought of the idea during a quarterly meeting of the Transmission Line Assessment Program committee. This working group of engineers, control center employees, operators and linemen analyze line conditions, draw up a plan for maintenance and decide where to spend capital dollars.
During the meeting, the group discussed how it could stop lines from cascading and galloping during severe weather events. At that point, the team thought of the idea of a stop-gap structure.
NPPD worked with FEMA to review the design and cost-effectiveness of the structures, and determine a proper distance between them. FEMA representatives then came to the job site to observe the linemen install one of the structures.
From that point on, FEMA required NPPD to take photos every time they installed a new cascade structure. Every year, the utility applies for a Hazard Mitigation Grant to install more of the structures throughout its system. The amount of dollars available is proportional to the amount of emergency assistance declared in a previous year. FEMA also required that this hazard be identified in the utility's Hazard Mitigation Plan before requesting grant assistance.
FEMA is reimbursing the utility for 65% of the cost of the structures through the Storm Mitigation Fund. The installation of the structures is a win-win for the government and for the utility. For example, if NPPD can prevent losing 40 miles or 50 miles per line, it will reduce the amount of necessary federal disaster funds required after an emergency.
NPPD is always looking for ways to cut costs through efficient use of staff and inventory. Over time, the utility reduced the distance between the stop-gap structures to 5 miles to 7 miles. As a result, the reduction affected the number of miles of line that a crew could build or restore in one week's time.
NPPD changed the distance between the structures based on its available manpower and materials. The utility wanted to have enough material in stock and enough crews on hand to quickly restore power within a week's time following a storm.
The utility stores a majority of its materials at the operation center in York, Nebraska. While other locations have a limited maintenance stock of a few poles, crossarms and insulators, this center stores acres and acres of poles and cross-arms. NPPD then ships materials out of its centrally located store room.
Installing Storm Structures
In general, NPPD often replaces downed wood poles with new wood poles. If the field crew is able to build a strong structure from wood and keep the replacement costs down, then they select wood over steel. In some cases, however, the engineering department advises NPPD to install lattice steel structures with heavy foundations. Linemen assemble the five-pole storm mitigation structure. They then install guy anchors. They also tension guy wires in both directions with adjustable grips to adjust the tension as they install the wire. The crew cuts the wire and deadends it both ways. That way, if a structure loses tension on one side, it won't collapse since it also has tension on the other side. This approach prevents the domino effect.
It typically takes a six- or seven-person crew less than a week to build one structure. When the linemen first started installing these cascade mitigation structures, it took more than a week, but now they can do it in about four days because of their familiarity with the design and installation. For many of the linemen, pressing deadends and knowing the tension on the wire is standard line work, so there wasn't much of a learning curve to install these structures.
NPPD is four transmission lines away from installing the cascade structures throughout its entire system. The linemen have already installed more than 230 of the structures across the utility's about 5,000 miles of transmission lines in the last 15 years.
So far, the utility hasn't had any problems with the cascade structures. In fact, a tornado recently tore through NPPD's service territory and initiated a cascade in 2010. The containment structure, however, limited damage to 18 miles of line instead of 45 miles.
NPPD is hardening its system through the installation of cascade containment structures. That way, if Mother Nature hits Nebraska with ice, high winds, snowstorms or tornadoes, the potential damage to NPPD's transmission system will be limited and service will be restored quickly.
Scott Walz (firstname.lastname@example.org) is the transmission line district superintendent for the Nebraska Public Power District. He has been with the utility for 25 years and works out of the York, Nebraska, operations center. This center handles all of the material for the transmission, substation and distribution divisions. Walz started in the industry as an apprentice lineman, then worked as a journeyman and foreman for six years. He was then promoted to a superintendent of distribution until he took on his current position.
Federal Emergency Management Association www.fema.gov
Hawkeye Helicopters | www.hawkeyehelicoper.net
Nebraska Public Power District | www.nppd.com