DUCTILITY FOR BRITTLE TRANSMISSION TOWERS

Many electric power transmission towers are designed with a relatively brittle lattice steel structure that is prone to failure when subjected to extraordinary single dynamic events caused by storms, wind, tree strike, ice load shedding, tornadoes, aircraft impact, stringing accidents, and vandalism. "Brittle is an odd way to describe a mild steel tower," explained Dr. Paul Ibanez of ANCO, "but this term refers to the transmission system as a whole, because tower systems are often designed to rely on tension of the power line to provide lateral and torsional stability. When this is lost, or extraordinary peak loads are experienced, the tower has only a few inches of ductility before catastrophic failure occurs. Several feet of ductility are needed to prevent tower failure." In the worst scenarios, miles of towers have been lost in a domino effect emanating from a single event.

ANCO DEVELOPS A LOAD LIMITER

Working with the Bonneville Power Administration and the Department of Energy (under the SBIR Program), ANCO has developed and performed full-scale tests on a Transmission Line Load Limiter (TL3) to provide existing and future tower designs with much needed ductility. The TL3, consists of a double flat dual helix that is placed between the tower and the insulator string. The TL3 is constructed from galvanized flame cut carbon steel and is designed to meet the following criteria: simple and rugged design necessary for line hardware, compact construction to avoid affecting tower design on retrofit, and very cost efficient to manufacture.

In a typical configuration, the TL3 weighs 40 kilograms (88 pounds) and is 15 centimeters (6 inches) long. The TL3 behaves as a rigid link under normal loads. In the event of a dynamic load that could fail the tower or tower arm, the TL3 extends plastically up to 2 meters (7 feet) to absorb the shock energy and limit the force applied to the tower or adjacent towers. The properties of the device can easily be adjusted by varying the thickness and material of the plates, width of the helix, and the preset force. The TL3 can also be locked out with a simple bolt to facilitate installation and tower servicing without activating the device.

PROOF TESTING AT THE EPRI TLMRF

Working with the Electric Power Research Institute tower test line at its Transmission Line Mechanical Research Facility (TLMRF) near Fort Worth, Texas, ANCO tested the TL3 on typical 345 kV towers with five Single Bluebird conductors and 305 meter (1,016 feet) spans. Broken conductor tests were made on the center and end spans of a nine tower line, with and without TL3 units installed (see photograph above). Without the TL3, the peak conductor force experienced was 150% of the insulator string dead weight load, sufficient to cause significant damage (though not collapse) to two towers. With the TL3 in place, the peak force was reduced to 50% of this value and no damage occurred; demonstrating the usefulness of the TL3 in providing significant additional margin to transmission lines experiencing failures.

COMMERCIALIZATION SOUGHT

Dr. Ibanez, the co-inventor of the TL3, indicated that ANCO has received a patent on the transmission line load limiter and is seeking to work with EPRI, line hardware manufacturers, and electric utilities to commercialize the device. He added that, "the ability to absorb energy adds a significant advantage to slip clamps, shear bolts, breakaway arms, and other approaches proposed in the past. In addition, the installation of the device to existing structures appears to be a more positive and less costly approach than replacing or reinforcing the structures."