In the present-day network of high-voltage overhead transmission lines, segments of cable are joined by a crimp splice joint, in which the locking of strands in the two cables relies upon a serrate sleeve that is crushed into the cable. The transmission line heats in temperature as it carries electric power, and the span of cable between towers stretches each time the temperature of the cable is increased. Repeated cycling to higher temperature in a transmission line that carries electric power with a day/night cyclic load fatigues the friction grip of the crimp joint, and it is doomed to fail after a large number of such cycles. The problem is made worse when a transmission line is required to carry an overload, either because of growing electric demand in an area or because of a temporary grid overload in a service region. Failures of splice joints are a recognized problem for grid security. Accelerator Technology Corp. (ATC) proposes to develop a cable splice technology that employs friction stir-welding (FSW) to securely bond the aluminum wires of two cable ends to one another. FSW joins the aluminum wires and an aluminum sheath together to make a metallurgical bond, and the bond exhibits excellent electrical conductance and purging of oxidation from the metal. The weld is made without heating the cable, without need for cover gas. The FSW splice is stronger than the cable that it connects, and is not prone to robust against temperature cycling, even to temperature much greater than can be used with conventional splices. The FSW splice is also robust against corrosion and wind-driven oscillations. In the Phase 1 effort the FSW process will be optimized to address two challenges that are unique to the cable splice application: the void space among strands must be crushed by compression before welding, and the dynamics of the rotary friction stir must be optimized to consolidate all remaining void space into two regions, so that the build of the weld is a solid strong aluminum bar. The effort will include subcontract efforts at Texas A&M University and at MTI, the most experienced company in FSW technology. ATC is undertaking the development in partnership with two utilities, one in Texas and one in Ohio, who are interested in placing the new splices into first use. The goal of the development is a splice that costs the same and takes the same time to install as conventional splices. But the FSW splice would be stronger than the cable it joins and would be more tolerant of occasional overloads on the transmission line. The benefits of the FSW cable splice are to eliminate the splice joint as the weak-sister of a transmission line. The FSW splice will make it the last element to fail instead of the first. That improvement would reduce the vulnerability of the electric grid to line failures when a transmission line is operated above rated capacity, either because of growing electric demand or occasional grid overloads.
Keywords: electric transmission line, cable splices, grid overloads, blackouts.
