Variable-resistance conductors (VRC) for power-line de-icing

Ice storms can result in accumulation of ice on structures, including overhead power transmission and distribution lines and associated poles and towers; this ice may reach thicknesses of many tens of millimeters. Icing can cause catastrophic damage which disrupts power transmission and is expensive to repair. Normal operation of a power transmission or distribution line entails Joule heating of the conductor as current flows through it. Lines are normally designed to have a constant, low resistance, so as to avoid excessive power losses and avoid excessive operating temperatures. Because the normal heating is low (by design), it is of limited value in preventing or recovering from an icing event. This paper describes power conductors that can switch their electrical resistance from a very low value, to transmit electric energy, to a much higher value, for de-icing. The switching in between two conductor resistances does not disturb the main conductor function, which is to provide a customer with uninterrupted electric power. A variable-resistance conductor (VRC) is built of N strands (or groups of strands) insulated from each other, where N is any odd integer greater than one. For instance, N = 3, 5 or 7, etc. In normal energy-transmission operations all the conductor strands (or strand groups) are connected in parallel, whereas in de-icing mode they all are connected in series. Switching from parallel to series connection increases the line resistance by a large factor of N-2, making the resistance sufficiently high for heating the line above the ice melting point. One important advantage of the method is that it uses low-voltage and, thus, low-cost switches. The design of a VRC de-icing system is described, including considerations for switches, conductors, control and deployment strategy. We also describe safety devices to return the line to normal operation if the electronics get damaged. Laboratory and full-scale prototypes have both successfully demonstrated the capability of VRC de-icing. (C) 2010 Elsevier B.V. All rights reserved.