that no oxide or gas pockets form under the heat of the welding opera- 

 tion which might weaken the structure of the conducting member. Also, 

 oxygen-free copper can be obtained with high levels of ductility and 

 conductivity. 



Aluminum . Most high-voltage/ampacity ocean or river-bottom cables 

 use copper conductors, although aluminum conductors have, at least in 

 underground cables, been shown to be more cost effective where conductor 

 weight is a critical factor. > Of approximately 260 high-pressure pipe- 

 type cable systems, operating at or above 69 kilovolts and energized be- 

 tween the years 1959 and 1968, only about seven systems can be regarded 

 as strictly underwater-cable types and, of these seven, only one uses 

 aluminum conductors. 3 The one aluminum conductor system, in this group 

 of seven, is a 2.5-mile river crossing. 



In recent years serious attention has been given to the idea of 

 using aluminum conductors in the majority of underground high-power sys- 

 tems; and presumably more and more high-power ocean cables will use alu- 

 minum in place of copper. The ampacities of aluminum-conductor and 

 copper-conductor cable systems are compared in Table III-l.^ Table \Yl-2 

 lists the physical and electrical properties of several conductors, 

 including copper and aluminum. 



Sodium. A third metal, which shows considerable promise for use as 

 the conductor in a high-power ocean cable, is sodium."' 7 '" Its three 

 major advantages are low density (0.97), high flexibility, and low cost 

 (about half the cost of aluminum and one third the cost of copper, on a 

 per-unit-weight basis"'''). Although sodium melts at relative low tempera- 

 tures (97.5°C) and is extremely chemically active (explosively reactive 

 to water if a large surface area of the metal is exposed), it has been 

 found that polyethylene, a commonly used cable insulation, serves as an 

 effective barrier to air and water while simultaneously providing good 

 mechanical strength. 7 Finally, even though the conductivity of sodium 

 is almost a third that of copper, copper is almost seven times as costly 

 as sodium for equivalent ampacity. 



Low Temperature Metals . The idea of operating high-power under- 

 ground cable systems at low temperatures and even in the superconducting 

 state has been examined in great detail in recent years . *■ »"> 10 > 11 There 

 has been considerable progress since about 1962 in the development of 

 high temperature superconductors* and superconducting transmission lines. 

 However, there are still major problem areas requiring a considerable 

 outlay of R&D funds before it will be possible to demonstrate the econo- 

 mic feasibility of superconducting power transmission systems. 1 Analysis 11 

 of a resistive cryogenic cable system (i.e., non-superconducting but 

 operating at very low temperatures, e.g., between 20 and 77°K) has shown 

 that such lines could compete on a cost basis with conventional oil-paper 

 pipe-type cables rated in excess of 1000 megawatts. Practical supercon- 

 ducting ocean-cable systems would probably use superconducting alloys of 



^Highest superconducting temperature is slightly above 20°K. 12 The alloy 

 is niobium/aluminum/germanium. 



20 



