to stop the inverter if this becomes necessary. The static capacitors are 

 pressure-compensated and are mounted outside the hull. Weights, volumes, 

 and dimensions were not given for the shore plant, since these factors are not 

 critical for this facility. 



CABLE SYSTEM 



The primary consideration in the selection of a cable system for use 

 in an underwater power transmission system is the reliability of its elements. 

 The cable, connectors, and support system must withstand the ocean 

 environment — that is, must be resistant to corrosion, erosion, marine fouling, 

 and water absorption — for the useful life of the power transmission system. 



Cable Conductor Material 



Two basic conductor materials were considered: aluminum and 

 copper. The conductivity of aluminum is 60% that of copper and would, 

 therefore, require a cross-sectional area approximately 1.6 times larger than 

 copper for the same current-carrying capacity. The tensile strength of 

 aluminum is 24,000 to 29,000 psi and that of copper 50,000 to 70,000 psi. 

 The modulus of elasticity of aluminum is 10 x 10 and that of copper 

 17 X 10 . Aluminum has a high contact resistance due to the oxidation of 

 its surface and exhibits a tendency to "creep" because of its high coefficient 

 of thermal expansion. Because of these qualities, aluminum was eliminated 

 as a conductor material from the study program. 



Cable Insulation Material 



The primary factors used in determining the selection of insulation 

 material for underwater cables are voltage stability and life, resistance to 

 moisture, resistance to temperature, dielectric properties, and flexibility. 

 Resistance to ionization or corona and mechanical strength are of secondary 

 importance. 



Of the available materials, polyethylene combines excellent electrical 

 characteristics with outstanding stability of both electrical and physical prop- 

 erties during long periods of exposure in seawater. Rubber and rubber-like 

 insulation materials require tinned conductors to prevent a detrimental 

 chemical reaction between the insulation and conductor. Rubber materials 

 have a higher water absorption rate than polyethylene. Varnished cambric 

 and impregnated papers have a long life, but require a continuous lead covering 

 for underwater use. Lead coverings would make the total weight of the cable 

 impractical for this installation. Polyethylene was, therefore, selected for the 

 cable insulation material. 



50 



