1.7 EXTRUDED DIELECTRIC POWER CABLES

Theuse of natural and synthetic polymers for industrial applications (during andafter World War II) led to this technology being applied for cable insulation;in contrast to paper-insulated cables, these polymers could be extruded.Natural rubber was used first and synthetic rubber followed; as developmentscontinued, butyl rubber became a material of choice for cable insulation for awhile. Further developments led to newer synthetic elastomeric (rubbery)polymers, such as Neoprene (1931), chloroprene, and Hypalon (1951). The newerpolymers, as they were developed, facilitated improve­ments in processing orproperties (e.g., longer term reliability on aging, or flame resistance) andall were used concurrently depending on the application. Ethylene-propylenepolymers (EPR) were employed in the 1960s as replacement for butyl rubber, buttheir usage, while steady, did not increase until the 1980s. It is to be notedthat improvement in the insulation properties of elastomers was related notonly to the polymer itself but also to the nature of the additives used.

High molecular weight polyethylene (HMWPE), which is not anelastomer, was extrudable and its development (starting in 1941) triggered adramatic change in the insulation of cables for the transmission anddistribution of electrical energy. Thermoplastic polyethylene was actuallyintroduced during World War II for high-frequency cable insulation. By 1947,HMWPE was furnished as 15 kV cable insula­tion. Wide usage began with theadvent of URD systems in the early 1960s. In the mid-1960s, conventional HMWPEwas the material of choice for the rapidly expand­ing URD systems in the UnitedStates. It was superior to butyl rubber for electrical properties and moistureresistance. These cables were used as loop circuits for #2 and #1/0 AWG cables.The domination of polyethylene lasted until the mid-1970s.

Furtherdiscussion of the more modern insulation materials such as HMWPE, cross-linkedpolyethylene (XLPE), and EPR is in Chapter 5, “Electrical InsulationMaterials.”