Ethylene Propylene Rubber and Crosslinked Polyethylene as Insulations for 90°C Rated Medium Voltage Cables

Amorphous ethylene propylene rubber insulated power cables are expected to be more stable under high temperature conditions than crystalline crosslinked polyethylene insulated cables. The properties of amorphous EP rubbers are little affected by temperature from −30 to 150°C; those of crystalline XLPE fall off as temperature exceeds the melting point (around 100°C) of the crystalline regions. Probably reinforced amorphous EP rubber insulated cables are expected to be more stable in wet locations than unfilled XLPE insulated cables. They do not fail by the formation of water trees as the primary mechanism, since the bonds between filler and polymer are stronger than the forces responsible for water trees forming in XLPE cables in service. The reinforcing effect of fillers in EP rubber makes questionable any extrapolation from high stress treeing studies of unreinforced PE, XLPE, or EP polymers. For the longest possible life in wet locations insulation should be protected by a water impermeable metallic sheath, which must be protected against corrosion, mechanical damage, fatigue cracks, etc. Any insulation will eventually fail on continued exposure to water and electrical stress.