Oil-filled, gas-filled, and pipe-type cables have been largely considered obsolete since the 1960s. Such cables are designed to have significant oil flow through the cable. Standard PILC cables are impregnated with oil but the oil is not designed to flow or cool the cable. Oil-filled cables are typically lead-insulated and can be purchased on reels. Pipe-type cables differ from oil-filled cables in that they are installed in a rigid pipe usually made of steel. With pipe-type cables, the pipes are constructed first, and then at a later date, the cable is pulled through. The cable may feature skid wires to prevent damage during the pulling process. The cross-sectional volume of oil in a pipe-type cable is significantly higher than in an oil-filled cable. These pipe-type cables are oil-filled at nominal low, medium, and high pressures. Higher voltages require higher oil pressures to prevent the formation of voids that would allow partial discharges within the cable insulation. Pipe-type cables will typically have a cathodic protection system driven off voltage where an oil-filled cable circuit would not. Pipe-type cable systems are often protected from forming through an asphaltic coating. There are still many of these pipe-type circuits in operation today. However, they have fallen out of favor due to the high front-end cost and massive O+M budget needed to maintain the fleet of pumping plants.
