Impregnated Cables are cables which have insulation which is impregnated with an insulating material. For instance Paper insulation can be impregnated with oil or synthetic fluid with good dielectric properties. The impregnated insulation is enclosed in a lead sheath to prevent the ingress of water.
Some cables have insulation which requires the insulating compound to be maintained at a particular pressure. This may require the installation of pumps at periodic locations.
Oil impregnated cables have special channels for the flow of oil. The oil which is pressurized fills up any void which may develop in the insulation.
Modern Impregnated Cables do not require pumps or any pressurizing devices.
Oil impregnated cables can have their own cooling systems if used at voltages above 525 kV