A process for producing an energy cable including at least one electrically conductive core and at least one thermoplastic electrically insulating layer, includes the steps of: impregnating a thermoplastic material in subdivided solid form, having a melting enthalpy equal to or lower than 70 J/g, with a dielectric fluid to obtain an impregnated thermoplastic material; feeding the impregnated thermoplastic material in subdivided solid form to a single-screw extruder; and extruding the impregnated thermoplastic material onto the at least one electrically conductive core, so as to form the at least one thermoplastic electrically insulating layer, whereby the impregnated thermoplastic material is not subjected to any mechanical homogenization step in a molten state. Energy cables having a large amount of the dielectric fluid in the electrically insulting layer, e.g. higher than 10 wt %, are obtained without showing any morphological defects in the layer itself and any drawbacks in the extrusion process, even when the rotation speed of the extruder screw, and therefore, the cable production speed, are high (e.g. higher than 20 m/min for medium voltage cable).