Extruded cables for high voltage direct current transmission : advances in research and development Fundamentals of HVDC Cable Transmission Main Principles of HVDC Extruded Cable Design Space Charge in HVDC Extruded Insulation: Storage Effects and Measurement Methods Improved Design of HVDC Extruded Cable Systems Life Modeling of HVDC Extruded Cable Insulation Main Realizations of HVDC Extruded Cable Systems in the World