With the rapid growth in the demand for large-scale utilization and long-distance transmission of new energy power

high-voltage direct-current (HVDC) cables are developing towards extra-high-voltage direct-current (EHVDC) cables with higher voltage levels and larger transmission capacities. This trend imposes higher requirements on the electrical

thermal

and mechanical properties of cable insulation systems. The development of EHVDC cable system is facing challenges such as higher insulation field strength

more difficult removal of crosslinking byproducts

and higher risk of thermal instability. This article systematically sorts out the key technical challenges that need to be solved and their causes from aspects such as insulation materials

cable insulation systems

and cable test methods. Firstly

the influences of residual byproducts in crosslinked polyethylene (XLPE) on its insulation property and the electric field distortion caused by the radial uneven distribution of byproducts in cable insulation are analyzed

and the control method of byproducts in thick XLPE cable insulation is proposed. Low-crosslinking and non-crosslinked insulation materials can reduce byproduct generation and improve electrical properties

but their thermal deformation resistance is weak. Therefore

the balance between electrical and mechanical properties should be ensured through the examination of high temperature mechanical property tests. Then

at the level of cable insulation system

the hazards and research status of scorch during extrusion are analyzed

emphasizing the importance of the anti-scorch property in the long-term extrusion of cable insulation. For cable joints

the reasons for the decline of electrical property at the insulation interface are clarified

and it is pointed out that attention should be paid to the matching of multi-layer insulation and the interface charge effect. Finally

in terms of the test methods

the problems of thermal stability decline

long prequalification test cycle and new overvoltage impulse in EHVDC cable are discussed. The necessity of introducing thermal stability tests

extended qualification tests

and special temporary overvoltage tests is explained. This research can provide valuable insights for the future development of EHVDC cables with high electrical strength

temperature stability

and mechanical load-bearing capacity.