Current rating methodology for mass impregnated HVDC cables

Calculating the current rating of HVDC cable systems is less straightforward than for ac cables, which are always thermally limited. While the permissible current in an HVDC cable circuit may be limited by the operating temperature of the dielectric, it may also be restricted by the electric stress within the dielectric. Conventional current rating calculations such as IEC 60287 are thus ill suited to HVDC applications. This paper demonstrates the importance of the electric stress constraint through the application of finite element modeling techniques which permit a coupling of the thermal and electrical properties within the cable. The results obtained demonstrate that changes within the installation environment cause the limiting factor to switch from thermal to electrical under steady state conditions, demonstrating the importance of developing calculation methods which can represent both cases.