Z. Y. Huang

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.

Thermal-Electric Rating Method for Mass-Impregnated Paper-Insulated HVDC Cable Circuits

Compared to conventional ac cables, the current rating of HVDC cable circuits is more complicated because the permissible current is not only limited by the operating temperature, but also by the electrical stress within the insulation. Therefore the IEC60287 current rating standard is considered insufficient for some HVDC applications because it fails to include the stress limitation. This paper presents an analytical methodology to calculate an electrical stress-limited rating for HVDC cables, which considers the maximum dielectric stress as the current limiting factor. This paper represents the first part of a project which has comprehensively reviewed cable rating calculations for mass impregnated HVDC cable systems.

Dielectric thermal-mechanical analysis and constrained high voltage DC cable rating

For most mass-impregnated (MI) paper insulated HVDC cables, the dielectric strength is found to be weakened when the cable cools, resulting from the creation of dielectric cavities. To better understand the cavity creation, two mechanisms are frequently compared, based on the imperfect impregnation process and sheath plastic deformation respectively. This paper presents an analytical calculation of cable internal pressure, which is applicable to both cavity creation mechanisms. It demonstrates that the two mechanisms can jointly place constraints on the MI cable rating, by limiting the overall rating-related thermal expansion.

Numerical thermo-mechanical stress analysis for HVDC cables

Calculating the current rating of paper insulated HVDC cables under low ambient temperatures can require additional mechanical considerations. Under rapid cable heating or cooling processes, an extremely high mechanical stress or a rapid pressure drop can develop due to the strong impregnant thermal expansion or contraction respectively. This may cause plastic deformation of the sheath or the creation of voids. This paper demonstrates the importance of this thermo-mechanical constraint through the application of finite element modelling techniques which permit a coupling of the thermal and mechanical properties within the cable. The results show that the FEA technique can be fully applied to analyze the internal thermo-mechanical stress distribution of the cable and calculate the resulting mechanical stress-limited rating, which provides an alternative to an analytical method previously developed by the same author.