Current Rating Considerations in Designing HVDC Cable Installations

Ampacity calculations for high voltage direct current (HVDC) cables are more difficult than those for the ac cables because of the absence of suitable analytical formulas. The current carrying capability of such cables is not only limited by the operating temperature, but also by the electrical stress within the insulation. The IEC60287 standard only considers the thermal limitations of the conductor current and, therefore, cannot be used for some HVDC applications because it fails to include the analytical derivations of the stress-limited rating equations. However, applications of the HVDC cables are becoming more common and so there is a need to extend the IEC standard methodology so that ampacity calculations can be performed for such installations. This paper considers various aspects of the HVDC cable design and how they influence cable rating. It focuses on the temperature difference across the electrical insulation and the entire cable as a means of representing the maximum dielectric stress as the current limiting factor when calculating ratings for HVDC cables.