In the transformation of an existing XLPE cable line from AC operation into DC

the design of DC current rating is one of the key issues which determines the transmission capacity and operation reliability of the transformed line.In this paper

the equivalent thermal circuits and analytical calculation formulas for the same cable line running under AC and DC are compared

the main factors resulting in the difference between AC and DC current rating of a cable line are analyzed

and the AC and DC current ratings of a 10 kV three-core cable in air are tested under same conditions.It is found that for XLPE cables involved in the AC-to-DC transformation

the insulation temperature gradient will not be a limiting condition

and the DC current rating can be calculated according to IEC 60287—2017 based on the requirement for the permitted conductor temperature.Among the main influencing factors

which are the resistance of conductor

the loss of metallic sheath

the number of cores with current carrying

the thermal resistance of surrounding circumstance and the permitted maximum temperature of conductor

the decrease of the conductor temperature from 90℃ under AC to 70℃ under DC is the most unfavorable condition for the cable line

which largely offsets the improving effect of other factors to the DC current rating.A good correspondence is found between the current rating results tested and c alculated

which verifies the validity of the analytical calculation method.According to the AC and DC current ratings analytically calculated for the 10 kV and 35 kV three-core as well as 110 kV single-core cables under typical installation conditions

the DC ratings of the three-core cables increase slightly while those of the single-core cables decrease compared with the AC ones

with the amplitudes of variation no more than 6.5%.