Electric field distribution based on radial nonuniform conductivity in HVDC XLPE cable insulation

The steady-state electric field distribution in a polymeric insulated HVDC cable is determined by the electrical conductivity, which is dependent upon the temperature and electric field in homogeneous cable insulation. In practice, the radial inhomogeneity of the morphological structure and crosslinking byproducts in XLPE insulation will lead to a radial nonuniformity of the conductivity, which may further result in the change of electric field distribution in the insulation. In this paper, the conductivity distribution characteristics in XLPE insulation of HVDC cable are investigated at various temperatures and electric stresses. It is found that the difference of conductivity across the insulation is from several times to two orders in magnitude at given temperature and electric stress. Besides, the standard deviation of conductivity indicates that the difference in magnitude is generally larger at lower temperature. Then, the electric field distribution is analyzed at operating and at test voltages with or without load based on above results. A nonuniformity coefficient C
_E
is introduced to characterize the degree of deviation of the actual electric field distribution from the mean electric stress; a distortion rate δ
_E
characterizes the difference of the field distribution based on the radial nonuniform conductivity from the uniform conductivity. It shows that C
_E
and δ
_E
are larger at no load than at full load. Under zero load at 1.85 U
₀
, C
_E
and δ
_E
can reach 2.26 and 92.25 %, respectively. The results show that the radial nonuniform conductivity has a significant impact on the electric field distribution in the application of HVDC XLPE cable especially with thick insulation. The effect of conductivity with radial nonuniformity on the modification of field distribution should be considered in the development and the qualification tests for HVDC cables.