Electric Field Distribution in HVDC Cable Joint in Non-Stationary Conditions

Abstract

Accessories such as joints and terminations represent a weak point in HVDC cable systems. Indeed, the DC field distribution is intimately dependent on the thermal condition of the accessory and on material properties. Moreover, there is no available method to probe charge distribution in those conditions. In this work, the field distribution in non-stationary conditions, both thermally and electrically, is computed considering different insulating materials assembled in a same geometry, with focus on the tangential field distribution. We show that the position of the maximum field varies in time in a way that is not easy to anticipate. The work points to the need of precise data on materials conductivity and to the need to probe field distribution in 3D.

Targeted Thermal and Electrical Properties of Rubber Materials for HVDC Cable Accessories

“…The field spreads more along the interface, in such a way that the maximum tangential field does not exceed 3 kV/mm in steady state. The shape of the profiles approaches those found when considering a full joint made of XLPE [4]. The trend could be expected considering the parallel behavior of the electrical conductivities at low field and high temperature for XLPE and EPDM-Case 3, cf.…”

“…3. The same feature holds in any thermal condition for a conductivity larger than that of XLPE and nearly field-independent [4].…”

“…Its variation is more difficult to anticipate than the radial field. We have shown that the axial contribution of the field under the stress cone is significantly reduced when the electrical conductivity of the joint increases [3], [4]. It seems due to a move of the field into the cable insulation followed by a redirection of the field lines to the radial direction.…”

“…Already, the XLPE conductivity is substantially nonlinear. The laws previously considered in [3], [4] were with lower non-linearity. The steeper-than XLPE non-linearity as used in Case 3 does not provide great improvement in the field distribution, compared to pure XLPE.…”

“…Fig. 1) obtained under nonstationary electrical conditions and different thermal conditions considering known properties of EPDM and SiR and HVDCgrade cross-linked polyethylene (XLPE) as cable insulation [3], [4]. The field distribution at short stressing time is governed by the permittivity of the insulations, while at long time it is controlled by the electrical conductivity which is dependent on temperature and electric field.…”