Abstract
Accessories such as joints and terminations represent weak points in HVDC cable systems. The DC field distribution is intimately dependent on the thermal conditions of the accessory and on material properties. Moreover, there is no available method to probe charge distribution in these conditions. In this work, the field distribution in non-stationary conditions, both thermally and electrically, is computed considering crosslinked polyethylene (XLPE) as cable insulation and different insulating materials (silicone, rubber, XLPE) for a 200 kV joint assembled in a same geometry. In the conditions used, i.e., temperatures up to 70 °C, and with the material properties considered, the dielectric time constant appears of the same order or longer than the thermal one and is of several hours. This indicates that both physical phenomena need to be considered for modelling the electric field distribution. Both the radial and the tangential field distributions are analysed, and focus is given on the field distribution under the stress cone on the ground side and near the central deflector on the high voltage side of the joint. We show that the position of the maximum field varies in time in a way that is not easy to anticipate. Under the cone, the smallest tangential field is obtained with the joint insulating material having the highest electrical conductivity. This results from a shift of the field towards the cable insulation in which the geometrical features produce a weaker axial component of the field. At the level of the central deflector, it is clear that the tangential field is higher when the mismatch between the conductivity of the two insulations is larger. In addition, the field grows as a function of time under stress. This work shows the need of precise data on materials conductivity and the need of probing field distribution in 3D.
Highlights
Accessories may represent a weak point in HVDC cable links, especially when moving to ever-higher voltages where the feedback on in-service behaviour is lacking [1,2]
We report mainly on the modelling of the electric field distribution in unsteady situations from the electrical and thermal point of view in cable accessories comprising an association of insulators of different nature in a specific geometry
We have considered three thermal conditions: a homogeneous temperature at T = 30 ◦C, a stationary condition with a thermal gradient due to the injection of a current of 1 kA in the conductor, and an unsteady condition both from an electrical and thermal point of view, keeping the same current flowing in the conductor and considering a homogeneous initial temperature of 30 ◦C
Summary
Accessories may represent a weak point in HVDC cable links, especially when moving to ever-higher voltages where the feedback on in-service behaviour is lacking [1,2]. Methods for probing charge and field distributions in localized areas are lacking and different insulating materials coexisting brings further difficulty. For this reason, thermal and electrical modelling is necessary. I.e., based on field and temperature dependencies of conductivity and permittivity, the numerical resolution of the problem is not a real difficulty. It must be based on reliable experimental data characterizing the materials, especially conductivity, and on the exploration of different practical combinations of thermal and electrical stresses that may be encountered
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