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

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.

Insulating Materials for HVDC Cable Accessories: Effects on the Electric Field in Nonstationary Situations

“…For that, finite element modelling is used and the accuracy of the modelling is driven primarily by the correctness of conductivity data provided to the model. Several recent works treat modelling of the field distribution in joints, including the transient states [5] [17] [28]. Also, modelling can be used to determine optimal properties of materials for field homogenization, being insulations or field grading layers [29].…”

“…Figure 4 provides a cross-section of the joint model used in the present simulations. Details of the joint construction and limit conditions are given elsewhere [28]. The field distributions were computed considering 3 associations of materials: XLPE/EPDM, XLPE/SiR and XLPE/XLPE, the latter being representative of an interface-free joint.…”

“…On the cone side, the temperature being minimum at the tip of the cone. A thermal gradient is set, spreading over ≈100 mm: it is about 3°C in case of EPDM joint and 7°C in case of SiR joint [28]. The effect of this thermal gradient could be to concentrate the field at the tip of the cone; however, this effect is not obvious in the results.…”

Electric Field Distribution and Dielectric Losses in XLPE Insulation and Semiconductor Screens of High-Voltage Cables

“…Many articles have described the effect of various factors on the electrical conductivity γ of cross-linked polyethylene [1,[4][5][6][7][9][10][11][12]18,. These factors include electric field stress, percentage of nanoparticles (most commonly Al 2 O 3 ), distance from the high-voltage conductor, temperature, aging time, degassing time, frequency, and insulation thickness.…”

“…of semiconductor screen and pure XLPE used to calculate the electric field distribution and dielectric losses in high-voltage cables[1,[4][5][6][22][23][24].…”