The design of transmission systems requires electric field distribution estimation, which, in case of HVDC application is strongly sensitive to thermal and electrical configuration as well as to the nature of dielectric materials being used owing to the resistive field distribution. In this paper, the field distribution in a dielectric bi-layer of XLPE and rubber materials, as representative of cable junctions, is estimated based on experimental data on field and temperature dependencies of conductivity. Through space charge measurements on bi-layer dielectrics, it is shown that the space charge density and electric field distributions are to a first order estimation consistent with data issued from conductivity measurements. Most notably, the interface charge building up between the two dielectrics changes sign, depending on field and temperature. However, in the high field range (order of 20 kV/mm), charge build-up in the bulk of dielectric materials introduces further distortion to field distribution.

The purpose of this work is to present and to discuss a methodology for the assessment of materials intended to be used as insulation in HVDC cables, particularly as regards the evaluation of the effects of material formulation on space charge accumulation within the insulation. Sample design, testing method and protocol and criteria used for the material evaluation are specifically considered. The criteria being put forward are based on available models of insulation life vs. stress with consideration of polarity reversal effects. It is shown through several examples using low density polyethylene and crosslinked polyethylene materials that criteria such as the Field Enhancement Factor and the space-averaged residual charge density are indeed sensitive to material formulation and especially to the presence of crosslinking by-products. When using a crosslinking process that does not produce residues, the material tends to behave like low density polyethylene regarding space charge features. Finally, the question of the representativeness of tests performed on flat specimens vs. model or full cables is discussed.

Abstract:The development of high voltage direct current (HVDC) technologies generates new paradigms in research. In particular and contrary to the AC case, investigation of electrical conduction is not only needed for understanding the dielectric breakdown but also to describe the field distribution inside the insulation. Here, we revisit the so-called Maxwell-Wagner effect in multi-layered dielectrics by considering on the one hand a non-linear field dependent model of conductivity and on the other hand by performing space charge measurements giving access to the interfacial charge accumulated between different dielectrics. We show that space charge measurements give access to the amount of interfacial charge built-up by the Maxwell-Wagner effect between two dielectrics of different natures. Measurements also demonstrate that the field distribution undergoes a transition from a capacitive distribution to a resistive one, under long lasting stress.

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Abstract:In the field of energy transport, High-Voltage DC (HVDC) technologies are booming at present due to the more flexible power converter solutions along with needs to bring electrical energy from distributed production areas to consumption sites and to strengthen large-scale energy networks. These developments go with challenges in qualifying insulating materials embedded in those systems and in the design of insulations relying on stress distribution. Our purpose in this communication is to illustrate how far the field distribution in DC insulation systems can be anticipated based on conductivity data gathered as a function of temperature and electric field. Transient currents and conductivity estimates as a function of temperature and field were recorded on miniaturized HVDC power cables with construction of 1.5 mm thick crosslinked polyethylene (XLPE) insulation. Outputs of the conductivity model are compared to measured field distributions using space charge measurements techniques. It is shown that some features of the field distribution on model cables put under thermal gradient can be anticipated based on conductivity data. However, space charge build-up can induce substantial electric field strengthening when materials are not well controlled.

The general objective of this work is to obtain a better understanding of polymeric-insulated HVDC cable characteristics, especially related with conduction current and space charge features. It goes with the building of a model of conductivity vs. field and temperature in order to forecast field distribution in cables. Conduction current was measured in miniature of HVDC power cables with construction 1.5mm polymer insulation thickness, 0.7mm inner semiconductor, 0.15mm outer semiconductor and conductor diameter of 1.4mm. Current measurements were performed with applied voltage varying from 2kV to 30kV and temperature in the range from 30°C to 90°C. The time of polarization (voltage applied) and depolarization (short-circuit) were 1h/1h.The results show that there is not a single conduction mechanism for all applied fields and in the all temperature range. The conductivity is clearly non-linear at voltages above about 10kV (field at inner semicon of 10kV/mm), the threshold being dependent on temperature. With the increase in thermal and electrical stresses, transient currents appear characteristic of processes with massive charge injection at the electrode and therefore the treatment of transport based on a homogeneous conductivity is no longer a sufficient hypothesis.

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.