B. Vissouvanadin

Correlating conductivity and space charge measurements in multi-dielectrics under various electrical and thermal stresses

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 bilayer 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.

Deconvolution techniques for space charge recovery using pulsed electroacoustic method in coaxial geometry

The Pulsed Electro-Acoustic method (PEA) is a powerful tool for assessing space charge accumulation in a large variety of polymeric materials having various thicknesses. However, the acoustic attenuation and dispersion phenomena, especially when dealing with relatively thick samples, affect the space charge resolution at positions far away from the sensor. Moreover, for cable systems, the PEA signals are affected by the electrostrictive effect, due to the divergent nature of the electric stress, which introduces extra-distortion on the PEA signal. In this work, we report on two different deconvolution techniques being temporal and frequential. Both methods correct, with a similar figure of merit, the wave attenuation, dispersion and electrostrictive effects. Application of such deconvolution algorithms on experimental signals from a model power cables enables to show injection of negative charges at the inner electrode which results in a built-up of negative heterocharges adjacent to the outer electrode during volt-on.

Electric field profile measurement and modeling in multi-dielectrics for HVDC application

One of the major issues to be considered for the development of polymeric-type DC cable systems is the accumulation of space charges and the consequent distortion of the electric field. These phenomena occur both in the cable and in the accessories, especially at interfaces between different materials. In this respect, accessories, like joints and terminations are key points for the reliability of DC links. They are made with polymeric materials of different nature, the respective dielectric properties having a significant impact on the field distribution and interfacial charge accumulation processes. Therefore, the design of such accessories requires accurate forecasting of the actual field distribution within the insulation, under the non-homogeneous temperature conditions of the cable in service. The aim of the present contribution is to investigate the influence of conductivity on the electric field distribution in multi-dielectrics by experimental measurements and simulation.

A deconvolution technique for space charge recovery in lossy and dispersive dielectrics using PEA method

The Pulsed Electro-Acoustic method (PEA) is widely used both in laboratory for fundamental research and in industry for components assessment and diagnosis. However, in most cases, the technique is used without paying enough attention for acoustic losses and dispersive effects, which render the interpretation of the material/product response somewhat uncertain, especially when dealing with relatively thick samples. Moreover, for cable systems, PEA signals are affected by the acoustic wave propagation in the material and the response of the measurement system. We developed a deconvolution technique that takes into account the divergent effects due to coaxial geometry and insulation thickness. The accuracy of the recovered charge profile depends on the system modeling (especially the estimate of the attenuation and dispersion coefficients). Furthermore, simulation results show that the recovered charge profiles can be affected, to some extent, by the Signal to Noise Ratio of experimental data. A comparison between experimental results on power cables with and without the deconvolution technique is reported and discussed.

Dielectric and electrical properties of radiation-cured epoxy

Dielectric and electrical properties of radiation-cured epoxy are investigated in various conditions of electric field and temperature for different initiator content. The impact of the onium salt initiator is particularly considered due to the formation of ionic species that can impact the electrical properties. Dielectric spectroscopy performed in the temperature range -80 °C to +200 °C and the frequency range 10-1 to 106 Hz shows that β-relaxation of the cured resin and relaxation due to ionic species are dominating the dielectric behavior at low and high temperature respectively. Water absorbed in the resin increases the dielectric losses and conduction current. Ionic and electronic charge carriers are seen to dominate the electrical behavior at low and high field respectively. No clear trend has been observed as regard the effect of initiator concentration on the dielectric and electrical properties of radiation-cured epoxy.

Charge front and negative differential mobility in HVDC model cables

Space charge measurements have been carried out by means of pulsed-electro-acoustic (PEA) method on mini-cables with 1.5 mm-thick cross-linked polyethylene (XLPE) as insulation. Measurements were realized at room temperature or with a temperature gradient of 10°C through the insulation (Tin = 70°C, Text = 60°C) under DC voltage of -30kV and -55kV. Space charge results highlight the enhancement of the field at the outer-electrode when combining thermal and electrical stresses. They also reveal systematic occurrence of a negative front of charges generated at the inner electrode that moves toward the outer electrode at the beginning of the polarization step. It is observed that the transit time of the front of negative charge increases, and therefore the mobility decreases, with the applied voltage. Further, the estimated mobility -in the range 10-14 - 10-13 m2V-1s-1 for the present results, increases when the temperature increases for the same condition of applied voltage. The possible origin of this charge front is discussed on the basis of transient charging current measurements realized on the same cables. The features substantiate the hypothesis of negative differential mobility used for modelling space charge packets.

Effect of initiator nature on ionic conduction in radiation cured epoxies

Radiation-cured epoxy networks are promising materials for a number of applications for which both high mechanical and dielectric strength are required. In this work, epoxy materials are manufactured via a cationic mechanism which requires an initiator, generally an onium salt, to trigger the cross-linking process under ionizing radiation. It has been established that both the nature of initiator and its content have an impact on the network properties in terms of thermal and mechanical behaviour. We show in this contribution that the increase in chain mobility (decrease in Tg) along with the presence of residual ions results in space charge polarization, increasing both dielectric permittivity and losses under low frequency AC field. This effect also promotes build-up of heterocharges at moderate DC field.

Correlating structural and electrical properties in radiation-cured polymers

Radiation-cured epoxy and acrylate-based materials are used in a number of applications where curing under ionizing radiations is preferred to the classical thermal curing process. Because mechanical properties are often the key features in these applications much less attention has been paid to their electrical properties. We report on the dielectric properties of two classes of radiation-cured materials being typical of two classes of polymerization: free radical mechanism (acrylate-based formulation) and cationic mechanism (epoxy-based formulation). Questions arise as regards the electrical properties due to the complex formulation aiming at controlling the network properties (mechanical properties, degree of cure, glass transition temperature) and to the need of initiators in cationic polymerization with the inclusion of pair of ions in the network. Samples are 1mm-thick plaques, being cured under electron-beam. Different formulations are tested with the aim to investigate the influence of the network structure on electrical properties with specific emphasis on the nature of initiator (iodonium or sulfonium salts with various counter ions) and its concentration in epoxy-based formulation and reactive diluents in acrylate-based formulation. Structural and thermo-mechanical characterization is correlated with the electrical properties being current-voltage characteristics, space charge distribution and dielectric spectroscopy. Mechanisms driving the observed properties are presented and discussed.