J. Castellon

Dielectric properties of XLPE/Sio2 nanocomposites based on CIGRE WG D1.24 cooperative test results

A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences.

Characterization of epoxy microcomposite and nanocomposite materials for power engineering applications

This article presents the results from round-robin tests performed on epoxy composite materials. These results show the potential of these materials for use as electrical insulation in some specific applications. A small section of the article addresses the health and safety issues related to the use of nanoparticles in the electrical power engineering industry. We define epoxy nanocomposites as epoxy-based materials containing exclusively nanosized filler particles. Epoxy microcomposites are defined as epoxy materials containing exclusively microsized filler particles, and epoxy micro+nano composites are materials containing both microsized and nanosized particles.

Nanostructured polymer microcomposites: A distinct class of insulating materials

Experimental evidence was produced and gathered to demonstrate the distinct nature of nanostructured polymer microcomposites. The case of a polymer composite consisting of a high-content of micrometric quartz with a small adjunct of nanoclay is discussed. Emphasis is put on dielectric behavior studies while some results on thermal characteristics are presented. Overall results strongly support the potential of this class of insulating material for electrotechnical applications.

Electrical properties analysis of micro and nano composite epoxy resin materials

This work deals with the study of micro and nanosilica filled epoxy resin samples carried out in the framework of CIGRE WG D1.24 cooperative test program. This program focused on chemical, electrical and electrostatic properties of epoxy based nanodielectrics for electrical engineering applications. Epoxy based samples filled with micro and/or nanoparticles of silica were characterized by transmission electron microscopy, dielectric spectroscopy, conduction current and space charge measurements. These mutually complementary techniques were used to examine the effect of the size and quantity of silica particles on the electrical properties of the analyzed materials. The analysis of charge injection, polarization, trapping and conduction phenomena has allowed the modeling of dielectric behavior of the studied materials under multiple stresses. The Schottky Injection and Space Charge Limited Current models were studied to explain conduction phenomena. A composition of micro and nano-sized silica particles accumulating the smallest amount of space charge is also proposed.

The thermal step technique: an advanced method for studying the properties and testing the quality of polymers

It is well known that the physical properties of a polymer alter with time. This phenomenon—ageing—is strongly related to the external factors acting upon the polymer and to the manufacturing process. An appropriate process could prevent the premature and undesired changes in the characteristics of a polymeric material. For this reason, considerable efforts are made to understand the mechanism of polymer degradation. Past studies have shown that the existence of electrical charges in the bulk of polymers can affect their properties significantly. These charges—usually called space charges or space charge—were first observed in polymers used for electrical insulation and therefore submitted to high electric fields. Some recent studies, particularly those presented in this paper, proved that space charge exists in almost every polymer. It has been shown that space charge can be present within the material from the very manufacturing process, without previous submission to stress. The increase of the amount of the space charge seems strongly related to the deterioration of the physical properties of polymers. Space charge measurements can therefore be considered as a test of quality for polymers. This paper is dedicated to a method for measuring space charge: the Thermal Step Method (TSM). The physical basis of the technique, as well as its applications, are presented. Results obtained on various polymers are presented and discussed. The results show that the features of this technique, particularly its high sensitivity, make it an appropriate tool for the characterization of a wide variety of materials. The TSM could also be associated with other physical, chemical or physico-chemical techniques.

Determination of Electric Field and Space Charge in the Insulation of Power Cables With the Thermal Step Method and a New Mathematical Processing

This paper proposes a technique for determining the distributions of the electric field and space charge in the insulation of power cables by using the data acquired with the thermal step method (TSM). The TSM consists of applying a low-temperature step to a short-circuited or dc-energized cable and of acquiring a transient capacitive current. The processing technique described in this paper is based on a series decomposition of the electric field, the coefficients of the series being identified via the measured current. The accuracy and the stability of the calculation method are evaluated by simulations performed using various distributions of the electric field and different noise levels. An application of the technique to space charge measurements in a dc conditioned power cable is then presented.

Computation of the Electric Field in Cable Insulation in the Presence of Water Trees and Space Charge

The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees.

Electric field computation in water treed polyethylene with space charge accumulation

Electrical degradation of the insulating systems of energy cables is close related to the water treeing and space charge development. In this paper the results of the calculations of the electric field repartition in the insulation of a medium voltage cable affected by water trees and their related space charge are presented. Continuous and individual water trees, growing from outer and/or inner semiconductor have been considered. The space charge density has been determined by measurements performed on plane samples of polyethylene by using the thermal step method. The repartition of the space charge in the water treed regions has been estimated from FTIR analysis on the repartition of the ions associated to water trees. The results show that the presence of water trees and of space charge determines an important increase of the electric field strength near to the semiconductor layers, which facilitate the inception of electrical trees in insulations and consequently the breakdown at lower voltage

Dielectric response of various partially cured epoxy nanocomposites

Insufficient crosslinking and water uptake during fabrication or manipulation are known to affect the dielectric response of epoxies. Post thermal treatment may result in the completion of cross-linking, partial removal of water, and aging. In order to study the effect of manufacturing imprecision on dielectric response, several under-cured epoxybased nanocomposite samples with modified nanoclay fillers were investigated. In addition, the influence of silane coupling agents and the use of ultrasonic waves on the nanoclay intercalation were also studied. The structure of the samples and the extent of cross linking were characterized using X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) respectively. It was found that surface silanization lead to improved clay intercalation and higher extent of intercalation/exfoliation. The influence of post thermal treatment on the dielectric response of the materials was investigated using Broadband Dielectric Spectroscopy (BDS). Once the samples were in a stable dielectric state, relaxation maps were performed. It was found that the samples with silanized nanoclay have the lowest activation energy and they also proved to be the “strongest” vitreous materials.

Space Charge Characterization of multi-stressed microcomposite nano-filled epoxy for Electrotechnical Applications

Many electrotechnical applications require the emergence of nanocomposite materials. However, these Lt new Gt materials must exhibit more performance than existing dielectrics. In this work, the study will concern dielectric properties of a composite material consisting of a quartz charge at the microscopic scale and nanoclay at the nanometric scale. Characterization will focus on the space-charge phenomenology at values of the electric field preceding breakdown. To do so, the original method developed at Montpellier university in the 80s, which uses a thermal step across the insulating material, will be utilized. The thermal step method (TSM) allows the space charge to be quantified and localized across the material. A comparative approach will be implemented. The properties measured for the nanostructured epoxy microcomposite will be compared to those observed for the base microcomposite. This will allow to evidence the benefit or no of the presence of the nanoclay in the role of space-charge accumulation. Preliminary results indicate that despite non-optimized materials conditions, electrical stresses could be pushed relatively high, e.g. some tens of kV/mm (about 30 kV/mm).