Petru Notingher

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.

Electric field and space charge measurements in thick power cable isulation

This paper describes the evolution of electric field and space charge in high voltage AC and DC cable polymeric insulation, followed using the thermal step method.

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.

Computational and experimental study of ionic space charge generated by combined corona-electrostatic electrode systems

Numerical computation of the electric field intensity and space charge density in electrode systems consisting of ionizing and nonionizing elements, connected at a same DC high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. The aim of the present paper is to demonstrate the effectiveness of an original method of field computation in the analysis of the factors that influence the distribution of the ionic space charge in such combined corona-electrostatic electrode systems. The computations and the experiments were carried out for an ionizing wire of diameter 0.3 mm, located at different distances h (10 to 30 mm) from a tubular support of diameter 25 mm. Several inter-electrode distances (20 to 45 mm) were simulated. The extension of the zone at the surface of the grounded electrode which is affected by the space charge diminishes when reducing the intervals between these elements of the electrode system, and - at similar applied voltage - the density of the corresponding corona current increases. The experimental data were in good agreement with the computed results, validating the accuracy of the numerical method of space-charge calculation in this special electrode configuration.

Application of the thermal step method to space charge measurements in inhomogeneous solid insulating structures: A theoretical approach

The thermal step method is a nondestructive technique for determining electric charge distribution across solid insulating structures. It consists in measuring and analyzing a transient capacitive current due to the redistribution of influence charges when the sample is crossed by a thermal wave. This work concerns the application of the technique to inhomogeneous insulating structures. A general equation of the thermal step current appearing in such a sample is established. It is shown that this expression is close to the one corresponding to a homogeneous sample and allows using similar techniques for calculating electric field and charge distribution.

Industrial installation for voltage-on space measurements in HVDC cables

This work is concerned with a facility for characterizing the insulation of power cables whilst submitted to high dc voltage. The presented tool allows to measure the real distribution of the electric field in the cable insulation, which can be significantly different from the capacitive (Laplace) field repartition, and is of interest in insulation design and survey. After reviewing the basics of the technique used (principle of the thermal step method), a detailed presentation of the device and of its validation is given. Results obtained with the installation in the frame of design of dc cables are then exposed.

Surface temperature measurement for space charge distribution measurements with thermal methods

The implementation of space charge measurements based on thermal perturbation on thin films requires an improvement of the temperature distribution estimation at the surface and in the depth of dielectric materials for getting reliable space charge profile measurements. Absolute temperature variations are needed, both in time and space. The present contribution addresses surface temperature measurements based on either thermoelectric or bolometric effects. Both responses have been measured on copper-coated silicon nitride layers and gold-coated polypropylene films heated with a Nd:YAG laser pulse. It is shown that high temporal resolution thermal response can be obtained through the bolometric response and that the information appears nearly independent on the nature of the coating electrode. The setup developed provides good signal to noise ratio for heated electrodes of a few ohm resistance. Strategies are still to develop to get the temperature profile in the insulating sample layer.

Charge-Decay Characteristics of Granular Materials Forming Monolayers at the Surface of Grounded Electrodes

Laboratory studies and in-field observations have shown that the charge-decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. This paper validates an indirect method of charge-decay characterization, based on the measurement of the electrical potential at the surface of a monolayer of granular insulating material. The study was performed on three materials - polyvinyl chloride, polyethylene, and rubber - extracted from chopped electric wire wastes. The granules (characteristic size in the range 1-4 mm) were disposed on the surface of a grounded plate electrode (layer area: 100 mm times 100 mm; electrode area: 200 mm times 200 mm). A wire-type corona electrode, energized from a dc high-voltage supply, was employed for charging the granules. The potential due to the charge at the surface of the granular layer was measured with the capacitive probe of an electrostatic voltmeter connected to a personal computer. Data acquisition and processing were done using the LabView environment. The influence of particles characteristics and of ambient factors was studied. The findings enabled a more accurate modeling of discharging phenomena that affect the performances of electrostatic separators. The method can be easily adopted in electrostatic discharge studies for material characterization.

Characterization of a Trench-Gated IGBT using the split C-V Method

The present paper proposes an application of the Split C-V method to Trench gated IGBTs. A model of the gate to emitter capacitance along with an illustration of the monitoring of injected charges in the trench oxide of a Trench gated IGBT during an accelerated ageing campaign are presented.