S. Le Roy

Description of bipolar charge transport in polyethylene using a fluid model with a constant mobility: model prediction

We present a conduction model aimed at describing bipolar transport and space charge phenomena in low density polyethylene under dc stress. In the first part we recall the basic requirements for the description of charge transport and charge storage in disordered media with emphasis on the case of polyethylene. A quick review of available conduction models is presented and our approach is compared with these models. Then, the bases of the model are described and related assumptions are discussed. Finally, results on external current, trapped and free space charge distributions, field distribution and recombination rate are presented and discussed, considering a constant dc voltage, a step-increase of the voltage, and a polarization–depolarization protocol for the applied voltage. It is shown that the model is able to describe the general features reported for external current, electroluminescence and charge distribution in polyethylene.

Relative Importance of Trapping and Extraction in the Simulation of Space Charge Distribution in Polymeric Insulators under DC Potentials

Under high DC voltages polymeric insulators tend to accumulate space charge, leading to a deviation from the designed electric field for the system. Consequently local fields may reach high enough values to cause ageing and eventual failure. A particularly dangerous instance is the appearance of heterocharge at one or both electrodes. The origin for such charge is not very clear, it may be due to the field separation of charged entities intrinsic to the insulation or alternatively to the injection and transit of charges. Here we present a simple simulation model for the latter situation in which we identify the conditions required for the formation of heterocharges. The model led to interesting results showing a high dependence of the space charge distribution on the extraction rate and on the trapping rate.

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.

Simultaneous measurement of electroluminescence and space charge distribution in low density polyethylene under a uniform dc field

We report for the first time the simultaneous measurement of electroluminescence and space charge (SC) distribution in low density polyethylene films under a high dc field, using a specially developed pulsed electro-acoustic apparatus. An interpretation of the observed behaviour is proposed with the help of a numerical model of transport including the effect of charge trapping and recombination. It is shown that the two complementary approaches give consistent information about SC dynamics and charge recombination. This new kind of set-up should open the way for studying hot carrier transport in insulating materials.

The effect of Contact Charge upon the Injection Current at an Electrode-Insulator Interface

The effect of contact charge upon carrier injection has been evaluated using a number of possible expressions for the contact charge distribution. It was found that the contact charge effects dominated below a specific field around those of typical service levels. Below this field injection of same polarity carriers took place from the contact charge reservoir with a form for the injection current that depended upon the contact charge distribution. Above this field the injection current followed the Schottky form. Injection of opposite polarity carriers followed a Schottky form at all fields, though with activation energies and electrode fields modified by the contact charge.

Knowledge-based modelling of charge transport in insulating polymers: From experiments to model optimization

More than fifty years after the publication of the early work on electrical transport in disordered insulating solids, we are still unable to describe quantitatively the electrical response of these materials. During this period of time, concepts derived from semiconductor physics have been transposed to the case of insulating solids, and among them, to polymers. In spite of this, there is still no agreement on how to describe charge transport and there is still some controversy as regards the applicability of semiconductors physics to the case of disordered insulating materials and in particular to polymers used in electrical engineering applications. Our starting point is to accept the semi-conductor like description for polymeric materials as a basis to develop models. Up-to-date measurement techniques are used to produce data set pertaining to different kinds of measurements with the objective to reduce the number of solutions for the set of adjustable parameters associated with the model. In a first part of the paper, we give the basis of the bipolar charge transport model developed for low density polyethylene under dc stress: it features trapping, detrapping and recombination of positive and negative charge carriers, each of them being generated by a Schottky injection at the metal-dielectric interface. In a second part, we introduce measurements used to evaluate the model: space charge distribution through the sample, external current and electroluminescence measurements, all being time dependent, at different fields. In a third part, we introduce the optimization method used to estimate parameter values that best fit the all set of experimental data. The algorithm is a combination of the Gauss-Newton algorithm and the method of gradient descent. Like other numerical minimization algorithms, the Levenberg-Marquardt algorithm is an iterative procedure used especially for nonlinear problems. In our case, this algorithm is used to minimize the sum of the squares of the deviations between experimental data and simulation data. We show that the optimization tool developed as an interface between the numerical model and the experimental data can readily produce a set of values of physical parameters for the model.

Numerical model for studying dynamic space charge behavior in polyethylene

A model providing time-dependent solutions for charge densities, field and electroluminescence in low density polyethylene (LDPE) films under uniform dc field has been recently proposed. It was developed to simulate experimental results that support the hypothesis of a charging controlled by injection and transport of electronic (as opposed to ionic) carriers. The model includes therefore bi-polar carrier production, trapping and recombination. The influence on the numerical results of the most critical model parameters are considered in this communication.

Impact of conditioning on space charge formation in XLPE under dc electrical stress

Whereas synthetic materials have been used for long in the insulation of HVAC cables, their use in the case for HVDC applications is still pending due to a lack of knowledge on the behaviour of space charge and on their impact on the reliability of cables. As volatile cross-linking by-products are known to have an impact on space charge accumulation, it is important to control the optimum outgassing conditions in respect to such application. In this work, the accumulation of space charge in cross-linked polyethylene (XLPE) samples is investigated under dc electrical fields at room temperature using the pulsed electroacoustic (PEA) method. Organic semiconducting screens were attached on both sides of the dielectric during the cross-linking procedure and were used as electrodes. Five different degassing conditions were studied. The samples degassed at room temperature developed heterocharges. Conditioning by holding at 50°C for 2 days decreased the accumulation of heterocharges. Surprisingly, no heterocharges were observed in some fresh samples and heterocharges only changed sign (the density remained almost the same) when the applied voltage was reversed. These results appear consistent with space charge build up resulting from a spatially inhomogeneous distribution of permanent dipole molecules, like crosslinking by-products, present in the material. A model, based on diffusion of volatile cross-linking by-products and polarization orientation features, was developed to describe the heterocharges formation in XLPE. Simplified assumptions (one dimensional infinite sample, desorption phenomenon) were adopted in order to obtain a simple expression of the polarisation charge density. Theoretical results predict, basically, that the heterocharges move in time from the electrodes into the bulk due to the permanent dipole gradient. The magnitude of the polarization charge is also affected by the temperature and the initial dipoles concentration.

Electrical characterization of PEN films using TSDC and PEA measurements

Thermo-stimulated depolarization current (TSDC) measurements have been performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic polyester. The aim is to develop the understanding of trapping mechanisms at play in this material, and particularly to understand the effect of temperature. Experimental results of TSDC are interpreted with the help of space charge measurements using the pulsed electroacoustic method (PEA) on PEN samples. For TSDC measurements, samples were polarized at temperatures of 130°C and 170°C. In both cases, the sub-glass transition and the glass transition relaxations are observed. However, in the case of a polarization temperature of 170°C, one more TSDC peak, so-called ρ-peak is observed at temperatures above the glass transition. From space charge results, it is shown that the ρ-peak has not a dipolar origin; it has been associated to charge detrapping in the material.

Simultaneous measurement of electroluminescence and space charge distribution in low-density polyethylene under DC field

We report for the first time the simultaneous measurement of the electroluminescence and the space charge distribution in cross linked polyethylene under high DC field using a specially-developed pulsed electro-acoustic apparatus. An interpretation of the observed behaviour is proposed with the help of a numerical model of transport including the effect of charge trapping and recombination.