Predicting the electric field distribution under dc stress within polymeric insulations remains a challenge, as space charge behaviour is still difficult to understand in these materials. Charge generation is often thought to arise from injection at the electrodes. Hence, surface roughness should be taken into account, as it strengthens the electric field locally and therefore promotes charge generation at some points. A charge transport model has been developed in 2D to account for surface roughness. The model is first validated with the help of previous results, obtained with a one dimensional charge transport model. Then, with simple shapes accounting for roughness, the simulated results show that surface roughness has a significant impact on the net space charge behaviour. The impact of shape, and size of protusions is presented, as well as a more realistic case were a large surface of the electrode is considered as rough.
One of the difficulties in unravelling transport processes in electrically insulating materials is the fact that the response, notably charging current transients, can have mixed contributions from orientation polarization and from space charge processes. This work aims at identifying and characterizing the polarization processes in a polar polymer in the time and frequency-domains and to implement the contribution of the polarization into a charge transport model. To do so, Alternate Polarization Current (APC) and Dielectric Spectroscopy measurements have been performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic polar polymer, providing information on polarization mechanisms in the time- and frequency-domain, respectively. In the frequency-domain, PEN exhibits 3 relaxation processes termed β, β* (sub-glass transitions), and α relaxations (glass transition) in increasing order of temperature. Conduction was also detected at high temperatures. Dielectric responses were treated using a simplified version of the Havriliak-Negami model (Cole-Cole (CC) model), using 3 parameters per relaxation process, these parameters being temperature dependent. The time dependent polarization obtained from the CC model is then added to a charge transport model. Simulated currents issued from the transport model implemented with the polarization are compared with the measured APCs, showing a good consistency between experiments and simulations in a situation where the response comes essentially from dipolar processes.
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, socalled 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.
Predicting the electric field distribution in polymers used as electrical insulating materials remains the Holy grail, as the presence of charges disturbs the Laplacian electric field. Charges arising from the electrodes are one of the dominant mechanism of charge generation, particularly in polyethylene-based materials. Hence, nanometric scale processes at play at the interface have a non negligible impact on charge injection. In the present study, a bipolar charge transport model developed in 2D is used to simulate the impact of several nanometric scale processes, such as the variation of the barrier height linked to the chemical structure of the material at the interface, as well as surface roughness. Simulation results as regards net charge density, current, but also recombination rate, will be compared to a case where no specific supplementary hypothesis is set at the electrodes. At last, simulations have been performed for a combination of roughness and barrier height variation along the electrode.
Polymers used as insulating materials are increasingly popular in many different fields. In electrical engineering - electronics, polymers are used in high-voltage transmission cables, capacitors, transformers, or as part of an embedded system in the IGBT module thanks to its superior thermal and electrical insulation properties. One of the disadvantages of polymers is the possible accumulation of space charge in the material volume for a long time, leading to an increase in the electric field compared to the original design value. Charge transport models in polymer materials have been increasingly developed to predict the conduction mechanisms under thermal-electrical stress. In this study, from a finite volume method (FVM), the authors developed a charge transport model in low density polyethylene (LDPE) based on the finite element method (FEM). The simulation results of this model are also compared to experimental results and to the FVM model under different electric fields for LDPE.
Understanding the space charge behaviour in solid organic insulations under thermo-electrical stress is of particular interest as charge dynamics is closely related to electric field distortion, charge accumulation and at last to materials ageing. Many polymers used as electrical insulators are polar materials, and have a non-linear behaviour as regards conduction processes. Hence, the underlying physics is not easily accessible and understandable. A charge transport model coupled with relaxation functions has been developed recently. In this paper, a comparison between the experimental measurements and the simulation results extracted from the coupled models is proposed, for two different materials: a polar material (PEN) and a weakly polar material (LDPE). From these results, we discuss the issues related to the prediction of the thermo-electrical behaviour of solid organic insulating materials, when the difficulty arises not only from the experimental observation (i.e. separating polarisation from transport processes), but also from the mathematical treatment required for modelling.
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