Modelling charge transport in solid dielectrics under electrical stresses and implication on materials reliability in electrical engineering – ModElec

The first part of the project is devoted to the evolution of an existing one-dimensional model of charge transport developed in the group DSF. It will focus on the mathematical and numerical development of this model. It will also focus on particular phenomena either observed experimentally (polarization) or known to have an impact on the charge dynamic but difficult to separate experimentally from other phenomena (diffusion and Langevin recombination), and not yet included in any model of charge transport in polymeric dielectrics under thermo-electrical stresses.
An inverse methods tool, or optimization tool, aiming at identifying parameters values of the models will be developed. This optimization tool will be used with the transport model to find the best parameters that fit all available experimental data.
The model will then be validated with a large panel of experimental data at room temperature, for two organic dielectrics. The outputs of the model will be directly used for comparison with experimental data (Space Charge measurements, DC and AC electroluminescence, current measurements). All the measurements described here will be performed at the LAPLACE, as it has the advantage to have a large number of experimental devices. The validity of the model with temperature will be tested by simulating TSDC and TSC measurements and comparing the simulated results with experimental data.
At last, the global tool (transport model + optimization tool) will be finalized. This tool will be tested as regards the uniqueness of the solutions. The optimization tool should be able to find a unique set of model parameters that correctly reproduces all the experimental data. The last part is devoted to the development of a user-friendly tool that encompasses the optimization and the transport model.

The global understanding of charge transport in disordered materials under different electrical stresses is one of the main scientific results of this project. It also leads to the identification of the special conditions where such or such behaviour is experimentally observed. Furthermore, the development of a robust model of charge transport in solid dielectrics is of relevance as it will be validated:
- mathematically and numerically at each step of its implementation;
- experimentally with a large panel of experimental results
- for two types of polymers having different electrical properties.
Moreover, the development of an optimization tool, dedicated to the identification of optimized parameters for the transport model remains one of the originality of this project, and will open the way to development of the package -transport model / optimization tool- for different applications.

The ability to develop such models of charge transport will enable to answer to economical, environmental and industrial issues. The knowledge of the field distribution in the insulation will allow pushing the system where the dielectric is employed to its limits (financial benefit). This will also lead to a better definition of the safety limits and reliability of the system, which is of industrial concern (more compact system with increasing voltages and currents). One may also expect from the model outputs to design new materials with optimized properties for a defined application, and/or explore potentialities of more environmentally insulating materials.


Among the potential follow-ups that would deserve to be built on the grounds of the current project one can name: extension and adaptation of the modelling to the case of other organic and inorganic dielectrics; development of an ageing model involving the presence of charge in the processes responsible of the system degradation; coupling the transport model to a discharge model in micrometric cavities, to understand the physics behind partial discharge and treeing in solid dielectrics; tools for systems conception and dimensioning, whenever there is the presence of a dielectric.

The results of fundamental research performed in this project will lead to publications in word leading scientific journals and communications at national, european and international conferences. One PhD thesis will be prepared and defended during the project.
From these scientific results, an easy-to-use software will be made available to the industrialists, protected by a licence. It will have the objective to be used on very specific conditions, depending on the type of insulation systems considered.

The project aims at understanding the behaviour of charge generation and transport in insulating disordered materials under electrical stresses, and to develop a one-dimensional model of charge transport able to predict the charge dynamic in such materials, whatever the electrical stress. This model should include the main processes known to have an impact on charge dynamic. The simulated results must be directly comparable to experimental data, allowing the evaluation of new physical hypotheses and to identify the conditions where such or such phenomenon is observed. Furthermore, an optimization tool dedicated to the identification of optimized parameters for the transport model will be developed. The main challenges linked to this project concern the identification of the important processes, their physical description and their mathematical implementation in the model; the validation of the transport model with the help of the optimization tool for a large panel of experimental data.
The main objectives of this project are as follow:
- The development of a transport model including physical mechanisms like polarization, recombination and diffusion;
- The understanding of the conditions where some particular phenomena are observed experimentally
- The validation of the transport model with the help of the optimization tool, for a large panel of experiments where different physical processes are active. This validation will be performed for isothermal and non-isothermal conditions
- A validation of the optimization tool, particularly in term of uniqueness of the solutions (optimized parameters of the transport model). A finite number of experiments should be defined to correctly describe and model the behaviour of the material.
The group ‘Solid Dielectric and Reliability’ (DSF) of the LAPLACE is specialized in electrical and physico-chemical characterization of insulating materials, especially polymers, and has an expertise in modelling the behaviour of charges in such materials. Each participant of the group DSF taking part in this project has an expertise in one particular measurement, or knowledge in theoretical processes linked to organic solid dielectrics. An external expert, previously professor at the Institut de Mathematiques de Toulouse (IMT) and now having a position in the department of Mathematics and Applications at Braga University (Portugal), will also be part of the project. He is specialized in numerical schemes implementation and in mathematical and numerical analysis, and will allow developing a robust model of charge transport.
One of the expected results of this project is the development of an easy-to-use software, including the transport model and the optimization tool, allowing using it for different applications.
The scientific results will allow answering to industrial problems like: the development of new insulating materials, with improved electrical properties, in order to replace more polluting dielectrics still in use; the increase of the reliability of the insulations and systems where they are used, especially as regards energy transport under HVDC where materials assessment tools are required.