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Tianbo ZHOU’s Thesis Defense

par Laurence Laffont - publié le

Tianbo ZHOU’s thesis defense, intitled "Conception de systèmes d’isolation haute tension en Électronique de Puissance : prise en compte de nouveaux matériaux et structures" (Design of High Voltage Insulation Systems in Power Electronics : Consideration of New Materials and Structures), will be taking place on July, 11th, 2022, ay 9:30 am, in amphithéâtre Maxwell at Université Paul Sabatier.

Jury :

Pierre BIDAN, Université Toulouse III - Paul Sabatier, Thesis Director

Stéphane DUCHESNE, Université d’Artois, Rapporteur

Alain SYLVESTRE, Université de Grenoble Alpes, Rapporteur

Séverine LE ROY, Research director, CNRS, LAPLACE, Reviewer

Lionel LAUDEBAT, INU Champollion, LAPLACE, Guest

Marie-Laure LOCATELLI, Research fellow CNRS, LAPLACE, Guest


Abstract

Dielectric materials are used in the insulation of electrical equipment, and their reliability and lifetime depend on their properties. Therefore, it is important to model these properties in order to simulate the behavior of these materials during the device design, especially their impact on the electric field and temperature distributions.
In this study, the equations, models and solving methods generally implemented in commercial design tools used for the numerical simulation of an insulation system are analyse. The main guideline conception’s are pointed out according to different studies. The limitations of current software are described and improvements are proposed by introducing additional modeling tools that can take into account of dielectric relaxation phenomena and certain non-linear properties.
The work then contributed to the study of the improvement of the insulation system of a power electronic module for high voltage ramping. In a module, the triple point between the ceramic substrate, the gel and the metallization has been identified as the place where electrical stresses are concentrated. The inclusion of a semi-resistive layer between the electrodes may be a solution to modify the electric field distribution in this area. Time-domain simulations were performed using the commercial tool COMSOL Multiphysics® to study the electric field distribution, as well as the current density distribution, for different combinations of permittivity and conductivity of the layer (linear or non-linear with field) under voltage step excitation, for the case (few studied in the literature) of very high dv/dt, such as induced by the new silicon carbide power devices. More faster is the switching, more conductivity and permittivity must be increased to ensure that the system is protected in all operating states (transient and steady state). This can lead to high transient leakage currents, and significant losses. The partial resolution of this problem by introducing a non-linear conductivity layer has led to the definition of a design approach for the optimization of the insulation system of a module.
The second point of the work is the consideration of relaxation phenomena, fundamental in dielectric materials, but delicate to manage in time domain in any conditions. The simple Debye model, allowing to consider only on relaxation mode, is most often insufficient in solid materials which can have several superposed modes. The underlying relaxation time distribution is complex to model in the time domain : The Diffusive Representation (DR) is a mathematical tool allowing both this modeling and the adaptation of the model by parametric identification using frequency or time measurements on the device or samples. The resulting models are introduced into COMOL to simulate the behavior of dielectric materials within any geometry and under any excitation, not only harmonic. Furthermore, under some assumptions, it is also possible to simulation the behavior at high temperature and materials with nonlinear conductivity.
In conclusion, taking into account of relaxation phenomena and nonlinear properties in dielectric materials in time domain simulation will allow to optimize the choice of geometries and materials in the design phase, with the help of previously adapted numerical simulation tools.