Physique des décharges dans des cavités internes à un diélectrique et implication pour le vieillissement électrique des isolants et son diagnostic – UltimatePD

Partial discharges (PD ' a discharge affecting a small distance as compared to the distance between electrodes to which the voltage is applied) in solid insulated electrical systems and the way they affect insulation quality has for decades been a rewarding subject for many researchers. This is because electrical systems reliability is deeply affected by PD occurrence in weakening the insulation ability to sustain functional stresses (electrical, thermal, mechanical, etc.). The slow erosion of the insulating material ' especially in the case of organic polymers that are highly sensitive to exposition of PD, can lead to breakdown of the electrical system with large practical consequences, for example; interruption in the energy supply (which is specifically critical for the electricity network where large electrical generators can be stopped); blackout in part of the European grid (breakdown of solid insulated power cable has been at the origin of major blackout within the European power grid); lost of part of power generation in systems where the failed components are embedded. Two important areas of research in LAPLACE are modeling gas discharge phenomena and studies related to aging in dielectrics. Up to now, these have been very separate activities, but it is clear that there is considerable interest in developing an activity which combines both. The reason is that the standard scenario describing aging in dielectrics is gas breakdown in voids ' small, gas filled voids inevitably present in dielectrics which, over the course of some years, and after repeated breakdown events in the voids, lead to damage and eventual failure of the dielectric. State of the art gas discharge modeling has not been applied to this situation. In the dielectric community, the modeling of gas breakdown in the voids has been limited to very approximate treatments. The gas discharge physics community is very actively looking microdischarges ' discharges generated in very small geometries. A few of the outstanding issues which could be addressed by gas discharge modeling are (a) quantification of electron multiplication in voids for different conditions typical of those encountered in dielectric operation for different missions (b) evaluation of radical and photon generation in voids (c) energy distributions of electrons and ions on the walls. Closely related is the issue of charge transport in dielectrics, where our current understanding is largely empirical. Models of charge transport in dielectrics have been developed, but these are still very macroscopic. They could be developed, but before that some systematic clean experiments are needed. Experience at LAPLACE in the gas discharge groups in microdischarges, plasma display panels will be a part of this project. Also participating in the project will be the local experts in dielectric aging, who will help define reference conditions and evaluate the range of parameters of interest. A young CR CNRS who has considerable experience already in the modeling of radical and UV generation in gas discharges in dielectric barrier discharges and in streamer breakdown will pilot the project. Other young researchers participating in this project are experts in charge transport in dielectrics and in measuring conductivity in dielectrics. This project will borrow extensively from existing activities being carried in the framework of other projects, and a slight reorientation and reorganization will provide valuable information. Although the proposed project is 3 years, longer-term objectives can be identified, the details of which will depend on the results obtained over the next few years. 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 modeling to the case of cavity size with a very high aspect ratio, i.e. treeing filaments with lengths of some 100 'm, and diameter of a few 'm; development of a model of tree propagation involving the physics of the driving process, i.e. discharges along the dielectric walls; development of a degradation model involving the chemistry induced by the discharge; investigation of charging mechanisms in cellular polymer electrets; development of diagnosis tools dedicated to low activity partial discharges.