In collaboration with the Laboratoire du Génie des Procédés Papetiers and Enedis, EDF is looking for a PhD candidate for a thesis on the modelling of the ageing of the insulating barrier in power transformers.
Distribution transformers are key components in the operation of electrical networks, transforming medium voltages (between 15,000 and 30,000 volts) into low voltages (230 volts) to supply power to the domestic network. Because of their position at the end of the chain, there are many such devices on the network. They are therefore mass-produced and their production is part of a highly competitive industrial sector where technical improvements and optimisations are continual.
In some transformers, the structural integrity and insulation of the conductive components are ensured by a barrier made of superimposed layers of paper impregnated with oil. This barrier must guarantee good temperature resistance and contribute to the dielectric protection of the low and high voltage windings at the heart of the transformer. The ageing and loss of efficiency of this barrier is one of the main limits to the lifetime of the transformer.
Today, the diagnosis of the integrity of the insulating barrier is made by dosing certain tracer compounds emitted by the paper and which diffuse into the oil during the operation of the system. Although this method is relatively satisfactory when the insulating barrier is made of traditional Kraft paper, recent observations show that it is very imprecise and even totally unsuitable for other types of paper currently being studied for improving transformer performance.
Paper ageing laws: how to evaluate the lifetime of power transformers?
EDF's research and development department, in collaboration with Enedis, wants to establish a thermomechanical ageing law in order to be able to model the ageing of these new papers without having to go through oil composition analyses. This law will have to integrate a broader and global modeling of the transformer.
In order to reproduce the thermomechanical stresses undergone by the material within the transformer and to establish the law for paper ageing, the candidate will have to design an original experimental device, designed by EDF, which will enable to reproduce the oscillating sinusoidal loading of the Laplace forces induced by the magnetic field within the transformer.
During the project, the candidate will also be in charge of the implementation of a "dynamic fault tree" model. The interest of this approach is to propose an ageing modelling chain that can be applied and transposed to any type of material or stress. It will make it possible to evaluate the residual service life margins on the basis of predefined operating scenarios, taking into account the loading profiles and temperature variations, which are the two main factors influencing transformer ageing.
Detail of the offer
The thesis is planned for a duration of 3 years starting from the September 2020. The post is located at the EDF Lab Les Renardières site in Ecuelles (77). Given the academic partnership with the LGP2 in Grenoble, regular travel is expected between the two sites.
Candidates must have an engineering degree or a research master's degree (Bac+5) in the field of polymer and/or cellulosic materials, and have experience in carrying out and monitoring laboratory experiments as well as listening skills, scientific curiosity and autonomy.