Abstract
The increasing needs of electric power and the placement of power plants at more and more remote locations with respect to the users promote the development of electric power transport by DC submarine or terrestrial links. In the case of high voltage direct current (HVDC) cables with polymeric insulation, reliability is strongly affected by the development, under high electrical and thermal stress, of electric charges in the bulk of the dielectric and at the insulator/semiconductor interfaces. These accumulations of charges (called space charges as they are distributed in an insulating space) may lead to premature ageing and breakdown. The setup of new materials and the optimization of insulating structures for HVDC cables require a precise follow-up of the space charge zones. It is therefore necessary to develop non-destructive space charge measurement techniques, as the thermal step method (TSM), which is based on the application of a low temperature step to an insulating structure and on the measurement of a transient current due to the redistribution of influence charges at electrodes.The present work concerns the feasibility and the experimental set up of a device, based on the principle of the TSM, aiming to measure the space charges in insulating samples with semi-conducting electrodes, with a resolution approaching the degree of homogeneity of the interfaces (i.e., of micrometric order).After having reviewed the problematic of the dielectrics subjected to high dc fields and the existing space charge measurement methods, this dissertation presents several aspects and parameters related to the principle of the TSM, as well as to the definition and to the estimation of spatial resolutions. Analytical and numerical simulations are then carried out to study the contributions of a TSM-based setup on insulating structures composed of 100 µm-thick cross-linked polyethylene slabs provided with 30 µm-thick semiconducting electrodes. They show that the application of temperature steps, with amplitudes that do not affect the electrical and surface states of the materials, leads to measurement signals of several nano amperes for variations of the remnant electric field of the order of the kV/mm per micron. Highly effective thermal exchanges between the heat source (heating or coolant liquid) and the insulator are required to generate such signals, needed for measurements of sufficient dynamics and for resolutions approaching the micrometer. A bi-dimensional multi-physics simulation study is made, in order to analyze the heat transfer through a direct contact between the coolant liquid and a sample of the aimed type. The study allows to define and to optimize an innovating structure of thermal diffuser, where the heat exchanges operates by direct contact, thus providing a much better efficiency than the existing structures.The development of a TSM-based measurement tool, using the fallouts of the simulations, is then presented. It associates a hydro pneumatic system, a high efficiency diffuser favoring the dynamics of the thermal transfer and a sensor system, thus increasing the amplitude of the signals, the reproducibility and the accuracy of the measurements. Calibration measurements on the aimed samples show, with respect to the resolutions available on this type of structures (close to 10 µm), enhancements by a factor of 2 to 5 for the positioning resolution an a discerning resolution close to the state of the art. The critical analysis of the developed bench, corroborated to the predictions of the simulations, also puts into focus a significant room for improvement of the tool, for which evolutions proposed in this work would allow to enhance the performance.