Nuclear magnetic resonance imaging of morphological changes during electrical treeing in polyethylene due to partial discharges

Abstract

The interaction of the morphology with the dielectric aging inside a Polyethylene insulation system has been subject of many investigations. The Nuclear Magnetic Resonance (NMR) technique offers new aspects of examining the morphology during Electrical Treeing which is the last phase of the dielectric aging process. Therefore LDPE-specimen in needle-plane arrangement are electrically aged until different treeing phases are reached. Using the partial discharge measurement technique the aging condition is possible to be detected electrically. The different treeing phases correlate to significant partial discharge patterns. The aged specimen are investigated with the NMR-Imaging-technique especially with regard to morphological changes in the treeing region. Neither cutting nor any other preparation of the samples is necessary so that the aged dielectric material remains unchanged. In principle this measurement technique detects the spatial distribution of nuclear magnetization on a macroscopic scale (micro- to millimeters). However, this magnetization can be prepared in such a way that it contains information of morphological features with a size of a few nanometers. In solids this can be achieved by so-called spin diffusion, which exploits the diffusion of an initially non-equilibrium distribution of the magnetization. Morphological information can then be gained by comparing the experimental result with analytical and solutions of the diffusion equation for boundary conditions reflecting the morphology. The aged LDPE samples show a significant change of the measured signal and the derived morphological parameters in the treeing region. This is a consequence of the increased local temperature driven by the partial discharges followed by an accelerated cooling compared to the production process. This is confirmed by additional experiments with shock cooled molten LDPE samples. As a result the morphology in the treeing region is changed depending on the treeing phase. The size of the crystalline and amorphous regions are changed, the interfaces between them appear to vanish. Additionally a variation of the LDPE model insinuates a change in the lamellar structure of the LDPE which is no longer simple to describe in the aged region. An upgraded model describes the changed lamellar structure considering the dimensionality of the new configuration.