Charge transport in polymer-based insulating materials for high voltage applications: effect of single- and multi-layered structures

Doktorsavhandling, 2016

Polymer composites have been widely employed as electrical insulating materials for high voltage components and devices such as power cables, gas-insulated systems, rotating machines, line and post insulators, etc. Such materials are usually made by introducing inorganic oxides (SiO2, TiO2, Al2O3, MgO) into polymeric matrices e.g. polyethylene (PE), epoxy, silicone rubber. Reported experimental evidences indicate improvements which can be achieved in dielectric strength, partial discharge resistance, and life time of the polymer composites as compared to their base materials. However, the reasons behind the advancements in materials’ electrical performance have not been clearly elucidated in many cases. In this context, the present thesis concentrates on analyzing the influence of filler particles on transport of charge carriers in two polymer-based insulations namely nanofilled PE and microfilled enamel, which are novel insulating materials for high voltage direct current (HVDC) cables and electric motors, respectively. The contribution of internal interfaces between insulating layers existing in multi-layered structures to their electrical conduction is also considered. As for PE nanocomposites and its unfilled counterpart, specimens of thin films were prepared, whereas multi-layered structures were made by pressing them together at high temperature. Charging currents measured at various temperatures indicated a remarkable reduction in DC conductivity of the nanofilled dielectrics as compared to PE, making the former preferable insulating materials for HVDC cables, especially for those working under enhanced electric stresses. The observed effect was associated with the decreased charge mobility and increased trap energy in the nanocomposites as compared to the corresponding properties of unfilled PE. By comparing the measured currents obtained on single-layered and multi-layered structures, the impact of insulation–insulation interface on electrical conduction was revealed and analyzed. Further, the field dependencies were established for the transient currents and the quasi-steady state currents that were utilized for examining the dominant conduction mechanisms in the studied materials. A computer model was employed for studying the generation and transport of charge carriers in the insulations at various temperatures. The simulations demonstrated that apart from the reasons mentioned above, the weakening charge transport process in the nanocomposites is in great extent caused by the suppression of charge injection at the electrode–insulation interfaces. On the other hand, the insulation coatings of enameled wires are inherently multi-layered structures consisting of at least two layers of different materials. This study focuses on partial discharge resistant enamel insulation that was created by introducing chromium oxide (Cr2O3) particles of micrometer size into the base polymer. Experiments were conducted mainly on multi-layered enamel coatings, while some tests were also performed on single-layered flat samples and filler powder. Thus, contributions of each phase (filler and polymer matrix) to the electrical conduction and dielectric response of the filled enamel were highlighted. Charge transport in the multi-layered enamel coatings was studied in details by analyzing the surface potential decay characteristics. Accordingly, potential decay mechanisms due to the combination of charge injection, polarization, and intrinsic conduction were revealed, each of them dominating within a certain time interval. Based on that, a computer model of charge transport in multi-layered insulating structures was developed to reproduce the measured surface potentials. In this model, the consideration of the barrier effect of the internal interface is of utmost importance.