PP/PP-HI/silica nanocomposites for HVDC cable insulation: Are silica clusters beneficial for space charge accumulation?

New potential High Voltage Direct Current (HVDC) cable insulation materials based on nanocomposites are developed in this study. The nanocomposites are produced by blending of polypropylene (PP), propylene-ethylene copolymer (PP–HI) and a modified fumed silica (A-silica) in a concentration of 1 and 2 wt %. The A-silica is successfully modified with (3-aminopropyl)triethoxysilane (APTES) via a solvent-free method, as proven by infrared spectroscopy, thermogravimetry and transmission electron microscope mapping. A-silica in the polymer matrix acts as a nucleating agent resulting in an increase of the crystallization temperature of the polymers and a smaller crystal size. Moreover, the silica addition modified the crystals morphology of the unfilled PP/PP-HI blend. The composite containing A-silica with 2 wt% contains bigger-size silica clusters than the composite filled with 1 wt%. The composite with the higher A-silica concentration shows lower space charge accumulation and a lower charge current value. Besides, much deeper traps and lower trap density are observed in the composite with 2 wt% A-silica addition compared to the one with a lower concentration. Surprisingly, the presence of silica clusters with dimensions of more than 200 nm exhibit a positive effect on reducing the space charge accumulation. However, the real cause of this improvement might be due to change of the electron distribution stemming from the amine-amine hydrogen bond formation, or the change of the chain mobility due to the presence of occluded polymer macromolecules constrained inside the high structure silica clusters. Both phenomena may lead to a higher energetic barrier of charge de-trapping, thus increasing the depth of the charge traps

Investigation of nanocomposite polypropylene for DC capacitors: A feasibility study

This paper presents a preliminary feasibility investigation, in the framework of a recent European project, GRIDABLE, aiming at the development of nanostructured Polypropylene, PP, materials as insulation for DC LV-MV capacitors and MV-HV cables. Results of electrical property characterization of various types of PP-SiO2materials for DC capacitors are presented in this paper, focusing on breakdown strength and space charge measurements. They indicate that the nanofiller may influence the degradation rate of materials, modifying the space charge accumulation dynamics and extent, as well as the threshold field value for space charge accumulation. Statistical analysis of breakdown voltage shows that the presence of nanoparticles may also improve noticeably the breakdown behavior

Biaxially oriented silica-polypropylene nanocomposites for HVDC film capacitors: Morphology-dielectric property relationships, and critical evaluation of the current progress and limitations

Dielectric polymer nanocomposites are considered as one of the most promising insulation material candidates for future capacitive energy storage applications, providing tailorability of charge trapping and transport properties at the nanometric level which is a key for increased dielectric performance of biaxially oriented polypropylene (BOPP) for metallized film capacitors in high-voltage direct current (HVDC) applications. In this study, a comprehensive investigation of morphology and dielectric performance of pilot-scale BOPP nanocomposites with hexamethyldisilazane (HMDS)-treated hydrophobic fumed silica nanoparticles was carried out, providing critical perspectives on the performance and challenges of PNCs for thin film capacitors also in a broader context. In non-oriented cast films, incorporation of nanosilica modified the crystallization kinetics and α/β-crystalline spherulitic morphology of polypropylene and reduced the accumulation of space charge under a DC electric field. The nanocomposites exhibited promising dispersion characteristics in the nano-scale, however, the low amount of micron-sized agglomerates inherently present in commercial fumed silica persisted in the compounds which can become critical for thin film applications. Subsequently, biaxial-stretching-induced morphology development and dielectric properties of silica-BOPP nanocomposites were evaluated, highlighting the role of precursor morphology and film processing in the silica-BOPP film morphology, defects and dielectric performance. Charge trapping and transport properties of silica-BOPP films were investigated by isothermal and thermally stimulated techniques under high DC electro-thermal stresses, indicating profound modification of the trap density of states brought about by nanosilica. This resulted in more homogeneous space charge distribution and reduced temperature- and field dependent DC conductivity at 100 °C in comparison to neat BOPP under moderate field stresses (<200 V μm-1), while simultaneously maintaining low dielectric loss. However, the localized weak points caused by silica agglomerates still remain a challenge for the structural homogeneity and dielectric breakdown performance of thin BOPP films under extreme-field stress, hence emphasizing the need for further advancements in the agglomerate and PNC film morphology control to provide high-reliability nanodielectric capacitor thin films for practical HVDC film capacitor applications

Feasibility of Mini-Scale Injection Molding for Resource-Efficient Screening of PP-Based Cable Insulation Nanocomposites

This study presents a critical evaluation of the effect of two different sample manufacturing techniques on the morphological and dielectric properties of polypropylene (PP)-based nanocomposites, namely mini-scale injection molding (IM) vs. pilot-scale cast film extrusion. Polarized light microscopy revealed that the IM specimen morphology exhibited a layered 'skin-core' type morphology, largely differing from the spherulitic morphology of the corresponding extruded cast films. Higher degree of crystallinity in the IM specimens was evidenced by calorimetric and X-ray diffraction methods. The processing-dependent morphological differences were found to affect the isothermal charging current (ICC) and thermally stimulated depolarization current (TSDC) characteristics due to differences in charge mobility and trapping, thus making direct comparison of IM and cast film specimens non-straightforward. Nevertheless, mini-scale injection molding can be seen as a resource-efficient sample manufacturing method for facilitating early-stage screening of the best-performing material candidates, given that the morphological features are carefully taken into account