Ilkka Rytöluoto

Large-area dielectric breakdown performance of polymer films - part i: measurement method evaluation and statistical considerations on area-dependence

A multi-breakdown measurement method for large-area dielectric breakdown characterization of polymer films is presented and evaluated. Based on the self-healing breakdown capability of metalized film, large amount of breakdown data can be obtained from a relatively large total film area, thus enabling the execution of detailed breakdown performance analysis. The studied films include non-metalized laboratory-scale, pilot-scale and commercial capacitor-grade bi-axially oriented polypropylene films in the thickness range of 14-25 μm. With the active measurement area of 81 cm2 per sample, breakdown distributions covering total film areas of 486-972 cm2 are presented. Various aspects encompassing the sample film preparation, measurement procedure, breakdown progression, discharge event characterization, breakdown field determination, data validation and statistical analysis are discussed. Comparative small-area breakdown measurements were performed in order to study the relationship between the large-area multi-breakdown measurement method and a conventional small-area (1 cm2) manual breakdown measurement method. Implications of the area-dependence and the applicability of the Weibull area-scaling are also discussed.

New approach to evaluate area-dependent breakdown characteristics of dielectric polymer films

A new dielectric breakdown measurement method for determining breakdown characteristics of polymer films has been developed and evaluated. The method is based on measurement of multiple breakdowns per sample area beyond the weakest point of the film by utilizing low-energy self-healing breakdown of metallized polymer film. A data selection process based on the discharge energy characteristics of each measured self-healing breakdown is utilized prior to the statistical analysis in order to validate the mutual independence of the results. Even with a relatively small sample area, the method yields a large amount of breakdown data from a wide voltage spectrum, thus enabling the formation of detailed material-specific breakdown fingerprints. The measurement system and the area dependence of the breakdown results have been evaluated with capacitor-grade metallized film. For the statistical analysis, additively mixed Weibull distributions are utilized as multiple breakdown mechanisms are found to be operative in the dielectric. Defects in the dielectric volume are found to have a profound effect on the structure of the mixed distribution with increasing area.

DC ramp rate effect on the breakdown response of SiO 2 -BOPP nanocomposites

The effect of voltage ramp rate on the short-term dielectric breakdown strength of polymer nanocomposites is not well-documented. In this paper, the effect of DC field ramp rate on the large-area breakdown performance of melt-extruded bi-axially oriented polypropylene (BOPP) films incorporating 4.5 wt-% of nano-silica is studied. By utilizing a self-healing multi-breakdown measurement method with a variable DC voltage ramp rate, a statistically large amount of breakdown data was obtained from a large total sample film area as a function of DC field ramp rate (0.1-50 Vs-1μm-1)). With a decreasing ramp rate, Weibull statistical analysis of the breakdown data suggests a systematically decreasing trend in the breakdown strength (Weibull α) and an increase in the Weibull shape parameter of time (>1) for the nanocomposite. The observed behavior is speculated to be attributable to highly altered internal charge dynamics of the silica-BOPP nanocomposite. The results exemplify the importance of careful breakdown strength assessment when dielectric films of more complex internal structure are studied.

Influence of low amounts of nanostructured silica and calcium carbonate fillers on the large-area dielectric breakdown performance of bi-axially oriented polypropylene

Influence of low amounts (1.0-2.0wt-%) of nanostructured silica and calcium carbonate fillers on the large-area dielectric breakdown performance of bi-axially oriented polypropylene (BOPP) is analyzed. A multi-breakdown measurement method based on the self-healing breakdown capability of metallized film is utilized for the breakdown characterization in order to cover relatively large total film areas, thus leading to results of higher relevance from the practical point-of-view. The dispersion and distribution qualities of filler particles at the nanoscale are evaluated with transmission electron microscopy (TEM) imaging. Weibull statistical analysis suggests that the breakdown distribution homogeneity can be improved with both the filler types. The 1.0wt-% silica-BOPP composite also shows a shift of the weakest points towards higher dielectric strength in comparison to the neat BOPP. However, with increasing filler content, new failure modes are introduced into the nanocomposites, hence decreasing the overall breakdown performance in the >5% breakdown probability region in comparison to the un-filled reference BOPP film.

Effects of thermal aging on the characteristic breakdown behavior of Nano-SiO 2 -BOPP and BOPP films

The effects of thermal stress on the breakdown behavior of biaxially oriented polypropylene (BOPP)-silica nanocomposite thin film and commercial BOPP film was evaluated with a step stress experiment in the temperature range of 50...110 °C. Weak spots were measured in thermally aged unfilled BOPP films but not in the PP-silica, which in turn displayed increasing scatter of the breakdown voltages and the scattering seemingly settled to a certain level. No weak spots were measured in un-filled BOPP films removed after highest temperatures. The behavior of un-filled films is speculated to result from either actual polymer aging in addition to the statistical nature of breakdown phenomena or from thermally-activated processes possibly involving antioxidants. The different behavior in the nanocomposite was speculated to result from polymer-nanoparticle interactions possibly countering the effects of antioxidants and hindering weak spot formation. The results demonstrate the different behavior of polymer-nanocomposites and traditional BOPP films under thermal stress and highlight the importance of evaluating the aging behavior of new materials.

Large-area approach to evaluate DC electro-thermal ageing behavior of BOPP thin films for capacitor insulation systems

A large-area electro-thermal ageing test setup was developed and utilized to age several laboratory-scale biaxially oriented polypropylene (BOPP)-hydrophobic silica nanocomposite films. The films were aged in test capacitors with self-healing metallized film electrodes, which enabled the ageing test to continue beyond the first breakdowns. Eight different films were aged for 1000 hours under 100 V/μm DC stress at 75 °C, and large-area DC breakdown measurements, dielectric spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and dielectric spectroscopy were used to detect localized and bulk degradation after the ageing period. The effects of antioxidant contents, different PP grades and compounder screw speed were evaluated. Material characterization indicates no bulk degradation had occurred during ageing, which was associated with moderate temperature stress and inert nitrogen atmosphere. On the other hand, low-field breakdowns (weak points) were observed in all but two of the aged materials, indicating that ageing was dominated by localized degradation which may have introduced new breakdown mechanisms. Weak points were also measured in a similarly aged commercial capacitor-grade BOPP film aged at a lower field, supporting this conclusion. The importance of long-term characterization in material development is demonstrated, and it is shown that long-term properties of the evaluated nanocomposites were at least on the same level compared to neat BOPP films.

Role of microstructure in dielectric properties of thermally sprayed ceramic coatings

Thermally sprayed insulating ceramic coatings can be utilized in conditions where polymers are inapplicable. The coatings exhibit a special lamellar microstructure with interfaces and some defects (e.g. voids, cracks) in between. The aim of this study was to analyze the relationship between the microstructural features and the dielectric properties of various thermally sprayed ceramics. The structural characterization of the ceramic coatings was made based on following properties: porosity, volumetric gas permeability and the characteristic size of crystalline lamella. High gas permeability of the coatings decreased the breakdown strength but similar effect cannot be seen in the DC resistivity and the permittivity results. Decrease of DC resistivity at high humidity did not correlate to microstructural properties; rather it is speculated to indicate the hydrophilic nature of the coatings. The characteristic crystalline domain sizes showed no clear correlation with the dielectric properties.

Differences in AC and DC large-area breakdown behavior of polymer thin films

A large-area multiple breakdown measurement using self-healing gold sputtered electrodes was evaluated in determining the AC breakdown characteristics of insulating biaxially oriented polypropylene (BOPP) and BOPP-silica nanocomposite thin films. Results were compared with conventional small area measurements and it is shown that the large-area approach could be extended into AC measurements as long as good quality film-electrode interface in ensured. Special electrode arrangements are needed due to the absence of electrostatic force. AC small-area breakdown strength of pilot-scale films was ramp rate dependent but no such effect was clearly noticed for a commercial product. Conventional DC small-area breakdown measurements were also conducted and the results were compared to previously published large-area multiple breakdown data. The consistent results support the dependency between ramp rate and DC breakdown strength in the studied BOPP-silica nanocomposite.

Nanocomposite polypropylene for DC cables and capacitors: A new European project

This paper presents the scientific background of a new European project, GRIDABLE, which was launched at the beginning of 2017 and has to deliver results in manufacturing and characterization of LV-MV capacitors and MV-HV cables for DC application. The innovation is in the development of nanostructured materials based on polypropylene and silica, and the relevant capacitor and cable manufacturing procedures. The initial results regarding the electrical properties of PP-SiO2 materials, which have brought to the proposal of this project, are presented in this paper, focusing on breakdown strength and space charge measurements performed on nanofilled PP films for capacitors.

Dielectric breakdown properties of mechanically recycled SiO 2 -BOPP nanocomposites

This paper reports on the effects of multiple compounding cycles on the structural, chemical, rheological and DC dielectric breakdown properties of bi-axially oriented silica-polypropylene nanocomposite films. While the effect of multiple compounding cycles on the silica particle agglomeration remains seemingly modest according to the quantitative structural analysis, the recompounding is found to markedly affect the steady shear viscosity and film processability. Fourier transform infrared spectroscopy is utilized to ascertain that no thermo-oxidative degradation is occurring during the reprocessing. The large-area multi-breakdown measurements indicate a ~13 % shift towards higher DC dielectric strength after the second recompounding cycle, an effect which is believed to be attributable mainly to the changes in film processability. Surprisingly, a 75:25 mixture of the virgin and three-times-reprocessed nanocompounds exhibits a similar increase in dielectric strength. Particularly from the processing perspective, the results emphasize the necessity of careful consideration of property modifications upon incorporation of nanofillers.