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DC Breakdown Strength of Polypropylene Films: Area Dependence and Statistical Analysis
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2007 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, no 2, 275-286 p.Article in journal (Refereed) Published
Abstract [en]

The dc breakdown strengths of 4-15 μm thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 cm2. Some samples werestudied with larger electrodes. Samples were not aged but were measured as received.The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters α and β were fitted to the data. Depending on the electrode area and the film grade, the α-parameter values obtained were between 450 and 850 V/μm. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrodeareas. The α-values for the large sample areas were extrapolated from the small areadata by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 m2 by 40-50% whereas the Weibull extrapolation gave anaccuracy of ±15%. The extrapolated breakdown strengths for the full-scale powercapacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples.

Place, publisher, year, edition, pages
2007. Vol. 14, no 2, 275-286 p.
Keyword [en]
polypropylene (PP), biaxially oriented PP (BOPP), breakdown strength, Weibull distribution, area dependence, extrapolation, capacitor, capacitor film, capacitor dielectric
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-5304DOI: 10.1109/TDEI.2007.344604ISI: 000245598900002Scopus ID: 2-s2.0-34147181678OAI: oai:DiVA.org:kth-5304DiVA: diva2:8365
Note
Uppdaterad från submitted till published: 20101027. QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Polypropylene: Morphology, defects and electrical breakdown
Open this publication in new window or tab >>Polypropylene: Morphology, defects and electrical breakdown
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Crystal structure, morphology and crystallization kinetics of melt-crystallized polypropylene and poly(propylene-stat-ethylene) fractions with 2.7 to 11.0 mol% of ethylene were studied by differential scanning calorimeter, wide- and small-angle X-ray scattering, polarized light microscopy, transmission electron microscopy and infrared spectroscopy. With increasing ethylene content the poly(propylene-stat-ethylene) fractions showed unchanged crystallinity, increased unit cell volume and constant crystal thickness in combination with a shortened helix length. This indicated that a fraction of ethylene defects were incorporated into the crystal structure. During the isothermal crystallization both α- and γ-crystals could be formed. The γ-crystal fraction increased with increasing ethylene content and increasing crystallization temperature. For samples with α- and γ-crystal contents, multimodal melting was observed and a noticeable γ- to α-crystal conversion was observed on slow heating. The spherulitic structure of the copolymers was coarser than that for the homopolymer.

The crystalline lamellae in copolymers exhibited profound curvature in contrast to the straighter cross-hatched α-crystals typical to the homopolymer. Area dependence of electrical breakdown strength was studied for thin polypropylene homopolymer films. The measurements were performed with an automatic measurement system equipped with a scanning electrode arm. Five different electrodes having areas between 0.045 cm2 and 9.3 cm2 were used and typically 40-80 breakdowns per sample and electrode area were collected. All measurements were performed on dry samples in air at room temperature. The data was analyzed statistically and the Weibull function parameters α and β, the first one related to 63% probability for the sample to break down and the second one to the width of the distribution were fitted to the obtained data. Different features concerning the measurement system and conditions, e.g. criteria for the automatic detection of the breakdowns, effect of electrode edge design, partial discharges, DC ramp speed and humidity were critically analyzed. It was concluded that the obtained α-parameter values were stable and repeatable over several years of time. The β-parameter values, however, varied ± 10-30%, more for the large than the small electrodes, and were also sensitive to the changes both in the sample itself and in the measurement conditions.

Breakdown strengths of over 50 capacitor grade polypropylene films were analyzed. The obtained α-parameter values were between 450 and 850 V/μm, depending on the film grade and electrode area. In addition to the high breakdown strengths, reflected by the obtained α-values, another, sparse distribution consisting of low breakdown strengths was revealed when the amount of measurement points was high enough. This means that more than one Weibull distribution could be needed to describe the breakdown strength behavior of a polypropylene film. Breakdown values showed decreasing area dependence with decreasing electrode area. Breakdown strengths for larger sample areas were predicted from the small area data by area- and Weibull extrapolation. The area extrapolation led to predicted α-values 50% higher than measured at 4 m2 whereas the Weibull extrapolation showed an accuracy of ±15 % when predicted and measured values were compared.

Breakdown strengths were also extrapolated for film areas similar to those in impregnated power capacitors. It turned out that the power capacitors, tested at the factory, performed much better than predicted by the extrapolation. However, a few weak spots with very low breakdown values were also found. For the poly(ethyelene terephtalate) dielectric, which is not swelled by the impregnation liquid, the large area breakdown strength was predictable. This indicates that for polypropylene film processing and impregnation led, in addition to the improved large area breakdown performance, also to sparse weak spots with low breakdown probabilities. Different Weibull distributions were responsible for the breakdown strengths for the processed and impregnated polypropylene than for the dry film samples.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. [71] p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2005:19
Keyword
Chemical engineering, polypropylene, poly(propylene-stat-ethylene), crystal structure, morphology, crystallinity, crystallization kinetics, lamellae, spherulite, defect, Kemiteknik
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-268 (URN)91-7178-091-2 (ISBN)
Public defence
2005-06-17, Sal E2, Lindstedtsvägen 3, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2010-10-27Bibliographically approved

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