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Influence of electrode area on dielectric breakdown strength of thin poly(ethylene terephthalate) films
KTH, Superseded Departments, Polymer Technology.
2004 (English)In: Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on, 2004, 563-567 p.Conference paper, Published paper (Refereed)
Abstract [en]

DC breakdown voltages of 8-20 p thickpoly(ethy1ene terephthalate) (PET) films were measuredas a function of electrode area. The areas of the measurement electrodes were between 0.045-9.3 cm2.Also 13-15 m2 capacitor elements were tested. Weibull distribution parameters, scale parameter a and shapeparameter p, were fitted to the obtained breakdownstrength data. Based on the small area results, short termbreakdown strength of large area capacitor elementswas predicted. Area extrapolation based on the scaleparameter a was compared with Weibull extrapolation,in which also the shape of the distribution is taken into account. It was concluded that the Weibull extrapolation better represented the large area breakdown strength than the area extrapolation.

Place, publisher, year, edition, pages
2004. 563-567 p.
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-5303DOI: 10.1109/CEIDP.2004.1364312Scopus ID: 2-s2.0-17744389267OAI: oai:DiVA.org:kth-5303DiVA: diva2:8364
Conference
2004 Annual Report Conference on Electrical Insulation and Dielectric Phenomena,17-20 Oct. 2004
Note
QC 20101027Available from: 2005-06-07 Created: 2005-06-07 Last updated: 2010-10-27Bibliographically 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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