Steering Capacitor Film Development with Methods for Correct and Adequate Dielectric Performance Assessment

The transition of electric power systems towards renewable generation has created an increasing market for power electronics using film capacitors as one of their key components. Size, weight, and cost reduction can be achieved with better capacitors – an objective achievable with advanced dielectric films. The current state-of-the-art biaxially oriented polypropylene (BOPP) films are already operated close to their fundamental limits, causing a growing demand for next-generation technologies. To perform well when used in a capacitor, a film needs to have a wide range of fundamental and applied properties, all of which should be evaluated during film development to ensure there are no unwanted trade-offs. Power capacitors are used in applications with high downtime costs, e.g. HVDC, thus especially the reliability aspects must be given scrutiny. This thesis work was inspired by the lack of knowledge of the long-term performance of next generation dielectrics, e.g. polymer nanocomposites. Equally important was to fill the gaps in published knowledge of measurement methods to evaluate long-term properties, voltage endurance, and surprisingly, also the dielectric permittivity of thin (≈10 μm) low-loss films. In this thesis, a suitable measurement for each three is presented along with examples of their capability and an approach to applying them to steer film development. The large-area multi-breakdown method developed in our research group is extended to measurements at realistic operating temperatures, and industrial BOPP films are shown to exhibit an 11–20 % decrease in the DC breakdown strength between room temperature and 100 ◦C. The results align with literature, which supports the validity of the approach. BOPP films made of base materials varying in terms of molecular weight are measured: these films exhibit similar short-term breakdown performance at room temperature, yet at 100 ◦C differences emerge. The difference did not correlate with the reduction of breakdown performance after DC electro-thermal aging, demonstrating the necessity of long-term tests. Electron beam evaporation in high vacuum (P<10−6 mbar) is established as a repeatable and suitable method to metallize electrodes on ultra low-loss BOPP films, solving earlier issues of abnormally high dielectric losses or unrealistically low real permittivity. Metallization process is identified as the crucial factor: no pre- or post-treatments are required, and valid results are obtainable with various electrode metals. The method was demonstrated by measuring true “literature value” dielectric permittivity of commercial BOPP films: E≈2.25 and tanδ≈10−4. The importance of successful metallization process for measuring the intrinsic losses is demonstrated: samples with sputter deposited electrodes exhibited abnormally high dielectric losses, as also did samples metallized using another e-beam evaporator. The multi-breakdown approach is also extended to times-to-breakdown tests, and accelerated ageing tests are conducted on an industrial BOPP film. High-field degradation and drastically reduced insulation life are observed. Analysis of the Weibull failure rate supports the notion that at current design stresses, BOPP is already operated close to the fundamental material limits, and also that the life in operating conditions cannot be determined by simple inverse power law extrapolation of accelerated rapid ageing data. Again, long-term ageing testing is advocated. Space charge measurements on “classic” BOPP films reveal charge accumulation at high fields, as expected. Interestingly, no space charge accumulation is detected in a novel nanostructured material under similar conditions, demonstrating the potentiality of nanofilled DC insulation. A DC electro-thermal ageing test method is presented to investigate long-term phenomena in realistic operating conditions. Two 1000 h DC electro-thermal ageing tests associate ageing with the formation of electrically weak points. Large-area breakdown behavior, being sensitive to local changes, is established as a recommended ageing indicator. Material characterization does not reveal ageing-induced changes in bulk properties, supporting the literature-backed conclusion that early ageing progresses by localized degradation. A trial with eight pilot-scale materials demonstrate that weak point formation may be inhibited in nanostructured materials, but also that material-specific optimization of film processing is required to reach optimal dielectric performance. Ultimately, the methods developed are fused into one resource-efficient approach to capacitor film development, in which the short-, mid-, and long-term properties are evaluated in three overlapping phases. Reliance on individual performance metrics to steer film development is discouraged: all properties need to be at an appropriate level for a film to perform in application, and there are trade-offs to be managed

Capacitor Technologies, Applications and Reliability

Various aspects of capacitor technologies and applications are discussed. Major emphasis is placed on: the causes of failures; accelerated testing; screening tests; destructive physical analysis; applications techniques; and improvements in capacitor capabilities

Lämpöikäännytyksen vaikutukset kondensaattorikäyttöön suunniteltuihin ohutkalvoeristyksiin

Ohutkalvoeristyksiä käytetään kondensaattorisovelluksissa, joissa vaaditaan luotettavuutta, korkeaa energiatiheyttä ja pieniä häviöitä. Itseparanevia metalloituja ohutkalvoeristyksiä käyttäen kondensaattorit saadaan ikääntymään hallitusti. Uudet sovelluskohteet ovat luoneet tarpeen ohutkalvokondensaattoreiden energiatiheyden parantamiselle, mihin voidaan päästä kentänvoimakkuutta tai suhteellista permittiviteettiä nostamalla. Parannusta on haettu uusien materiaalien, kuten nanokomposiittien sekä valmistusprosessin parametrien säädön avulla. Kondensaattoreilta vaaditaan usein luotettavaa yli 30 vuoden elinikää, ja uusien materiaalin ikääntymiskäytös täytyy tuntea ennenaikaisen ikääntymisen varalta. Vaikka lyhytaikaisten ominaisuuksien määrittäminen onkin verraten yksinkertaista, tiedon puute uusien eristemateriaalien ikääntymiskäytöksestä on yksi alan haasteista. Diplomityön tavoitteena oli laatia kattava kirjallisuusselvitys polymeeriohutkalvoeristysjärjestelmistä ja kokeellisesti vertailla perinteisen BOPP-kalvon ja PP-silika nanokomposiitin ikääntymiskäytöstä lämpörasituksessa. Kappaleisiin 2–4 jakautuva kirjallisuusselvitys käsittelee polymeerien kemiallisia, fyysisiä ja sähköisiä ominaisuuksia, joiden pohjalta siirrytään tarkastelemaan kondensaattorisovelluksissa merkittäviä ikääntymismekanismeja sekä niiden vaikutusten mallintamista. Kokeellisessa osuudessa ohutkalvoja ikäännytettiin 1008 tuntia suojakaasussa ja askeleittain korotetussa 50…110 °C lämpötilassa. Ikääntymistä kartoitettiin läpilyöntikäytöksen muutoksien avulla, uutta MultiBreak-mittausmenetelmää hyödyntäen. Puhtaassa PP-kalvossa mitattiin 50…70 °C lämpörasituksesta lähtien pääjakaumasta poikkeava defektipopulaatio, mitä kirjallisuusselvityksen pohjalta pidettiin merkkinä paikallisiin epäideaalisuuskohtiin keskittyvästä ikääntymisestä. PP-silikassa vastaavaa kehitystä ei havaittu, mikä nähtiin merkkinä erilaisesta, polymeeri-nanopartikkelien vuorovaikutusalueessa tapahtuvasta ikääntymisestä. Puhtaissa PP-kalvoissa esiintynyt heikkojen pisteiden jakauma katosi 90…100 °C jälkeen, minkä tulkittiin viittaavan erilliseen, vain korkeilla lämpötiloilla toimivaan mekanismiin. Aiheuttajaksi esitettiin rakenteellisia muutoksia tai antioksidanttien kulumista, mutta aihe vaatii lisätutkimusta. Tutkimustulosten pohjalta esitetään, että puhtaiden PP-ohutkalvojen ikääntymiskäytöstä reaalimaailman sovelluksissa pitää tarkastella enintään 60…65 °C lämpötiloissa, kun taas nanosilika–materiaalien ikääntymistä voidaan kiihdyttää ainakin 110 °C:ssa. Lisäksi hallittu lämpökäsittely ja lämpöön liittyvien prosessointiparametrien säätö pitää nähdä mahdollisina keinoina parantaa nykyisten PP-kalvojen läpilyöntikestoisuutta

Large-Area Multi-Breakdown Characterization of Polymer Films: A New Approach for Establishing Structure–Processing–Breakdown Relationships in Capacitor Dielectrics

The ever-growing need for high-energy density and high operation temperature capacitive energy storage for next-generation applications has necessitated research and development on new dielectric materials for film capacitors. Consequently, various new approaches offering unique ways to tailor dielectric properties of polymers have recently emerged, and new materials such as dielectric polymer nanocomposites (PNC) are envisioned as potential next-generation dielectrics. Establishment of optimized formulation and processing conventions is however necessary in order to achieve improvement in dielectric breakdown properties. Importantly however, such material development puts dielectric breakdown strength assessment of polymer films in a central role in guiding material development process towards highly optimized functional materials. This is not a trivial task though, as the current state-of-the-art breakdown strength measurement techniques rarely provide statistically sufficient amounts of breakdown data from the application point-of-view, thus leading to impaired evaluation of the practical breakdown performance in film capacitors.In this thesis, a new large-area multi-breakdown measurement method enabling detailed dielectric breakdown characterization of polymer films is developed and evaluated. Various aspects encompassing sample film preparation, measurement procedure, breakdown progression, discharge event characterization, breakdown field determination, data validation and statistical analysis are systematically and critically discussed. A data qualification process based on the self-healing discharge energy and breakdown voltage characteristics is developed and shown to be a sensible and convenient way to exclude non-breakdown events from the measurement data prior to Weibull statistical analysis. The measurement method is shown to enable high-statistical-accuracy breakdown characterization of both metallized and non-metallized polymer films of different nature, including laboratory-scale, pilot-scale and commercial-grade capacitor films. Statistical aspects on the area dependence are discussed and the problematic nature of Weibull area-extrapolation of small-area breakdown data to represent larger film areas is exemplified. The fundamental differences between the large-area multi-breakdown and the small-area single-breakdown measurement methods and the statistical aspects thereof are analyzed in more detail by the Monte Carlo simulation method.The large-area multi-breakdown method is utilized to carry out a comprehensive analysis on structure--processing--breakdown relationships in conventional polymer and polymer nanocomposite films. Analysis on the effects of film processing, structure and morphology on the large-area multi-breakdown response of cast- and bi-axially oriented isotactic polypropylene (PP) films emphasizes the determining effect of processing-dependent film morphology in large-area dielectric breakdown response of PP films. Commercial capacitor-grade bi-axially oriented polypropylene (BOPP) films are shown to exhibit differences in breakdown distribution structure and weak point behavior in comparison to the laboratory-scale BOPP films, presumably due to differences in raw material, additives, thermal history and processing. Breakdown characterization of commercial metallized polymer films as a function of inter-layer pressure also emphasizes the importance of careful breakdown characterization under authentic operation stresses in order to ensure proper design and operation in practical applications.BOPP-based polymer nanocomposite (PNC) films are studied with a particular emphasis on the effects of various compositional, structural and film processing factors on the breakdown behavior of laboratory- and pilot-scale melt-compounded BOPP nanocomposite films incorporating silica and/or calcium carbonate nanofillers. The optimum nano-filler content is found to reside at the low fill-fraction range (~1 wt-%), however, the fill-fraction itself is not the only determining factor, as compounds with equal nanoparticle content but with differences in e.g. compounder screw speed are found to exhibit large differences in breakdown response. Indications of possible silica-antioxidant interaction are also reported. Structural imaging of the films shows that nano-structural features cannot solely explain the observed large-area breakdown behavior – this aspect points towards large-area approach being necessary for the imaging techniques as well in order to reliably establish a link between structural properties and engineering breakdown strength. The results point out that up-scaling of PNC production is sensible with conventional melt-blending technology, although further development and optimization of nanocomposite formulations and processing are seen as necessary. Analysis on the ramp-rate-dependence of the breakdown response in dielectric polymer nanocomposite films also provides perspective on the importance of careful breakdown assessment when dielectric films of more complex internal structure are studied

Techniques of Final Preseal Visual Inspection

A dissertation is given on the final preseal visual inspection of microcircuit devices to detect manufacturing defects and reduce failure rates in service. The processes employed in fabricating monolithic integrated circuits and hybrid microcircuits, various failure mechanisms resulting from deficiencies in those processes, and the rudiments of performing final inspection are outlined