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

Temperature rise measurement for power-loss comparison of an aluminum electrolytic capacitor between sinusoidal and square-wave current injections

DC-link capacitors are a major factor of degrading reliability of power electric converters because they usually have a shorter lifetime and higher failure rate than those of semiconductor devices or magnetic devices. Characteristics of the capacitors are usually evaluated by a single sinusoidal current waveform. However, actual current flowing out of the converter into the capacitor is a modulated square current waveform. This paper provides experimental comparison of the power loss dissipated in an aluminum electrolytic capacitor between sinusoidal and square-wave current injections. Power loss is estimated by temperature rise of the capacitor. Experimental results confirm that power losses of the square-wave current injection were always lower than those of the sinusoidal current injection by 10–20%. Moreover, the power losses of the square-wave current injection can be estimated by a synthesis of fundamental and harmonic currents based on the Fourier series expansion, which brings a high accuracy less than 1% when more than fifth harmonic current is introduced. This comparison will be useful for estimating power loss and life time of electrolytic capacitors

A New Evaluation Circuit with a Low-Voltage Inverter Intended for Capacitors Used in a High-power Three-phase Inverter

DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current rating and full-scale voltage rating inverter.2016 IEEE Applied Power Electronics Conference and Exposition (APEC 2016), March 20-24, 2016, Long Beach, California, US

A New Evaluation Circuit with a Low-Voltage Inverter Intended for Capacitors Used in a High-power Three-phase Inverter

016 IEEE Applied Power Electronics Conference and Exposition (APEC 2016), March 20-24, 2016, Long Beach, California, USADC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current rating and full-scale voltage rating inverter

DC-bias-voltage dependence of degradation of aluminum electrolytic capacitors

Attention has been paid to reliability-related issues for dc-link capacitors such as monitoring methods, powerloss estimation, and ageing tests. The degradation of the capacitors depends on their operating condition including temperature, ripple current, and dc-bias voltage, which has a strong influence on failures as well. In design stages of power converters, it is desirable to know the relation between the degradation and electrolytic parameters. This paper makes an intensive discussion on the voltage dependence of the degradation of a small aluminum electrolytic capacitor with an ageing test and a leakage-current measurement. The ageing test reveals that a higher dc-bias voltage brings a faster increase in ESR but results in a slower drop in capacitance in a range within the rated voltage. This result implies that either capacitance or ESR cannot be a unique indicator of the lifetime. Attention should be paid both to the ESR and to the capacitance when one monitors the capacitor condition. On the other hand, more than the rated voltage leads a rapid degradation of the capacitor, which can be confirmed by a leakage-current measurement instead of the ageing test

Design and Analysis of a New Evaluation Circuit for Capacitors Used in a High-Power Three-Phase Inverter

DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current-rating and full-scale voltage-rating inverter. Theoretical analysis and simulated results verify the effectiveness of new evaluation circui

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

Comparative Reliability Analysis for Resonant Converter Operation under Component Ageing

This paper is concerned with reliability analysis in relation to optimal switching of resonant power converters, aimed at minimizing their power loss and operational temperature and thus reducing their lifetime degradation. In most cases, high performance resonant converter control requires a continuous adaptation of the power device switching instances depending on the power input source, load current, or parameter variation in passive components. If the resonant converter is switched before or after the resonant swing ends, more loss is added to the system, with effects in operating temperature rise and reliability deterioration. Even if the switching frequency is adjusted to keep the output voltage constant, it may influence the operational temperature. This article is quantifying this problem with a calculation of the lifetime implication for both cases of Zero Voltage Transition (ZVT) and Zero Current Transition (ZCT) buck converters. A broad range of parameter variation in passive components is considered due to ageing or aggravated stress. A thorough performance (lifetime / reliability) evaluation is achieved through a comparative analysis of results obtained for resonant converters with ideal operation, resonant converters with adjusted timing due to parameter variation, and hard-switched converters without any resonance. Conclusions are formulated in the end. The results reported in the paper should be of high interests to the power electronics community, given that resonant converters are used in a wide range of applications due to their ability to reduce power semiconductor losses

Experimental Verification of a New Evaluation Circuit for DC-link Capacitors Used in a Three-Phase Inverter

DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper presents experimental verification of a new evaluation circuit for dc-link capacitors used in a three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The circuit consists of a full-scale current-rating and downscale voltage-rating three-phase inverter, a low-voltage dc supply, and a high-voltage dc supply, which brings smaller power rating than that of a full-scale voltage-rating inverter by a factor of ten. A 1.2-kV 6400-W prototype was designed and constructed using a 120-V 640-W inverter, which verifies that the new circuit provides the same ripple current waveform as that of the full-scale three-phase inverter.電力技術／電力系統技術／半導体電力変換技術合同研究会, 3月8日-9日, 2016年, 九州工業大学　戸畑キャンパス, 福岡県