Glass as dielectric for high temperature power capacitors
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Glass as dielectric for high temperature power capacitors Timothy J Patey, Christoph Schlegel, and Emmanuel Logakis ABB Corporate Research, Segelhofstrasse 1K, CH-5405 Baden-Dättwil, Switzerland. ABSTRACT Modern polypropylene film power capacitors are state of the art for power factor correction and many DC link applications, but their long-term commercial use is limited to temperatures of less than 85°C. The temperature limit is given by the dielectric polypropylene which has a melting point in the range of 140 to 170°C, while glass is much higher. Thus, the temperature limit could potentially be overcome by use of thin, alkali-free glass as dielectric. “Glass capacitors” employing ultra-thin and high purity glass layers are promising devices for high temperature applications in oil, gas, aerospace, hybrid electric vehicles, DC transmission, and pulsed power systems. This includes emerging power electronic systems using silicon carbide switches and diodes. This work analyzes and compares various glasses with a thickness of less than 50 µm by dielectric spectroscopy and elemental analysis. It is demonstrated that glass is attractive as dielectric for a wide frequency range up to 200°C. It argues that the dielectric losses are currently too great for thin glass to be used within a commercial power capacitor. While high temperature prototypes already exist, we demonstrate through our analysis that further developments are required to integrate this promising device into commercial systems. It is seen that even trace amounts of alkali materials can have an impact on losses. These losses must be further reduced through fundamental research into polarization/conduction mechanisms of various glass components. INTRODUCTION Polypropylene (PP) is the current dielectric of choice for power capacities. For high energy pulse power capacitors with PP as dielectric, which require additional packaging and voltage derating, an energy density of 2 J/cm3 is ideal [1,2], although maximum energy density is around 6.5 J/cm3 [3]. Thin, alkali-free glass has been reported to have an energy density approaching 35 J/cm3 based on a high dielectric breakdown strength (12 MV/cm) and a high permittivity (6) in an alkali-free barium boroaluminosilicate glass [4]. Murata et al investigated the breakdown strength of thin, alkali-free glass with a steel pin electrode and metallic plate and measured a characteristic electrical breakdown field strength of 400 to 1100 MV/m as the glass substrate thickness decreased from 58 to 5 µm [5]. They suggest that randomly distributed bulk defects contribute to the breakdown of the glass. Recently, Manoharan et al examined the high temperature behaviour of alkali-free glass and write that a commercially available glass has a low dielectric loss of 0.25 to 0.35 % up to 300°C, although experimental data are presented up to 180°C [6]. In this work, we investigate experimentally the thermal properties of commercially available, thin, alkali-containing and alkali-free glasses by dielectric spectroscopy. Analysis of t
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