A89D381Ed01 - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 1; February 2008 1070-9878/08/$25.00 © 2008 IEEE 277 Transient Finite Element Computation of the Temperature Rise in Metallized Film Capacitor End Connections Caused by Underdamped Discharge Xiaoguang Qi* and Steven Boggs* + Electrical Insulation Research Center Departments of Physics* and Electrical Engineering + University of Connecticut 97 North Eagleville Road Storrs, CT 06269-3136, USA ABSTRACT Loss of end connection integrity is the most common failure mode for metallized film capacitors under pulse discharge conditions as a result of electrothermal effects associated with the intermittent nature of the contact between the sprayed metal end connection and the edge of the capacitor film. In this paper, the temperature rise as a result of varying polarity reversal is analyzed through the use of transient finite element analysis which shows that polarity reversal is unlikely to cause greater damage to the end connection than discharge without polarity reversal. This suggests that the adverse effect of polarity reversal is dielectric or electrical, but not electrothermal in nature. Index Terms Polarity reversal, end connection, metallized film capacitor, finite element. 1 INTRODUCTION METALLIZED film capacitor technology has the advantage of self clearing, which provides recovery from defect-induced breakdowns. This allows metallized film capacitors to operate relatively near to the breakdown field of the capacitor film. Such capacitors windings are made with offset films which also have unmetallized margins on the side to which no connection is made. The end connections of such capacitors are usually formed by wire arc metal spray. The metal spray particles are typically in the range of 50 μ m in diameter while the capacitor films are in the range of 10 μ m in thickness. The edge of the film is not metallized, so to form an effective electrical connection, the spray must penetrate between the film layers and adhere to the surface metallization of the film. This process does not involve a metallic bond but rather the hot metal spray particles cause the film to shrink around the spray particles which results in a degree of adhesion. However the penetration of the 50 μ m spray particles into the 10 μ m gap between films is sporadic, with substantial worst case distances, in the range of mm, between effective connections to the film. As the current flowing through the capacitor must go through the limited number of end connection “spots”, the current density around these spots can be very large, resulting in heating of the film on a microsecond time scale, thermal expansion, etc., which can cause failure of end connection spots, resulting in greater current density in the remaining spots. This process can cause progressive failure of the end connection, and end connections are the weak link when metallized film capacitors are operated at high discharge currents. We have studied this process for discharge current waveforms without polarity reversal [1].
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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A89D381Ed01 - IEEE Transactions on Dielectrics and...

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