Development of Predictive Tools and the Role of Electrode Area For Self-clearing Behavior in Coated-glass Systems

Open Access
- Author:
- Pyrz, Matthew Joseph
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 01, 2015
- Committee Members:
- Michael T Lanagan, Thesis Advisor/Co-Advisor
- Keywords:
- Self-clearing
Capacitor
Dielectric
Breakdown - Abstract:
- In metallized capacitor systems, self-clearing behavior refers to the process in which an electrical arc is extinguished via electrode vaporization. Upon dielectric breakdown, a defect location within the sample rapidly increases in conductivity, forming a short between the two electrodes. This can result in catastrophic failure of the device as a result of positive feedback between localized joule heating and material degradation. However, graceful failure can be observed when sufficient thermal energy is directed to the electrode, which vaporizes and severs the connection between the current source and the conductive filament. The role of a fixed electrode area on the resulting clearing regions was examined using both experimental and computational approaches. By constructing a mulit-area breakdown fixture, self-clearing in Al-coated polypropylene films was examined for five distinct electrode diameters (5-mm, 8-mm, 10-mm, 12-mm, and 15-mm). Though these areas are proportional to the amount of energy stored within the capacitor, it was theorized that they may play a restrictive role in controlling the amount of charge that can migrate into the conductive filament before the arc is extinguished. The results show that for small diameters (5-mm), the clearing behavior is highly predictable, which indicates that this electrode size does limit the amount of vaporization that can occur. However, for larger fixed areas, the clearing regions tend to increase accordingly, but their sizes become much less consistent between trials. Finally, by increasing the fixed electrode size, the energy released from the capacitor begins to dominate the self-clearing process, while the energy contribution from the source and cables is reduced. Two separate approaches were taken to simulate self-clearing behavior in coated-glass systems. First, an energy summation model considered the amount of energy stored within the capacitor of a given area, and assumed that all this energy is converted into heat at the epicenter of the breakdown event. This model concluded that capacitive energy discharge must be directed towards the coating, instead of the glass dielectric material for sufficient temperatures to be reached. Therefore, an electrode heating model was developed to simulate the pattern of joule heating in the coating. This model predicts that the electrode area has a direct impact on the size of the clearing region, but tends to overestimate the importance of this trend when compared to the experimental findings.