Modeling of stresses in cylindrically wound capacitors: Characterization and the influence of stress on dielectric breakdown of polymeric film

• Main Author: Tandon, Shalabh
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1997
• Subjects: Polymers|Materials science
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9809407

This dissertation investigates the elastic constants of the polypropylene (PP) film, the radial and circumferential stress states of the layers in the wound roll and the influence of compressive stress on the dielectric breakdown of the metalized polypropylene film. The metalized polypropylene film was mechanically and thermally characterized to determine 7 of its 9 elastic constants and 3 linear coefficients of thermal expansion. The results show that the in-plane tensile moduli (E$\sb{11}$ = 2.7 GPa, E$\sb{22}$ = 5.7 GPa) of the film are quite different and smaller than the out-of-plane modulus (E$\sb{33}$ = 13.0 GPa) of the film. Similarly, the out-of-plane thermal expansion coefficient (CTE) of the film is much larger than the in-plane CTE ($\alpha\sb3 \approx$ 10 $\alpha\sb2$). This large anisotropy in the moduli and the expansion coefficients will influence the winding and thermal stresses generated in the wound rolls. The radial and circumferential stresses in the layers of the wound roll were evaluated using the elastic constants of the film obtained in chapter 2. Expressions were derived to determine the influence of elastic constants of the film and the core on the radial and circumferential stresses in the roll. Stresses generated due to the thermal expansion of the assembly during operating temperature changes were also evaluated. The analysis showed that because of the applied winding stress, the layers near the core have compressive radial stresses. The circumferential stresses in the layers also decrease, becoming compressive in some cases for the layers near the core. The influence of the interfacial pressure (compressive stress) on the dielectric behavior of the film was the subject of chapter 4. Applying interfacial pressure, parallel to the electric field, changes the apparent dielectric breakdown strength of the film. At pressures of 0-4 MPa, the PP film has a catastrophic failure at 40% lower potential than its intrinsic breakdown potential. However, for slightly higher pressures ($\approx$5 MPa) the dielectric recovers the loss and can sustain potentials much higher than its intrinsic breakdown potential. The metalized PP films have a surface roughness imposed upon them to increase the interlayer frictive forces. This minimizes the axial slippage of layers in the roll. The surface roughness leads to entrapment of air (voids) within the layers of the roll. The air pockets are harmful for the film since they lead to premature failure of the dielectric. The capacitors are prepared by encapsulating the wound PP roll in a PP case surrounded by a dielectric fluid. At temperatures above 80$\sp\circ$C, the dielectric fluid diffuses through the PP containers. (Abstract shortened by UMI.)