Calculation and measurement of metalized film capacitor's inner pressure and its influence on self-healing characteristics

Self-healing is the main cause to capacitance loss of metalized film capacitors, and it might finally lead to the failure of a capacitor. Teardown analysis shown that capacitance loss decreased gradually from the outer layers to inner layers in a capacitor, and it is said that the elastic films add compressive radial force to every wound wrap, the pressure brought up by the radial force has an significant effect on self-healing, with the increasing of the pressure in the inner layers, the capacitance loss become stabilized. This paper analyzed the essential relationship between capacitance loss and pressure between film layers firstly. After that, the inner pressure was calculated based on the film theory, result shows that the inner pressure of a capacitor is related to the tension in the film, the width of the film and the location of the calculated film layer in a capacitor. Finally, the inner pressure was measured to confirm the calculated results. It shows that the measured and theoretical value are almost consistent at outer layers, but the theoretical value become large than that of the measured value when near to the inner layers, and both of them are show that the outer layers' pressure is less than that of the inner layers, and cleared area at the out layers is larger than that at the inner layers.