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Self-healing of metallized film capacitors

Release time:

2024-01-05 17:26

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Metallized film capacitors possess a self-healing property that is not found in film/foil configurations. When subjected to a sufficiently high voltage, point defects causing short circuits between the metallized electrodes will vaporize due to the high temperatures generated by the electric arc. This is because both the dielectric plastic material at the breakdown point and the metallized electrodes surrounding the breakdown area are extremely thin—typically ranging from about 0.02 to 0.05 micrometers. The point defect responsible for the short circuit is burned away, and the resulting vapor pressure helps blow the arc away. This process can be completed in less than 10 microseconds and usually does not interrupt the useful operation of the affected capacitor. This self-healing capability allows single-layer windings made of metallized films to be used without any additional defect protection, thereby reducing the physical space required to meet specified performance criteria. In other words, it enhances the so-called volumetric efficiency of the capacitor. The self-healing ability of metallized films is utilized multiple times during the manufacturing process of metallized film capacitors. Typically, after the metallized film has been cut to the desired width, any defects that may have formed can be burned out (or repaired) by applying an appropriate voltage before winding begins. The same method is also employed after surface metallization (schopage) to eliminate any defects introduced during the secondary metallization process. The pinholes created by the self-healing arc may slightly reduce the capacitance of the capacitor. However, this reduction is minimal; even if thousands of defects need to be burned out, the overall reduction typically remains well below 1% of the capacitor’s total capacitance. For large film capacitors designed with exceptionally high stability and long service life—such as buffer capacitors—special fault-isolation patterns can be used for metallization. The image on the right shows such a T-shaped pattern being formed. Each of these T-shaped patterns intentionally narrows the cross-section of the conductive metallized layer. These constrictions act like miniature fuses: if a point defect causes a short circuit between the electrodes, the high current flowing through the short will only burn out the fuse surrounding the fault. As a result, the affected section is disconnected and isolated in a controlled manner, without any explosive propagation around the larger short-circuit arc. Consequently, the affected area is confined, the fault is mitigated and controlled, significantly reducing internal damage to the capacitor. In field installations of power distribution equipment, multiple capacitors are often connected in parallel to enhance the fault-tolerance of the capacitor bank, with each capacitor protected either by an internal or external fuse. If a single capacitor experiences an internal short circuit, the resulting fault current—amplified by the discharge of capacitance from neighboring capacitors—will melt the fuse, thereby isolating the faulty capacitor from the rest of the system. This technique is similar to the aforementioned T-pattern metallization approach but operates on a much larger physical scale. More complex series and parallel arrangements of capacitor banks are also employed to ensure continued service availability, even in the event of a single capacitor failure on this larger scale.

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