Self- healing is the ability of a metallized capacitor to clear a fault area where a momentary short occurs due to dielectric breakdown under voltage. The conditions that lead to a fault vary. In the production of the dielectric film, contamination can occur or a process control problem can result in compromised dielectric strength.
Film capacitors have the capability to self-heal, as some of them are able to remove or mitigate fault areas through a process referred to as self-healing. The ability of a film capacitor to self-heal is mainly determined by its dielectric and electrode materials.
Image courtesy of KYOCERA AVX. Metallized film capacitors exhibit a self-healing property that significantly improves their lifetime reliability characteristics. Figure 4 depicts the basic process wherein a dielectric defect results in a high current, high-temperature short circuit that quickly demetallizes the surrounding area.
In high voltage, high energy applications such as electric trains and solar power grids, the safety and reliability of capacitors are paramount. Catastrophic failures and associated explosions or fires are unacceptable. Just as importantly, service lifetime and predictability for optimizing up-time are critical to the product's success.
Unfortunately, this mechanism can be dificult to control, and in the worst case, a run-away process can result, causing the destruction of the entire capacitor in short order. To avoid this, KYOCERA AVX developed a controlled self-healing process in 1974 based on the segmentation of overall capacitance into elementary cells protected by fuse gates.
High voltage capacitors for energy storage are generally divided into two distinct technologies: aluminum electrolytic and metal film. Electrolytic capacitors rely on an aluminum oxide dielectric grown on aluminum foil electrodes to form the basic structure.