The expected life of a film capacitor is dependent on a number of factors, which generally apply to paper-oil capacitors as well. Many of the causes listed below apply mainly to high-voltage capacitors where life problems are of the greatest concern.
- Voltage stress: Electrons, in the form of leakage current, collide with polymer molecules causing them to break down. Given time, the dielectric will fail. Any kind of defect in the film or between windings will speed this process by creating voltage gradient “hotspots”. The defects may actually be required to cause damage. It is believed that in high voltage AC applications, UV light generated by charge movement deteriorates the dielectric. This has been observed in clear dielectrics like unfilled polyethylene, and no doubt happens in opaque ones as well.
- Corona: The worst form of voltage stress is “corona”, a current flow in ionized gas in a void, either in the dielectric, or between layers. Corona does not constitute a true voltage breakdown, but once corona starts, rapid deterioration of the polymer will lead to breakdown. Corona is similar to “partial discharge”, except that corona, by definition, always involves the emission of light and partial discharge may not.
- Moisture: Moisture absorption accelerates damage from voltage stress. Some film dielectrics, like polycarbonate and polysulfone tend to absorb more moisture than others (polypropylene and polystyrene),
- High-voltage pulses and voltage reversal: Stress on the dielectric depends on how fast voltage it is applied, as well as its magnitude. For example, when the voltage applied to a high-voltage DC capacitor is reversed, accumulated charge in the dielectric (called space charge) adds to the voltage stress. This must be allowed for in certain applications. Space charge is implicated in all sorts of capacitor behavior and failure mechanisms. Many high-voltage capacitors are designed for pulse operation. There is no rigorous way to calculate reliability. Manufacturers apparently depend on testing and experience.
- Thermal stress: Elevated temperatures will cause a slow deterioration of the dielectric, especially film.
- Contamination: Because the film is often only microns thick, microscopic conductive particles can cause shorts between the conductive layers. The short will not necessarily occur at the time of manufacture, but can occur hundreds or thousands of hours later when cold flow has allowed the particle to penetrate the film.
- Film defects: Thin spots, pinholes, cracks, voids, and impurities in the film can cause early failure. Because these are just about impossible to avoid in ultra-thin plastic films, film capacitors are often wound using several thin film layers instead of one thick one. It is unlikely that a pinhole or other defect in one layer will line up with a defect in another layer. Oil impregnation greatly reduces the problem.
- Voids in the windings: Voids in the windings, like voids in the dielectric, cause higher than normal voltage stress, and points where corona or voltage breakdown can more easily occur. This can also be mitigated by oil impregnation, which can greatly increase the voltage need to start corona.
- Other problems: Film capacitors tend fail by shorting, but some defects can cause other kinds of failures. Poor lead attachment can cause opens for example.
Read more – A Technical Guide to Capacitor