Problem: Ionization / Glow discharge
In the above photo the glow discharge, usually hidden inside the capacitor, becomes visible. The film, metal layer to the outside, runs on a metal roller with a potential of 5kV DC. As the film approaches the roller the air gap between roller and film narrows. Once this gap becomes smaller than the breakdown voltage of air an arc spreads to the reverse side of the film. Because of the high film speed there is a subsequent submission of new unloaded film leading to a constant glow.
Inside a capacitor winding this phenomenon will happen with AC voltage once the ionization point in an air gap between two film layers is exceeded.
Solution: both sides zinc metallization
Steiner now also offers both sides zinc metallized OPP film for non-impregnated AC film capacitors to diminish electrical aging by corona discharge in a capacitor.
This can be as well combined with a multiple inner series construction.
Please contact us for further information.
The following pdf download gives you more technical information:
At very high electric fields glow discharge in an air gap will also happen for DC capacitors. It is lower in magnitude compared to AC because it is fed by the rather small leakage current through the dielectric. The generated ions cause a corrosion of the anode only. The metal layer of the cathode stays unaffected.
Electrical weak spots test / Self-healing properties
The self-healing properties are a major advantage of metallized film capacitors. At weak spots of the dielectric film the conductive metal-layer will evaporate in an arc discharge and thus a permanent short-circuit is avoided. While inside a capacitor this effect is only audible it can be made visible on a special rewinding-machine that applies a voltage up to the destruction level of the film.