Failure Mechanism of Metallized Paper Dielectric Capacitor

The electrode of a metallized paper capacitor is a metal film evaporated in vacuum on the surface of the capacitor paper.

A, electrical parameter deterioration failure.

"Self-healing" is a unique advantage of metallized capacitors, but the process of self-healing is quite complex. However, the phenomenon of metal particle migration and thermal cracking of the dielectric material will certainly occur in the self-healing part. Capacitor paper is made up of fiber, cellulose is a kind of carbohydrate macromolecule material. At high temperature, the cellulose of the capacitor is decomposed into free carbon atoms or carbon ions, which increases the surface conductivity of the self-healing part, resulting in the decrease of the capacitor resistance, the increase of the loss and the decrease of the capacitance. In severe cases, the capacitor can be invalidated by the deterioration of electrical parameters beyond the scope of technical conditions. When the metallized paper dielectric capacitor works below the rated working voltage, the self-healing energy is insufficient, and the conductive impurity in the paper forms a low resistance path under the action of electric field, which can also lead to the decrease of the insulation resistance and the increase of the loss of the capacitor.

Capacitor paper is a porous polar organic material, easy to absorb moisture. Although the capacitor core is impregnated, the insulation resistance of capacitor will be reduced and the loss will be increased if the process is improper or impregnated is not pure, or the phenomenon of impregnation and aging occurs after working under the electric field for a certain period of time.

A failure form of metallized paper dielectric capacitors produced by overcapacity failure. The metallized paper dielectric capacitors may fail because of the increase of capacitance when stored at high temperature, and when the voltage is added at high temperature. When stored at high temperature, semi-sealed metallized paper capacitor can not avoid moisture absorption, water is a strong polar material, its dielectric constant is close to 20 times of that of impregnated capacitor. Therefore, a small amount of moisture intrusion into the capacitor core, will also cause a significant increase in capacitance. The capacitance will decrease after baking and dehumidification. If the capacitor works in the high temperature environment, the joint action of water and electric field will cause electrolytic corrosion of the metal film electrode, reduce the effective area of the electrode plate and increase the resistance of the plate, resulting in a large reduction of the capacitance. If the lead is corroded in contact with the metal film, the contact resistance will increase and the effective capacitance of the capacitor will be further reduced. The capacitance of individual capacitors can be reduced to close to open circuit.

B, lead fracture failure.

When metallized paper dielectric capacitors work in high humidity environment, the positive end lead of the capacitor will be seriously corroded. This kind of electrolytic corrosion will lead to the reduction of the mechanical strength of the lead and the failure of the lead when it is serious.