In the previous article we focused on one of the two different mechanisms of self-healing in metalised film capacitors: discharge self-healing, also known as high-voltage self-healing. In this article we will look at the other type of self-healing, electrochemical self-healing, also often referred to as low-voltage self-healing.

Electrochemical Self-Healing

Such self-healing often occurs in aluminium metalised film capacitors at low voltage. The mechanism of this self-healing is as follows: if there is a defect in the dielectric film of the metalised film capacitor, after the voltage is added to the capacitor (even if the voltage is very low), there will be a large leakage current through the defect, which is expressed as the insulation resistance of the capacitor is much lower than the value specified in the technical conditions. Obviously, there are ionic currents and possibly electronic currents in the leakage current. Because all kinds of organic films have a certain water absorption rate (0.01% to 0.4%) and because capacitors may be subject to moisture during their manufacture, storage and use, a significant part of the ionic current will be O2- and H-ion currents resulting from water being electrolysed. After the O2-ion reaches the AL metalised anode, it combines with the AL to form AL2O3, which gradually forms an AL2O3 insulation layer over time to cover and isolate the defect, thus increasing the insulation resistance of the capacitor and achieving self-healing.

It is obvious that a certain amount of energy is required to complete the self-healing of a metalised organic film capacitor. There are two sources of energy, one is from the power supply and the other is from the oxidation and nitriding exothermic reaction of the metal in the blemish section, the energy required for self-healing is often referred to as self-healing energy.

 
Self-healing is the most important feature of metalised film capacitors and the benefits it brings are major. However, there are some disadvantages, such as the gradual reduction in the capacity of the capacitor used. If the capacity is working with a lot of self-healing, it will lead to a significant decrease in its capacity and insulation resistance, a significant increase in the loss angle and a rapid failure of the capacitor.

If you have insights into other aspects of the self-healing properties of metalised film capacitors, please discuss them with us.