The self-healing capacitor is characterized by its self-healing performance. When the dielectric breaks down, the short-circuit current causes the metal film around the breakdown site to melt and evaporate, thus restoring the insulation and thus providing high operational reliability. The time required for self-healing after media breakdown is only a few microseconds, and when the weak point of the capacitor internal media breakdown occurs to form a pathway, an arc is formed within a very short period of time, so that the local temperature and pressure rises sharply, the metal layer evaporates violently, the self-healing radius expands, the arc is pulled off, and a circular area is formed on the surface of the media with the breakdown point as the center of the lost metal coating, and the self-healing process is completed. When the damaged self-healing capacitor is opened, it can be found that the self-healing effect is most obvious in the edge part of the medium inside the capacitor.
Gas pressure created by metal vapor out of the area failed drives and thus an isolated area non-conductive and non-metalized layer is created in this location. And this time the condenser is running normally and no significant current is drawn from the grid. This characteristic will provide extra protection against high voltage transients across the device. To enhance this action some new designs have divided the metal into segments where the shorting current will fuse open the bad segment.
Self-healing type low voltage shunt capacitor is suitable for the AC power system with working frequency 50Hz or 60Hz, rated voltage 1000V and below and is connected in parallel with load, in order to improve the power factor of system and reduce line loss, improving the voltage quality. It is widely used to improve the efficiency of power supply equipment, local compensation of the inductive equipment such as motor and transformer and electric furnace, electric traction engine, UPS systems and power system harmonic filtration and other aspects.
Self-healing type capacitors are characterized by self-healing properties. When the medium is broken down, the short circuit current will melt the metal film around the breakdown area, thus restoring the insulation, so it has high operational reliability. The required time for healing after breakdown is only a few microseconds. When the capacitor internal vulnerabilities occur during circuit formation, arc is formed in a very short time, increasing the local temperature and pressure sharply and making metal layer evaporate intensely with self-healing radius enlarged and arc broken, thus forming a circular area with a breakdown point as center and a metal coating lost on the surface of medium, which indicates self-healing process is complete.
Why capacitor will loss self-healing ?
Dielectrics always have weak spots or defects and thinner zones which are more sensitive to breakdowns than the ordinary material. A breakdown, i.e. a short circuit through the dielectric, leads to local energy generation which transforms the material in the breakthrough canal into a plasma and vaporizes the thin metallization around the breakthrough hole. Around this hole with a diameter of 5…100 μm (0.2…4 mils) there is created a metal-free insulation area approximately 0.1 to 3 mm (4 to 120 mils) in diameter. The instantaneous short circuit thus is turned into an open circuit. The phenomenon is called self-healing. The condition for self-healing is a certain minimum energy estimated to be at least 10 μJ. Small part capacitances or low working voltages, therefore, may lead to a lack of self-healing if a breakthrough nevertheless should occur in a weak spot, e.g. under extreme heat when the dielectric has grown weaker.
What’s the resulting effects of a self-healing ?
During self-healing the polymeric dielectric is decomposed. Carbon-rich compounds generate amorphous carbon which will be deposited on cavity walls and, unfortunately, also on the insulated burned off surfaces around the breakthrough hole and its canal walls.
Compounds very rich in carbon, such as polystyrene, generate so much carbon that they are impossible to use in a customarily metallized design. The carbon deposits destroy the IR after a self-healing.
Other materials have a varying degree of carbon generation (Table 1). That information, however, must be combined with information on the clearing chemistry of the materials. Best is PP followed by PET and the poorer PPS and PEN.
What’s the advantage of self healing capacitor ?
The benefits of self-healing capacitors 1- lower weight and volume compared to the older generation capacitors.2- Because of the simple construction and low consumption, it is economical. 3- The low dielectric loss and capacitance reduces the heat generated and thus more capacitor life.
What’s the disadvantages of self-healing capacitors ?
There are problems associated with self healing. the healing will leave residues of free carbon which can create excessive leakage. Second as the metal area is reduced with each healing the capacitor value will decrease and can cause a circuit failure.
Why Capacitor’ capacity decreases?
The capacity drop of self-healing capacitor in operation is a normal phenomenon, the drop is not too big, so it is not a fatal defect. The domestic self-healing capacitor drops 3%~5% in the early stage of putting into operation. Assuming that the capacitance current of each self-healing capacitor cabinet is 184A at the early stage of operation, and basically stabilizes at about 174A after a period of operation, the main reasons for the decrease of capacitor current in operation are as follows:
(1) The self-healing effect of the capacitor causes the reduction of the effective area of the plate. The duration of the self-healing process is about 1-10 uS, and the capacity of the capacitor is reduced by about 20-100F each time. A small amount of self-healing has little effect on the capacitor, but if the self-healing energy is too large, it will cause the effective area of the plate. If it is reduced too much, the capacitance will drop too fast.
(2) The electrical corrosion of the metal plate causes the plate resistance to increase. Under the action of electric field, temperature, and moisture, aluminum produces electrical corrosion to form Al2 0 3, making the resistivity as high as I0<sup>16</sup>2/cm . If the corrosion points appear in batches, the area of the electrode plate is reduced, which has a great influence on the reduction of the capacitance. The residual air in the element is the main factor causing the galvanic corrosion of the aluminum film.
(3) Edge erosion of the metal layer plate/ Due to the high working electronic field strength at the edge of the electrode plate, the corrosion rate is higher than that of the other parts,, and the phenomenon of edge receding will occur, reducing the electrode plate area.
