What is a Self-healing Low Voltage Shunt Capacitor?
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.
The component used in a dry self-healing type high-voltage shunt capacitor is the self-healing type capacitor element. Ordinary aluminum foil capacitor components are sandwiched between two layers of aluminum foil with insulating medium (paper-film compound or whole film) through rolling, pressing, welding, vacuum drying, and liquid impregnating. Due to the electric weakness of medium, the medium must be more than two layers, and the electrical weakness of each layer is staggered, so as to avoid the short-circuiting of two circuits caused by medium breakdown.
However, the self-healing capacitor is different, its medium is a monolayer polypropylene film, and the surface is steamed and coated with a thin layer of metal (less than 1/100 μm) as conductive electrode. When the voltage is applied, the weakness point of polypropylene film electrical is broken down, and the breakdown current will pass through the breakdown point. Because the current density of conductive metallized coating increases sharply, and make the temperature of metallized layer increase rapidly, the metal conductor around breakdown points evaporate and dissipate rapidly, forming the metal coating blank area, which make insulation breakdown points recover automatically. The insulation film produces a very small hole with a diameter of about a few micrometers, and the disappeared area of metallized coating during healing process is about a few millimeters in diameter.
According to the requirements of self-healing performance, the thinner metallized capacitor plate coating is better. The thinner coating produced in the lower the energy of healing, the smaller the temperature rises, thus, the smaller the damage of capacitor is, also for self-healing. But, according to the relationship of medium strength and coating resistance, the thinner coating the thinner, the greater the contact resistance, the stronger the breakdown field. If the contact resistance is great, the temperature will be seriously high, which will cause overheat damage to contacting area under the effect of switching inrush current, resulting in capacitor failure. ATO 5 Kvar, 10 Kvar, 15 Kvar, 20 Kvar... self-healing shunt capacitors use a unique coating edge thickening technology, making the metallized coating area gradually thinning, electrode protruded edge thickening, which will not only improve the breakdown field strength and reduce the capacity loss, but also improve the inrush capacity of component. For more information about them, please go to the capacitors shopping page.
