the self-healing capacitor uses a single layer of polypropylene film as the dielectric, and a thin layer of metal is evaporated on the surface as the conductive electrode. when the high voltage is applied, the electric weakness of polypropylene film is broken down, the impedance of breakdown point is obviously reduced, the current density is increased rapidly, which makes the metallized coating produce high heat, the metal conductor around the breakdown point evaporates and dissipates rapidly, forming a metal coating blank area, and the breakdown point automatically recovers the insulation.
the priority values of capacitor rated voltage are as follows: 0.23, 0.4, 0.525 and 0.69kv. the rated voltage of capacitor is generally 5% higher than that of electrical equipment.
the capacitance of a capacitor is the ratio of the charge on the electrode plate to the voltage between the plates, which is related to the area of the plate, the insulation thickness between the plates and the dielectric coefficient of the insulating medium. the formula is c = 14 π ε × sd, where ε is the dielectric coefficient of the insulating medium between the plates; s is the area of the capacitor plate; d is the thickness of the insulation layer of the capacitor.
generally, after the static capacitor is put into operation, the reduction rate of capacitance value in the first year shall be within 2%, and that from the second year to the fifth year shall be between 1% and 2%. after the fifth year, due to dielectric aging, the capacitance value will decrease rapidly. when the capacitance value drops to less than 85% of the original value, the life of the capacitor can be considered to be over.
in ac circuit, the reactive power qc = uisin φ, because the dielectric loss angle of capacitor is very small, φ = 90 °, so sin φ = 1, then reactive power qc = ui = ω cu2 × 10-3 = 2 π fcu2 × 10-3 (μ f). from this formula, it can be seen that the reactive power of capacitor is not only determined by capacitor c, but also directly related to power frequency f and terminal voltage u. the accurate definition of rated reactive power of capacitor should be under standard frequency no power corresponding to static capacitance c when external rated voltage is connected.
when the output voltage of the capacitor is lower than the rated voltage of the capacitor. therefore, if the rated voltage of the capacitor is too high and the actual operating voltage of the capacitor is lower than the rated value for a long time, it is likely that the reactive power of the power grid will be short because the reactive power output of the capacitor is lower than the design value.
under the action of applied voltage, the dielectric breakdown of the capacitor is caused by the existence or development of impurities or air gap in the medium to form a conducting circuit; then a pulse current with a very steep front edge flows through the metal layer within a small range near the conduction circuit. the current on the metal layer near the breakdown point suddenly rises and is inversely proportional to the distance from the breakdown point. at the time t, the temperature of the metal layer in the region with radius rt reaches the melting point of the metal, so the metal in this range melts and generates an arc. the current results in the release of capacitance energy and the sudden increase of temperature and pressure in the local area of arc channel.
with the effect of discharge energy, the metal layer in the region with radius rt evaporates violently, accompanied by splashing. in the process of increasing the radius of the region, the arc is broken, the metal is blown away and oxidized and cooled, which destroys the conductive path, and forms a circular insulation area with the breakdown point as the center, and the self-healing process of the capacitor ends.