In solid dielectrics that are highly clean and flawless, the dielectric strength is high and on the order of 10 MV / cm. The highest dielectric strength obtained under carefully controlled conditions is know as the “inherent strength” of the dielectric. Dielectrics often fail at voltages well below inherent resistance, generally due to one of the following causes. The solid dielectric used in power apparatus with suitable examples and sketches about solid dielectric used in power apparatus adavancedly its recommended you to read it carefully.
(a) Electro-mechanical breakdown
(b) Breakdown due to internal discharges
(c) Rupture of the surface (monitoring and erosion)
(d) Thermal rupture
(e) Electrochemical rupture
(f) Chemical deterioration
They are now covere in the following sections.
Electro chemical Breakdown – solid dielectric used in power apparatus
Since no insulating agent is completely free of ions, a leakage current flows when an electric field is applied. Ions can arise from the dissociation of impurities or from a slight ionization of the insulating material itself. When these ions reach the electrodes, reactions occur according to Faraday’s law of electrolysis, but on a much smaller scale. The insulation and metal of the electrode can be attacked, gas can develop or substance can deposit on the electrodes. The products of the electrode reaction can be chemically or electrically harmful and, in some cases, cause rapid failure of the insulation. Due to the much smaller currents, the reactions are much slower than in normal electrolysis processes. The products of the reactions can be harmful electrically and chemically because the insulation and electrodes can be attacked and because harmful gases can be generated.
In general, a 1 F paper capacitor operating at 1 kV at room temperature would take 2 to 3 years to generate 1 cc of hydrogen. At elevated temperatures, the electrolysis products would form much faster. Since contamination leads to an increase in ion concentration, care must also be take to avoid contamination during manufacturing.
Electro chemical Breakdown Ctd
The electrolysis rate is much higher with direct charge than with changing charge. This is due to the fact that the reactions can be totally or partially reversed if the polarity changes and the scope of the reaction depends on the reaction rate and the diffusion time of the reaction products outside the electrodes, as well as the type reaction. . However, at the network frequency, the electrochemical effects can be severe and are often responsible for long-term insulation failure. The most common source of ions are ionizable contaminants in insulation. Therefore, contamination of the insulation during manufacture and during installation in devices must be avoided with great care. Furthermore, contamination in polar insulating materials should be avoid with greater care due to the higher degree of dissociation of the ionic substance in solution.
The long life of capacitors containing chlorine impregnants under direct charge can be significantly extended by adding small amounts of certain stabilizers that are hydrogen acceptors and act as depolarizers on the cathode. Hydrogen ions released at the cathode react easily with the stabilizer and not with the impregnating agent, a more difficult chemical process. In the absence of the stabilizer, hydrogen reacts with the chlorine in the impregnating agent to form hydrochloric acid, and there is rapid deterioration due to acid attack on the electrodes and cellulose. The extension of the useful life caused by the stabilizers is proportional to the amount of stabilizer added. For example, with 2% azobenzene stabilizer, the average shelf life can be extended 50 times.