Breakdown of Solid Insulating Materials

Information about Breakdown of Solid Insulating Materials

Published on September 8, 2011

Author: krish5292

Source: authorstream.com

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Breakdown in solid dielectrics: Breakdown in solid dielectrics Sudhi.M.M 08012014104 Final EEE B Breakdown of Solid Insulating Materials: Breakdown of Solid Insulating Materials In solid dielectrics, highly purified and free of imperfections, the breakdown strength is high, of the order of 10 MV/cm. The highest breakdown strength obtained under carefully controlled conditions is known as the "intrinsic strength" of the dielectric. Breakdown of Solid Insulating Materials: Breakdown of Solid Insulating Materials Dielectrics usually fail at stresses well below the intrinsic strength due usually to one of the following causes. (a) electro-mechanical breakdown (b) breakdown due to internal discharges (c) surface breakdown (tracking and erosion) (d) thermal breakdown (e) electro-chemical breakdown (f) chemical deterioration Electro-mechanical breakdown: Electro-mechanical breakdown When an electric field is applied to a dielectric between two electrodes, a mechanical force will be exerted on the dielectric due to the force of attraction between the surface charges. This compression decreases the dielectric thickness thus increasing the effective stress. Electro-mechanical breakdown: Electro-mechanical breakdown Electro-mechanical breakdown: Electro-mechanical breakdown Electro-mechanical breakdown: Electro-mechanical breakdown By differentiating with respect to d, it is seen that the system becomes unstable when ln (do/d) > ½ or d < 0.6 do . Thus when the field is increased, the thickness of the material decreases. At the field when d < 0.6 do, any further increase in the field would cause the mechanical collapse of the dielectric. The apparent stress (V/do) at which this collapse occurs is thus given by the equation Breakdown due to internal discharges: Breakdown due to internal discharges Solid insulating materials sometimes contain voids or cavities in the medium or boundaries between the dielectric and the electrodes. These voids have a dielectric constant of unity and a lower dielectric strength. Hence the electric field strength in the voids is higher than that across the dielectric. Equivalent circuit of dielectric with void: Equivalent circuit of dielectric with void Thermal Breakdown: Thermal Breakdown Heat is generated continuously in electrically stressed insulation by dielectric losses, which is transferred to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surfaces. If the heat generated exceeds the heat lost to the surroundings, the temperature of the insulation increases. Thermal breakdown: Thermal breakdown The power dissipated in the dielectric can be calculated Thermal breakdown: Thermal breakdown Thermal breakdown: Thermal breakdown Electro-chemical Breakdown: Electro-chemical Breakdown Since no insulant is completely free of ions, a leakage current will flow when an electric field is applied. The ions may arise from dissociation of impurities or from slight ionisations of the insulating material itself. When these ions reach the electrodes, reactions occur in accordance with Faraday's law of electrolysis, but on a much smaller scale. The insulation and the electrode metal may be attacked, gas may be evolved or substance may be deposited on the electrodes . Electro-chemical Breakdown: Electro-chemical Breakdown The products of the electrode reaction may be chemically or electrically harmful and in some cases can lead to rapid failure of the insulation. The reactions are much slower than in normal electrolytic processes due to the much smaller currents. The products of the reactions may be electrically and chemically harmful because the insulation and electrodes may be attacked, and because harmful gases may be evolved

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