BREAKDOWN IN SOLID

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.  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)  chemical deterioration

• 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.

Process of breakdown

• 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.  Thus even under normal working voltages, the field in the 

voids may exceed their breakdown value and breakdown may occur.

Equivalent circuit of dielectric with void

When the voltage Vv across the void exceeds the critical voltage Vc, a discharge is initiated and the voltage collapses.  The discharge extinguishes very rapidly (say 0.1 s).  The voltage across the void again builds up and the discharges recur.  The number and frequency of the discharges will depend on the applied voltage.

The voltage and current waveforms (exaggerated for clarity)

• Surface Breakdown

Surface flashover is a breakdown of the medium in which the solid is immersed.  The role of the solid dielectric is only to distort the field so that the electric strength of the gas is exceeded. If a piece of solid insulation is inserted in a gas so that the solid surface is perpendicular to the equipotentials at all points, then the voltage gradient is not affected by the solid insulation.  An example of this is a cylindrical insulator placed in the direction of a uniform field.  Field intensification results if solid insulation departs even in detail from the cylindrical shape.  In particular if the edges are chipped, or if the ends of the cylinder are not quite perpendicular to the axis, then an air gap exists next to the electrode, and the stress can reach up to 0r times the mean stress in the gap. [0r is the dielectric constant of the cylinder].  Discharge may therefore occur at a voltage approaching 1/0r times the breakdown voltage in the absence of the cylinder, and these discharges can precipitate a breakdown.

The three essential components of the surface flashover phenomena are;

1. The presence of a conducting film across the surface of the insulation
2. A mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks
3. Degradation of the insulation must be caused by the sparks.

• 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.

Equilibrium will be reached at a temperature 1 where the heat generated is equal to the heat lost to the surroundings

Variation of heat generated by a device for 2 different applied fields and the heat lost from the device with temperature

• Chemical Deterioration

Progressive chemical degradation of insulating materials can occur in the absence of electric stress from a number of causes;

1. Chemical Instability
2. Oxidation
3. Hydrolysis
4. Other processes

Dependence of life of paper on temperature