Introduction to Insulation breakdown in gases, liquids and solids

INTRODUCTION TO INSULATION BREAKDOWN IN GASES, LIQUIDS AND SOLIDS

In practical applications, the failure or breakdown of a dielectric material is of grat concern to engineers. There are different mechanisms by which dielectric breakdown takes place.

INSULATION BREAKDOWN IN GASEOUS DIELECTRICS

The breakdown in gaseous dielectrics is due to ionization caused by collision of electrons. When a strong electric field is applied the accelerated free electrons act energy greater than the ionization of the gas.

The breakdown is initiated with a spark discharge. Then this develops to a arc discharge. It results in a high current density and direct short circuit. The dielectric strength of gaseous dielectrics depends on

(i) Pressure: The dielectric strength is higher at very high and at very low pressure.

(ii) Uniformly of applied electric field: In non-uniform 26g boqq electric fields to the field intensity tends to concentrate in certain points. This stress concentration initiate collision ionization at relatively low voltage.

(iii) Polarity of electrodes: The breakdown voltage deb depends on the polarity of electrodes. Thus if point and plain electrodes are used, the breakdown voltage will gabe higher when the point electrode is negative.

(iv) Frequency of applied field: When the frequency is increased the breakdown voltage decreases. But, breakdown voltage increases after reaching a minimum value at about 1 MHz.

(v)Distance between the electrodes.

(vi)Chemical composition of the gas.

DIELECTRIC BREAKDOWN IN LIQUIDS

In impure liquids there are small conductive particles in suspension and coalesce end to end. Thus, impurity particles form a conducting bridge between the electrodes. This leads to discharge.

• In some liquids, the discharge initiates as partial discharges in gas bubbles entrapped in the liquid. These partial discharges can raise the temperature and vaporize more of the liquid. (Fig. 1.17)

Thus the size of the bubble increases. The eventual discharge can be a series of partial discharges in entrapped gas bubbles.

• Moisture absorption and absorption of gases generally bas nio deteriorate the dielectric strength. The oxidation of lliw oge certain liquids, such as oils, produces more acidic and hence higher conductivity eventually give discharge.

• In some liquids, the discharge involve the emission of a large number of electrons from the electrode into the liquid due to field emission at high fields. This is a discharge process by electrode injection.

DIELECTRIC BREAKDOWN SOLIDS

There are various major mechanisms that can lead to dielectric breakdown in solids. The most likely mechanism depends on the dielectric material's condition and sometimes on extrinsic factors such as the ambient conditions, moisture absorption being a typical example.