BREAKDOWN IN SOLID DIELECTRIC
The requirements of good dielectric:
Low dielectric loss,
High mechanical strength,
Free from gaseous inclusions, and moisture,
Resistant to thermal and chemical deterioration.
Solid dielectrics have higher breakdown strength compared to liquids and gases. When breakdown occurs, solids get permanently damaged while gases fully and liquids partly recover their dielectric strength. Breakdown mechanism varies depending on the time of application of voltage as shown in Figure 1 and can be classified as follows:
Maximum strength usually obtainable ranges from 5 - 10 MV/cm.
Intrinsic breakdown depends upon the presence of free electron which capable of migration through the lattice of the dielectric. Usually small numbers of conduction electrons are present, with some structural imperfections and small amounts of impurities. The impurity atoms or molecules act as traps for the conduction electrons up to certain ranges of electric fields and temperatures. When these ranges are exceeded, additional electrons and trapped are released and participate in the conduction process.
Two types of intrinsic breakdown mechanisms:
Assumed to be electronic in nature (occurs in time 10-8 s)
Initial density of conduction (free) electrons assumed to be large and electron-electron collisions occurs.
When electric field is applied, electrons gain energy and cross the forbidden gap from the valency to the conduction band. This process repeated, more and more electrons available in
conduction band, eventually leading to breakdown.
a. Intrinsic or ionic breakdown
b. Electromechanical breakdown
c. Thermal breakdown
d. Electrochemical breakdown
e. Treeing and tracking
When voltage is applied only for short durations of the order 10-8Sec, the dielectric strength of a solid dielectric increases very rapidly to an upper...
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