Types of dielectric breakdown

Types of dielectric breakdown

Some important types of dielectric breakdown in solid are

(i) Intrinsic breakdown and avalanche breakdown

(ii) Thermal breakdown

(iii) Chemical and electrochemical breakdown

(iv) Discharge breakdown

(v) Defect breakdown

(i) Intrinsic breakdown

In dielectric, the charge displacement increases with increasing electrical field strength.

Beyond a critical value of electrical field strength, there is an electrical breakdown due to physical deterioration in dielectric material (fig. 1.19).

When the applied electrical field is large, some of the electrons in the valence band cross over to the conduction band across the large forbidden energy gap. They become conduction electrons producing large conduction current.

As a result, a large current flows through the dielectric and breakdown occurs. This type of breakdown is called intrinsic breakdown.

Avalanche breakdown

The conduction electrons are accelerated to very high Jos at wo velocity and energy on further application of electrical field.

These conduction electrons collide with valence electrons in the covalent bond and they transfer their energy to valence electron. The valence electrons on acquiring this energy, jump from the valence band to the conduction band.

This process continues as more and more valence electrons jump to the conduction band by breaking the covalent bond.

As a result, a large current flows through the dielectric and breakdown occurs. This type of breakdown is called Avalanche breakdown.

Characteristics

• Intrinsic and avalanche breakdowns require large electrical field.

• They occur even at low temperatures (about room temperature and at lower temperature).

• They do not depend on the size and shape of the dielectric material and configuration of electrodes.

• They can occur in thin samples.

• They occur within a short span of time (microseconds).

(ii) Thermal breakdown

When an electric field is applied to a dielectric material, some amount of heat is produced. This heat must be dissipated from the material.

In some cases, the amount of heat produced is very large as compared to the heat dissipated. Due to excess of heat, the temperature inside the dielectric increases and may produce local melting in the dielectric material.

During this process, a large amount of current flows through the material and causes the dielectric to breakdown. This type of breakdown is known as thermal breakdown.

Characteristics

• This type of breakdown occurs only at high temperatures.

• The strength of the electrical field to create dielectric breakdown depends upon the size and shape of the dielectric sample.

• The breakdown time is of the order of few milliseconds.

• It requires moderate electrical fields.

(iii) Chemical and Electrochemical breakdown

Electrochemical breakdown is similar to thermal breakdown. When the  temperature of a dielectric material increases, mobility of ions increases and hence the electrochemical reaction may take place.

This leads to leakage current and energy loss in the material and finally dielectric breakdown occurs. add to aqada bas sie bas ssie sdt no basqeb

This type of breakdown is known as chemical and electrochemical breakdown.

Characteristics

• It occurs even at low temperatures.

• It depends on the concentration of ions and magnitude Ishots of leakage current.

(iv) Discharge breakdown

Discharge breakdown occurs when a dielectric contains occluded gas bubbles (fig. 1.20). When this type of dielectric is subjected to electrical field, the gases present in the material will easily ionise and thus produce large ionisation current. 

The gaseous ions bombard the solid dielectric. This causes electrical deterioration and leads to dielectric breakdown. This is known as discharge breakdown.

Characteristics

• This breakdown occurs at low voltages.

• It occurs in the dielectric material where there are a large number of occluded gas bubbles.

• When discharge takes place at a point, the surrounding places are burnt and hence their electrical properties are affected.

Thus, the life of an insulating material depends upon the number of discharges which are taking place inside the material. That is, it depends upon the frequency of applied voltage.

(v) Defect breakdown

The surface of the dielectric material may have defects such as cracks, porosity and blow holes (fig. 1.21). Impurities like dust or moisture may present at these discontinuities (defects). This leads to a breakdown in a dielectric material.

Partial discharges can locally melt the insulator and can easily cause chemical transformations.