neutron radiation

Breakdown of Solid Insulating Materials

In solid dielectrics that are highly clean and flawless, the dielectric strength is high and on the order of 10 MV / cm. The highest dielectric strength obtained under carefully control conditions is know as the “inherent strength” of the dielectric. Dielectrics often fail at voltages well below inherent resistance, generally due to one of the following causes.The solid dielectric materials with suitable examples and sketches about solid dielectric materials adavancedly its recommended you to read it carefully.

(a) Electromechanical rupture

(b) Rupture due to internal discharges

(c) Surface Breakdown (monitoring and erosion)

(d) Thermal rupture

(e) Electrochemical rupture

(f) Chemical deterioration

They are now covered in the following sections.

Surface Breakdown

Surface flashover

The overturning of the surface is a division of the medium into which the solid is immerse. The role of the solid dielectric is just to distort the field so that the electrical resistance of the gas is exceed.

If a piece of solid insulation is inser into a gas so that the solid surface is perpendicular to the equipotentials at all points, the voltage gradient is not affect by the solid insulation. An example of this is a cylindrical insulator arrange in the direction of a uniform field. Field reinforcement is obtain if the solid insulation also deviates from the cylindrical shape in detail. Especially if the edges are broken or if the cylinder ends are not completely perpendicular to the axis, there is an air gap next to the electrode and the voltage can reach up to 0 times the average voltage in the gap. [0r is the dielectric constant of the cylinder]. Therefore, the discharge can occur at a voltage close to 1/0 times the breakdown voltage in the absence of the cylinder, and these discharges can trigger a failure.

The three main components of surface flashover phenomena are

(a) the presence of a conductive film on the surface of the insulation

(b) a mechanism by which the leakage current through the conductive film is interrupt by the generation of sparks,

(c) Deterioration of insulation must be cause by sparks.

Surface flashover Ctd

Conductive film is generally moisture from the atmosphere that is absorb by some form of pollution. Moisture is not absolutely necessary, as a metal path can also result from metal dust due to wear on moving parts. Sparks are generate between the moisture films, which separate by drying the surface due to the heating effect of the leakage current and act as extensions of the electrodes. {For a discharge to occur, a voltage must be available for the respective gas state that corresponds to at least the Paschen minimum. For example, the minimum Paschen in air at 380 V is NTP, while monitoring can be done below 100 V.

This does not depend on gas degradation.] Insulation degradation is almost entirely the result of heat. of the spark, and this heat is characterized when a chase occurs or evaporates when erosion occurs. Charring leads to permanent extension of the electrodes and generally takes the form of dendritic growth. Increasing the creepage distance during design prevents tracking, but in most practical cases, moisture films can eliminate the designed creepage path.

Tracking

Tracking is the formation of a permanent conductive path through an insulation surface, and in most cases conduction (carbon path) results from deterioration of the insulation itself, leading to a bridge between the electrodes. The insulation material must be organic to be able to follow it.

Erosion

If the decomposition products are volatile during a surface discharge and there is no conductive residual carbon on the surface, it is simply chipping. This is erosion, which in turn occurs in organic materials.

If surface discharges are likely to occur, it is preferable to use materials with erosion properties rather than tracking properties. As tracking will immediately render the insulation completely ineffective. While erosion will only weaken the material but allow operation until replacement can take place later.

Reference – solid dielectric materials

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