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SURFACE BREAKDOWN

Surface flashover

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. 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

(a) The presence of a conducting film across the surface of the insulation

(b) A mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks,

(c) Degradation of the insulation must be caused by the sparks.

The conducting film is usually moisture from the atmosphere absorbed by some form of contamination. Moisture is not essential as a conducting path can also arise from metal dust due to wear and tear of moving parts. Sparks are drawn between moisture films, separated by drying of the surface due to heating effect of leakage current, which act as extensions to the electrodes. {For a discharge to occur there must be a voltage at least equal to the Paschen minimum for the particular state of the gas. For example, Paschen minimum in air at N.T.P it is 380 V, whereas tracking can occur at well below 100 V. It does not depend on gaseous breakdown.] Degradation of the insulation is almost exclusively the result of heat from the sparks, and this heat either carbonizes if tracking is to occur, or volatilizes if erosion is to occur. Carbonization results in a permanent extension of the electrodes and usually takes the form of a dendrite growth. Increase of creep age path during design will prevent tracking, but in most practical cases, moisture films can eliminate the designed creep age path.

Tracking

Tracking is the formation of a permanent conducting path across a surface of the insulation, and in most cases the conduction (carbon path) results from degradation of the insulation itself leading to a bridge between the electrodes.

The insulating material must be organic in nature for tracking to occur.

Erosion

In a surface discharge, if the products of decomposition are volatile and there is no residual conducting carbon on the surface, the process is simply one of pitting. This is erosion, which again 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 makes insulation immediately completely ineffective, whereas erosion only weakens the material but allows operation until replacement can be made later.

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