Solid insulation and breakdown

A good dielectric should have low dielectric loss, high mechanical strength, should be free from gaseous inclusions and moisture, resistant to thermal and chemical deterioration. Solid dielectrics have higher breakdown strength compared to liquids and gases. The mechanism of breakdown is a complex phenomena in the case of solid and varies depending on the time of application of voltage, as figure 24 below

Figure 24 Variation of breakdown strength with time after application of voltage

The variation of breakdown mechanisms can be classified as follows
            1. Intrinsic or ionic breakdown
            2. Electromechanical breakdown
            3. Failure due to treeing and tracking
            4. Thermal breakdown
            5. Electrochemical breakdown
            6. Breakdown due to internal discharges

Electromechanical breakdown

When an electric field is applied to a dielectric between two electrodes, a mechanical force will be exerted on the dielectric due to the force of attraction between the surface charges. This compression decreases the dielectric thickness thus increasing the effective stress. As shown in figure 25 below

Figure 25 Process of breakdown

Compressive force Pc = ½ D E = ½ ɛ_o ɛ_r V2/d2, and From Hooke's Law for large strains, Pc = Y ln (do/d)

At equilibrium, equating forces gives the equation,

By differentiating with respect to d, it is seen that the system becomes unstable when ln (do/d) > ½ or d < 0.6 do.

Thus, when the field is increased which the thickness of the material will decreases. At the field when d < 0.6 do, any further increase in the field would cause the mechanical collapse of the dielectric. The apparent stress (V/do) at which this collapse occurs is thus given by the equation.

Breakdown due to internal discharges

Solid insulating materials sometimes contain voids or cavities in the medium or boundaries between the dielectric and the electrodes. These voids have a dielectric constant of unity and a lower dielectric strength. Hence the electric field strength in the voids is higher than that across the dielectric. Thus even under normal working voltages, the field in the voids may exceed their breakdown value and breakdown may occur. The mechanism can be explained by considering the following equivalent circuit of the dielectric with the void, shown in figure 26 below

Figure 26 Equivalent circuit of dielectric with void

When the voltage Vv across the void exceeds the critical voltage Vc, a discharge is initiated and the voltage collapses. The discharge extinguishes very rapidly (say 0.1 µs). The voltage across the void again builds up and the discharges recur. The number and frequency of the discharges will depend on the applied voltage. The voltage and current waveforms (exaggerated for clarity) are shown in figure 27 below

Figure 27 Internal discharges

In each of the discharges, there will be heat dissipated in the voids which will cause carbonization of the surface of the voids and erosion of the material. The gradual erosion of the material and consequent reduction in the thickness of the insulating material eventually leads to breakdown.

Breakdown by this process is slow and may occur in a few days or may take a few years.

Surface breakdown

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 ɛ_r times the mean stress in the gap. [ɛ_r is the dielectric constant of the cylinder]. Discharge may therefore occur at a voltage approaching 1/ɛ_r 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.

 

 

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.

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.

Thermal breakdown

Heat is generated continuously in electrically stressed insulation by dielectric losses, which is transferred to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surfaces. If the heat generated exceeded the heat lost to the surroundings, the temperature of the insulation increases.


In practice, although the heat lost may be considered somewhat linear, the heat generated increases rapidly with temperature, and at certain values of electric field no stable state exists where the heat lost is equal to the heat generated so that the material breaks down thermally. The rapid increase is due to the fact that with rise in temperature, the loss angle of the dielectric increases in accordance with an exponential law (loss α e^-A/T, where T is the absolute temperature).

Figure 28 Thermal breakdown

Figure 28 above shows the variation of heat generated by a device for 2 different applied fields and the heat lost from the device with temperature.

For the field E2, a stable temperature θ_A exists (provided the temperature is not allowed to reach θ_B). For the field E₁, the heat generated is always greater than the heat lost so that the temperature would keep increasing until breakdown occurs.

The maximum voltage a given insulating material can withstand cannot be increased indefinitely simply by increasing its thickness. Owing to thermal effects, there is an upper limit of voltage V_θ, beyond which it is not possible to go without thermal instability. This is because with thick insulation, the internal temperature is little affected by the surface conditions. Usually, in the practical use of insulating materials, V_θ is a limiting factor only for high-temperature operation, or at high frequency failures.

Electrochemical breakdown

Since no insulation is completely free of ions, a leakage current will flow when an electric field is applied. The ions may arise from dissociation of impurities or from slight ionisations of the insulating material itself. When these ions reach the electrodes, reactions occur in accordance with Faraday's law of electrolysis, but on a much smaller scale. The insulation and the electrode metal may be attacked, gas may be evolved or substance may be deposited on the electrodes. The products of the electrode reaction may be chemically or electrically harmful and in some cases can lead to rapid failure of the insulation. The reactions are much slower than in normal electrolytic processes due to the much smaller currents. The products of the reactions may be electrically and chemically harmful because the insulation and electrodes may be attacked, and because harmful gases may be evolved.

The rate of electrolysis is much greater with direct stress than with alternating stress. This is due to the fact that the reactions may be wholly or partially reversed when the polarity changes and the extent of reaction depends on the reaction rate and the time for diffusion of the reaction products away from the electrodes as well as on the nature of the reaction products. However at power frequency, electrochemical effects can be serious and are often responsible for long-term failure of insulation. The most frequent source of ions is ionizable impurities in the insulation. Thus contamination of insulation during manufacture and during assembly into equipment must be avoided with great care.

Also, contamination in polar insulating materials should be avoided with still greater care because of the greater degree of dissociation of ionic substance in solution.

The long term lives of capacitors containing chlorinated impregnants under direct stress may be greatly extended by adding small quantities of certain stabilizers, which are hydrogen acceptors and act as depolarizers at the cathode.

Hydrogen ions discharged at the cathode readily react with the stabilizer rather than with the impregnant, a more difficult chemical process. In the absence of the stabilizer, the hydrogen reacts with the chlorine of the impregnant to produce hydrochloric acid, and rapid deterioration occurs due to attack of the acid on the electrodes and cellulose. The extension of the life caused by the stabilizers is proportional to the amount of stabilizer added. For example, with 2% of the stabilizer Azobenzene, mean life may be extended 50 times.

Chemical Deterioration

Progressive chemical degradation of insulating materials can occur in the absence of electric stress from a number of causes.

Chemical Instability

Many insulating materials, especially organic materials, show chemical instability. Such chemical changes may result from spontaneous breakdown of the structure of the material. Under normal operating conditions, this process is very slow, but the process is strongly temperature dependant. The logarithm of the life t of paper insulation can be expressed as an inverse function of the absolute temperature θ. Log10 t = A/θ + B where A & B are constants.

Oxidation

In the presence of air or oxygen, especially ozone, materials such as rubber and polyethylene undergo oxidation giving rise to surface cracks, particularly if stretched and exposed to light. Polythene also oxidises in strong day light unless protected by an opaque filler.

Hydrolysis

When moisture or water vapour is present on the surface of a solid dielectric, hydrolysis occurs and the materials lose their electrical and mechanical properties. Electrical properties of materials such as paper, cotton tape, and other cellulose materials deteriorate very rapidly due to hydrolysis. Polyethylene film may lose its mechanical strength in a few days if kept at 100 % relative humidity.

Other processes

Progressive chemical degradation of insulating materials can also occur due to a variety of processes such as, incompatibility of materials (ex: rubber ages more rapidly at elevated temperatures in the presence of copper, and cellulose degrades much more rapidly in the presence of traces of acidic substances), and leaching (washing out of a soluble constituent) of chemically active substances (ex: glass fabrics made from glasses of high sodium content lose their strength rapidly due to leaching of sodium to the surface of the fibres and the subsequent chemical attack of the strong alkali on the glass surface).

Cited from the research papers, base on the figure 29 below

Figure 29 Illustration of void-to-free transition

As the figure 29 above, shows that illustrates the void-to-free transition. Conduction a carriers (electrons) can be penetrate the void either through the ionization of gas in the void or by de-trapping from loose chain ends. Electron production is accelerated by the intensive electric field within the void following the development of electron avalanche. The electric field further divides electrons and positive ions and the electron cloud strikes the wall of the void.

 

In the meantime, positive ions drift to the opposite side. The wall is eventually damaged as a result of either carbonization or ohmic heating or both. After the elapse of time, the carbonized area shapes a new void. As new void is considerably smaller than the original, the electric field within the new void is stronger than before, ultimately giving birth to electrical treeing.