neutron radiation

Solid Dielectric Breakdown

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 controlled 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 breakdown with suitable examples and sketches about solid dielectric breakdown adavancedly its recommended you to read it carefully.

(a) Electro-mechanical breakdown

(b) Breakdown due to internal discharges

(c) Rupture of the surface (monitoring and erosion)

(d) Thermal rupture

(e) Electrochemical rupture

(f) Chemical deterioration

They are now covere in the following sections.

Electro-mechanical breakdown – solid dielectric

When an electric field is apply to a dielectric between two electrodes, a mechanical force is exerted on the dielectric due to the attractive force between the surface charges. This compression reduces the dielectric thickness, which increases the effective voltage. This is show in Figure.

solid dielectric breakdown
solid dielectric breakdown

Process of breakdown

solid dielectric breakdown
The equilibrium forces result in the equation:

The equilibrium forces result in the equation:

solid dielectric breakdown
equation

When differentiating with respect to d, it can be seen that the system becomes unstable when ln (do / d)> 1/2 or d <0.6 do.

So when the field is enlarge, the thickness of the material decreases. With the field if d <0.6, any further increase in the field would cause the dielectric to decompose mechanically. The apparent voltage (V / do) at which this collapse occurs is given by the equation

solid dielectric breakdown
equation

Breakdown due to internal discharges – solid dielectric

Solid insulating materials sometimes contain gaps or voids in the medium or boundaries between the dielectric and the electrodes. These cavities have a dielectric constant of one and a lower dielectric strength. Therefore, the intensity of the electric field in the cavities is greater than that of the dielectric. This means that even under normal operating voltages, the field in the cavities can exceed its breakdown value and breakdown can occur. The mechanism can be explain taking into account the following equivalent circuit diagram of the dielectric with the cavity (see Figure 2.3).

Equivalent circuit of dielectric with void
Equivalent circuit of dielectric with void

Equivalent circuit of dielectric with void

When the voltage Vv through the cavity exceeds the critical voltage Vc, a discharge is initiate and the voltage is broken. The download exits very quickly (for example, 0.1 s). The voltage across the cavity builds up again and the discharges are repeate. The number and frequency of discharges depend on the applied voltage. The voltage and current waveforms (exaggerated for clarity) are show in Figure.

Internal discharges

In each of the discharges, the heat dissipates in the cavities, resulting in charring of the cavity surface and material erosion. The gradual erosion of the material and the resulting reduction in the thickness of the insulating material eventually leads to rupture. The breakdown of this process is slow and can occur in a few days or in a few years.

Deterioration due to internal discharges

In organic liquid-solid dielectrics, internal discharges lead to gradual deterioration.

(a) Decay of the solid dielectric under attack by electrons that are release by the discharges.

(b) chemical effect on the dielectric of gaseous ionization products.

(c) high temperatures in the discharge area.

All cavities in the dielectric can be remove by careful impregnation, leading to an increase in discharge voltage Ei. The final value Ei depends on the electrical processes that lead to gas formation.

These are on oil-impregnated paper.

(a) Decomposition of moisture on paper

(b) Local electrical decomposition of the oil.

The voltage at which gas is formed from paper containing significant amounts of moisture may be less than 10 V / m, but it increases continuously with increasing dryness and may be greater than 100 V / m when the paper is fully dry. Except under very dry conditions, the first gas formed is the electrochemical decomposition of water on paper.

When a gas bubble is formed in a paper and oil dielectric at the discharge start voltage Ei, the discharges in the bubble break down the oil molecules, leading to increased gas formation and rapid bubble growth . As long as the bubble remains in the dielectric, the initial voltage Ei is low, often lower than the nominal voltage, but if the dielectric stays long enough for the gas to dissolve in the oil, the initial discharge voltage is restored initially high. Although the egg improves at rest, the discharges have caused permanent damage, which is reflected in an increased angle of loss and is due to ion formation by the discharges. Widespread charring also occurs due to discharges.

Surface Breakdown – solid dielectric

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.

Thermal Breakdown – solid dielectric

Also In electrically charged insulation, heat is continuously generate by dielectric losses, which are transmitted from its external surfaces to the surrounding medium by conduction through the solid dielectric and by radiation. When the heat generated exceeds the heat lost to the environment, the temperature of the insulation increases.

The power consumed in the dielectric can be calculate as follows.

Uniform direct stress

Equation

Uniform alternating stress

Example
Example

The simplest case is when the loss of heat by cooling is linearly related to the increase in temperature above the environment and the heat generate is independent of the temperature. (i.e. resistivity and loss angle do not vary with temperature).

Thermal breakdown
Thermal breakdown

Heat lost = k (thita – thita0), where  thita =  ambient temperature

Thermal Breakdown Ctd solid dielectric

In practice, the heat generate increases rapidly with temperature, although the heat loss can be considered somewhat linear, and at certain values of the electric field there is no stable state in which the heat loss is equal to the heat generate, so the material is thermal. is broken down The rapid increase is due to the fact that as the temperature increases, the dielectric loss angle increases according to an exponential law (loss ∝ e-A / T, where T is the absolute temperature).

Thermal breakdown

Figure shows the variation of the heat generated by a device for 2 different fields and the heat loss of the device with the temperature.

There is a stable temperature A for field E2 (as long as the temperature must not reach B). For the E1 field, the heat generate is always greater than the heat lost, so the temperature would continue to rise until a fault occurs.

The maximum voltage that a particular insulation material can withstand cannot be increase indefinitely simply by increasing its thickness. Due to thermal effects, there is an upper limit for voltage V, which cannot be exceeded without thermal instability. This is because with thick insulation, the internal temperature is only slightly influence by the surface conditions. Typically, V is a limiting factor in the practical use of insulating materials only for high temperature operation or in the event of high frequency failures.

Electro chemical Breakdown – solid dielectric used in power apparatus

Since no insulating agent is completely free of ions, a leakage current flows when an electric field is applied. Ions can arise from the dissociation of impurities or from a slight ionization of the insulating material itself. When these ions reach the electrodes, reactions occur according to Faraday’s law of electrolysis, but on a much smaller scale. The insulation and metal of the electrode can be attacked, gas can develop or substance can deposit on the electrodes. The products of the electrode reaction can be chemically or electrically harmful and, in some cases, cause rapid failure of the insulation. Due to the much smaller currents, the reactions are much slower than in normal electrolysis processes. The products of the reactions can be harmful electrically and chemically because the insulation and electrodes can be attacked and because harmful gases can be generated.

In general, a 1 F paper capacitor operating at 1 kV at room temperature would take 2 to 3 years to generate 1 cc of hydrogen. At elevated temperatures, the electrolysis products would form much faster. Since contamination leads to an increase in ion concentration, care must also be take to avoid contamination during manufacturing.

Electro chemical Breakdown Ctd

The electrolysis rate is much higher with direct charge than with changing charge. This is due to the fact that the reactions can be totally or partially reversed if the polarity changes and the scope of the reaction depends on the reaction rate and the diffusion time of the reaction products outside the electrodes, as well as the type reaction. . However, at the network frequency, the electrochemical effects can be severe and are often responsible for long-term insulation failure. The most common source of ions are ionizable contaminants in insulation. Therefore, contamination of the insulation during manufacture and during installation in devices must be avoided with great care. Furthermore, contamination in polar insulating materials should be avoid with greater care due to the higher degree of dissociation of the ionic substance in solution.

The long life of capacitors containing chlorine impregnants under direct charge can be significantly extended by adding small amounts of certain stabilizers that are hydrogen acceptors and act as depolarizers on the cathode. Hydrogen ions released at the cathode react easily with the stabilizer and not with the impregnating agent, a more difficult chemical process. In the absence of the stabilizer, hydrogen reacts with the chlorine in the impregnating agent to form hydrochloric acid, and there is rapid deterioration due to acid attack on the electrodes and cellulose. The extension of the useful life caused by the stabilizers is proportional to the amount of stabilizer added. For example, with 2% azobenzene stabilizer, the average shelf life can be extended 50 times.

Chemical Deterioration – solid dielectric definition

In addition progressive chemical degradation of insulation materials can occur without electrical stress for several reasons.

Chemical Instability

Many insulating materials, especially organic materials, show chemical instability. Such chemical changes can result from a spontaneous breakdown of the material’s structure. This process is very slow under normal operating conditions, but is highly dependent on temperature. The logarithm of the insulation life of the paper t can be expressed as the inverse function of the absolute temperature.

solid dielectric definition
solid dielectric definition

When t is expressed in weeks, the constants for vacuum-dried paper immersed in oil in contact with nitrogen are A = 7000 and B = – 16.0.

solid dielectric definition
solid dielectric definition

Dependence of life of paper on temperature

Also In the presence of oxygen or moisture, the life of the insulation decreases much faster. As the amount of moisture present increases, B decreases, so the life of the paper also decreases. When there is approximately 0.1% humidity, B decreases to 0.8, so t decreases by a factor of approximately 6. This means that the presence of approximately 0.1% humidity reduces the useful life of the insulation 6 times. Figure 2.7 shows the variation.

Oxidation

In the presence of air or oxygen, especially ozone, materials like rubber and polyethylene oxidize, causing surface cracks, especially when stretched and exposed to light. Polyethylene rusts even in broad daylight, unless protected by an opaque filler.

Hydrolysis

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

Other processes – solid dielectric definition

Progressive chemical degradation of insulating materials can also occur due to a variety of processes, such as. B. Also Incompatibility of materials (eg, Rubber ages faster at elevated temperatures in the presence of copper and cellulose, decomposes much more rapidly in the presence of traces of acid) and the washing (washing of a soluble component) of substances chemically active (eg B. Glass cloth of glasses with a high sodium content quickly loses its resistance due to the washing of sodium on the surface of the fibers and the subsequent chemical attack of strong alkali on the glass surface ).

Reference – solid dielectric breakdown

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