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.
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.
Process of breakdown
The equilibrium forces result in the 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
Breakdown due to internal discharges
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
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.
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.