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 definition with suitable examples and sketches about solid dielectric definition 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.
Chemical Deterioration – solid dielectric definition
In addition progressive chemical degradation of insulation materials can occur without electrical stress for several reasons.
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.
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.
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.
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.
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 ).