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. Also The solid dielectric insulation with suitable examples and sketches about solid dielectric insulation 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
Also They are now covere in the following sections.
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
Uniform alternating stress
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).
Heat lost = k (thita – thita0), where thita = ambient temperature
Thermal Breakdown Ctd solid dielectric insulation
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).
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.