Dielectric strength and Breakdown Voltage is very important for design high voltage Transformer, Motor, Generator. and also important for high voltage transmission line and FACTS (Flexible AC Transmission).
An Insulator or Dielectric Material there are no free electrons to electric conduction, as in normal condition there are no current can be flowing through it. However when the voltage applied between them increase and increase, such an insulator exceeds a certain value, it break down and allow a heavy electric current to flow through it, the value of this current much larger than the usual leakage current may be as equal to short circuit current. for solid insulator its get cracked also. this certain value of the voltage is Breakdown Voltage.
Breakdown Voltage of an insulator is the minimum voltage required to break it down.
Dielectric Strength of an insulator is the maximum potential difference (voltage applied) which a unit thickness of the medium can withstand without breaking down.
Unit of Dielectric Strength-
normally its Volt/meter, although its Express in KV/mm
example: Dielectric Strength of AIR is 3kv/mm, its implies that the maximum potential difference ( Voltage) which one millimeter of air can withstand across it without breaking down is 3 kilo volt (3000 volt), and if the potential difference exceed this value the air insulation is break down and allow large electric current flowing through it.
FACTOR OF DIELECTRIC STRENGTH:
Its value Depends on the Thickness of the insulator, temperature, moisture, shape of the insulator and several other factors.
THICKNESS: if then thickness increase dielectric strength also increase and vise versa, but double the thickness of the insulator does not double the dielectric strength or breakdown voltage.
The relation between the breakdown voltage V and the thickness of insulation is approximately V=At²⁄³. (this statement also known as Baur’s Law) here A is a constant depends on the nature of the medium.