ELECTRIC FIELD  STRESSES

·        ELECTRIC FIELD  STRESSES

Like In mechanical .designs where the criterion for design depends on the mechanical strength, of the materials and the stresses that are generated during their operation, in high voltage applications, the dielectric strength of insulating materials and the electric field stresses developed in them when subjected to high voltages are.
The important factors in high voltage systems in a high voltage apparatus the important materials used are conductors and insulators. While the conductors carry current the insulators prevent the flow of currents undesired paths the electric, stress to which an insulating material is subjected to is numerically equal to the voltage gradient, and is equal to the electric field intensity 


Where E is the electric field intensity,

φ is the applied voltage,

And (read Del) operator is defined as

 


 

Where ax, ay, and aZ are components of position vector

 

.

As already mentioned, the most important material used in a high voltage apparatus is the insulation. The dielectric strength of an insulating material can be defined as the maximum dielectric stress which the material can

Withstand. It can also be defined as the voltage at which the current starts increasing to very high values unless controlled by the external impedance of the circuit.

 

The electric breakdown strength of insulating materials depends on a variety of parameters, such as pressure, temperature, humidity, field configurations, nature of applied voltage, imperfections in dielectric materials, material of electrodes, and surface conditions of electrodes, etc. An understanding of the failure of the insulation will be possible by the study of the possible mechanisms by which the failure can occur.      

 

The most common cause of insulation failure is the presence of discharges either within the voids in the insulation or over the surface of the insulation. The probability of failure will be greatly reduced if such discharges could be eliminated at the normal working voltage. Then, failure can occur as a result of thermal or electrochemical deterioration of the insulation.

 

·        Gas/vacuum as Insulator

Air at atmospheric pressure is the most common gaseous insulation. The breakdown of air is of considerable practical importance to the design engineers of power transmission lines and power apparatus. Breakdown occurs in gases due to the process of collisional ionization.

Electrons get multiplied in an exponential manner, and if the applied voltage is sufficiently large, breakdown occurs. In some gases, free electrons are removed by attachment to neutral gas molecules; the breakdown strength of such gases is substantially large. An example of such a gas, with larger dielectric strength, is sulphur hexafluoride (SF6).

 

High pressure gas, provides a flexible and reliable medium for high voltage insulation using gases at high pressures, field gradients up to 25 MV/m have been realized. Nitrogen (N2) was the gas first used at high pressures because of its inertness and chemical stability, but its dielectric strength is the same as that of air. Other important practical insulating gases are carbon dioxide (CO2), dichlorodifluor9methane (CC12F2) (popularly known as Freon), and sulphur hexafluoride (SF6). The breakdown voltage at higher pressures in gases shows an increasing dependence on the nature and smoothness of the electrode material. It is relevant to point out that, of the gases examined to-date, SF6 has probably the most attractive overall dielectric and arc quenching properties for gas insulated high voltage systems.

 

However, in recent years pure SF6 gas has been found to be a green house gas causing environmental hazards and therefore research efforts are presently focussed on finding a replacement gas or gas mixture which is environmentally friendly. Pure nitrogen, air and SF6/N2 mixtures show good potential to replace SF6 gas in high voltage apparatus. In the next few years, SF6/N2, SF6 gas has to be replaced by a new gas and lot of research is being done to find such a gas.

Ideally, vacuum is the best insulator with field strengths up to 107 V/cm, limited only by emissions from the electrode surfaces. This decreases to less than
105 V/cm for gaps of several centimeters. Under high vacuum conditions, where the pressures are below 10-4 ton, the breakdown cannot occur due to collisional processes like in gases, and hence the breakdown strength is quite high. Vacuum insulation is used in particle accelerators, x-ray and field emission tubes, electron microscopes, capacitors, and Circuit Breakers.

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