介质气体

ವಾಹಕ ಪದಾರ್ಥಗಳು ಮೂಲತಃ ಶುದ್ಧ ಮತ್ತು ಪ್ರಾಮಾಣಿಕ ವಿದ್ಯುತ್ ಅವರೋಧಕಗಳು. ಯೋಗ್ಯವಾದ ವಿದ್ಯುತ್ ಕ್ಷೇತ್ರವನ್ನು ಪ್ರಯೋಗಿಸಿದಾಗ, ವಾಹಕ ವಾಯುಗಳನ್ನು ವಿಭಜಿಸಬಹುದು. ಶೂನ್ಯ, ಘನ ಪದಾರ್ಥಗಳು, ದ್ರವಗಳು ಮತ್ತು ವಾಯುಗಳು ವಾಹಕ ಪದಾರ್ಥಗಳಾಗಿರಬಹುದು. ವಾಹಕ ವಾಯು ಅನೇಕ ಸಾಧಾರಣವಾಗಿ ವಿದ್ಯುತ್ ಅವರೋಧಕ ವಾಯು ಎಂದೂ ಕರೆಯಲಾಗುತ್ತದೆ. ಇದು ವಾಯು ಅವಸ್ಥೆಯಲ್ಲಿರುವ ವಾಹಕ ಪದಾರ್ಥವಾಗಿದ್ದು, ವಿದ್ಯುತ್ ನಿಸ್ತಾರವನ್ನು ತಡೆಯಬಹುದು. ಶುಶ್ರುಷಿತ ವಾಯು, ಸುಲ್ಫರ್ ಹೆಕ್ಸಾಫ್ಲೋರೈಡ್ (SF6) ಆದಂತಹ ವಾಹಕ ವಾಯು ಉದಾಹರಣೆಗಳು.
ವಾಹಕ ವಾಯುಗಳು ವಾಸ್ತವವಾಗಿ ವಿದ್ಯುತ್ ಆಧಾರಿತ ಪಾರ್ಚಿಕಲ್ಗಳಿಂದ ಶೂನ್ಯವಾಗಿರುವುದಿಲ್ಲ. ಒಂದು ಪೆರಿಫೆರಲ್ ವಿದ್ಯುತ್ ಕ್ಷೇತ್ರ ವಾಯುಗಳ ಮೇಲೆ ಪ್ರಯೋಗಿಸಿದಾಗ, ಸ್ವತಂತ್ರ ಇಲೆಕ್ಟ್ರಾನ್‌ಗಳು ರಚಿಸಲ್ಪಡುತ್ತವೆ. ಈ ಸ್ವತಂತ್ರ ಇಲೆಕ್ಟ್ರಾನ್‌ಗಳು ವಿದ್ಯುತ್ ಚಾಪದ ಮೂಲಕ ಕಥೋಡ್ ಮಾಧ್ಯಂತರದಿಂದ ಐಡೋಡ್ ಮಾಧ್ಯಂತರದಿಂದ ವೇಗವಾಗಿ ಪ್ರವೇಶಿಸುತ್ತವೆ.

ಈ ಇಲೆಕ್ಟ್ರಾನ್‌ಗಳು ಗಾಸ್ ಅಣುಗಳ ಅಥವಾ ಮೋಲೆಕ್ಯುಲ್‌ಗಳ ಇಲೆಕ್ಟ್ರಾನ್‌ಗಳನ್ನು ತಿರುಳಿಸಲು ಯಾವುದೇ ಶಕ್ತಿಯನ್ನು ಪಡೆದಾಗ, ಆ ಇಲೆಕ್ಟ್ರಾನ್‌ಗಳು ಮೋಲೆಕ್ಯುಲ್‌ಗಳಿಂದ ಅನುಕ್ರಮಿಸಲಾಗದಿದ್ದರೆ, ಇಲೆಕ್ಟ್ರಾನ್ ಸಂಖ್ಯೆ ಘಾತಕವಾಗಿ ಬೆಳೆಯುತ್ತದೆ. ಫಲಿತಾಂಶವಾಗಿ ಬ್ರೇಕ್ಡównDown occurs. A few gases such as SF6 are strongly attached (the electrons are powerfully attached to the molecule), some are weakly attached for e.g., oxygen and some are not at all attached for e.g. N2. Examples of dielectric gases are Ammonia, Air, Carbon dioxide, Sulphur hexafluoride (SF6), Carbon Monoxide, Nitrogen, Hydrogen etc. The moisture content in dielectric gases may alter the properties to be a good dielectric.

Breakdown in Gases

Actually, it is the fall in resistance of the insulating gases. This will happen when the applied voltage increases than the breakdown voltage (dielectric strength). As a result of this, the gas will begin to conduct. That is, there will be a strong voltage rise in a small area in the gas. This area of strong voltage rise is the reason of partial ionisation of nearby gas and starts conduction. This is made intentionally in low pressure discharges (in an electrostatic precipitator or in fluorescent lights).

The Paschen’s law approximated the voltage which causes electrical breakdown (V = f(pd)). It is actually an equation which explains the breakdown voltage as the function of product of pressure and gap length. In that a curve is obtained, this is called Paschen’s curve. The Paschen’s curve for air and argon is represented in figure 1.
Here, as pressure is decreased, the breakdown voltage also reduced and then gradually increases which exceeds the original value. At standard pressure, the breakdown voltage reduces with the gap length up to a point.

When the gap length is reduced beyond that point, then the breakdown voltage start to increase and exceeds its original value. At high pressure and increased gap length condition, the breakdown voltage is more or less proportional to the product of the two. This is roughly proportional because of electrode effects (microscopic irregularity of electrodes may cause breakdown). The breakdown voltage of dielectric gases is also roughly proportional to density.

Breakdown Mechanism

The mechanism of breakdown will directly depend on the nature of the dielectric gases and the electrode polarity in which the breakdown begins. If breakdown begins at cathode, then the supply of initiatory electrons is by the electrode itself. Then the electrons will get accelerates, numerous electrons formation occurs and it results in breakdown. If breakdown begins at anode, then the supply of initiatory electrons is by the gas itself. For e.g. air and SF6 gas. A tiny sharp point in a gas may also be the reason of breakdown of gas gap. This happens as a result of step-by-step breakdown processes. Corona formation (i.e. corona discharge) can be related to this. It is actually a short energy release (discharge) and it results in feebly ionized gas channels. When the field is too high, one of these channels will conduct.

Properties of Dielectric Gases

The preferred properties of an excellent gaseous dielectric material are as follows

• Utmost dielectric strength.

• Fine heat transfer.

• Incombustible.

• Chemical idleness against the construction material used.

• Inertness.

• Environmentally non poisonous.

• Small temperature of condensation.

• High thermal constancy.

• Acquirable at low cost

Application of Dielectric Gases

It is used in Transformer, Radar waveguides, Circuit Breakers, Switchgears, High Voltage Switching, Coolants. They are usually used in high voltage application.

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