- Dielectric Gas Definition: A dielectric gas is defined as an insulating gas that prevents electrical discharge and can be polarized by an electric field.
- Breakdown in Gases: Breakdown in gases occurs when the applied voltage exceeds the breakdown voltage, causing the gas to conduct electricity.
- Paschen’s Law: This law states that the breakdown voltage depends on the product of the gas pressure and the gap length between electrodes.
- Breakdown Mechanism: The breakdown mechanism varies based on the type of dielectric gas and the polarity of the electrodes; corona discharge is one such mechanism.
- Applications of Dielectric Gases: Dielectric gases are used in high voltage applications such as transformers, radar waveguides, and circuit breakers due to their insulating properties.
Dielectric materials are pure electrical insulators. When an electric field is applied, dielectric gases can be polarized. Dielectric materials can be in the form of a vacuum, solids, liquids, or gases. A dielectric gas, also known as an insulating gas, is a gas that prevents electrical discharge. Examples of dielectric gases include dry air and Sulphur hexafluoride (SF6).
Gaseous dielectrics contain some electrically charged particles. When an electric field is applied, free electrons are created and move from the cathode to the anode due to electric pressure.
When these electrons achieve adequate energy to bang off the electrons of the gas atoms or molecules and after that, the electrons are not involved by the molecules, and then the electron concentration will begin to build up exponentially. As a result, breakdown 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
Breakdown in gases occurs when the insulating gas’s resistance decreases because the applied voltage exceeds the breakdown voltage (dielectric strength). As a result, the gas starts to conduct electricity. A high voltage rise in a small area of the gas causes partial ionization and begins conduction. This effect is intentionally used in low-pressure discharges, such as in electrostatic precipitators and 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.
