Breakdown in gases (HV Engineering), Lecture notes of Power Distribution and Utilization

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CONDUCTION AND BREAKDOWN

IN GASES

Dated : 07-11-

 Gases as insulating media

• The simplest and the most commonly found dielectrics are gases. Most of the electrical apparatus use air as the insulating medium, and in a few cases other gases such as nitrogen, carbon dioxide, freon and sulphur hexafluoride are also used.
• Various phenomena occur in gaseous dielectrics when a voltage is applied. When the applied voltage is low, small currents flow between the electrodes and the insulation retains its electrical properties.

 Gases as insulating media

• The maximum voltage applied to the insulation at the moment of breakdown is called the breakdown voltage.
• In order to understand the breakdown phenomenon in gases, a study of the electrical properties of gases and the processes by which high currents are produced in gases is essential.

 Gases as insulating media

• The electrical discharges in gases are of two types, i.e. (i) non-sustaining discharges, and (ii) self-sustaining types.
• The breakdown in a gas, called spark breakdown is the transition of a non-sustaining discharge into a self-sustaining discharge.
• The build-up of high currents in a breakdown is due to the process known as ionization in which electrons and ions are created from neutral atoms or molecules, and their migration to the anode and cathode respectively leads to high currents.

 Ionization Processes

• A gas in its normal state is almost a perfect insulator. However, when a high voltage is applied between the two electrodes immersed in a gaseous medium, the gas becomes a conductor and an electrical breakdown occurs.
• The processes that are primarily responsible for the breakdown of a gas are ionization by collision, photo-ionization, and the secondary ionization processes.
• In insulating gases (also called electron-attaching gases) the process of attachment also plays an important role.

 Ionization by collision

• The process of liberating an electron from a gas molecule with the simultaneous production of a positive ion is called ionisation.
• In the process of ionisation by collision, a free electron collides with a neutral gas molecule and gives rise to a new electron and a positive ion.
• If we consider a low pressure gas column in which an electric field E is applied across two plane parallel electrodes, as shown in Figure1.

 Ionization by collision

• Any electron starting at the cathode will be accelerated more and more between collisions with other gas molecules during its travel towards the anode.
• If the energy (E) gained during this travel between collisions exceeds the ionisation potential, Vi, which is the energy required to dislodge an electron from its atomic shell, then ionisation takes place.

 Ionization by collision

• This process can be represented as
• Where, A is the atom, 𝐴+^ is the positive ion

and 𝑒−is the electron.

 Photo Ionization

• The phenomena associated with ionisation by radiation, or photo-ionisation, involves the interaction of radiation with matter.
• Photo-ionisation occurs when the amount of radiation energy absorbed by an atom or molecule exceeds its ionisation potential.
• Just as an excited atom emits radiation when the electron returns to the lower state or to the ground state, the reverse process takes place when an atom absorbs radiation. This reversible process can be expressed as

Secondary Ionisation Processes

Dated : 13-11-

 Electron Emission due to Positive Ion Impact

• Positive ions are formed due to ionisation by collision or by photo-ionisation, and being positively charged, they travel towards the cathode.
• A positive ion approaching a metallic cathode can cause emission of electrons from the cathode by giving up its kinetic energy on impact.

 Electron Emission due to Positive Ion Impact

• If the total energy of the positive ion, namely, the sum of its kinetic energy and the ionisation energy, is greater than twice the work function of the metal, then one electron will be ejected and a second electron will neutralise the ion.
• The probability of this process is measured as 𝛾𝑖 which is called the Townsend's secondary ionisation coefficient due to positive ions.

 Electron Emission due to Metastable & Neutral Atoms

• A metastable atom or molecule is an excited particle whose lifetime is very large (10−3𝑠) compared to the lifetime of an ordinary particle (10−8𝑠).
• Electrons can be ejected from the metal surface by the impact of excited (metastable) atoms, provided that their total energy is sufficient to overcome the work function.
• This process is most easily observed with metastable atoms, because the lifetime of other excited states is too short for them to reach the cathode and cause electron emission, unless they originate very near to the cathode surface.

 Electron Attachment Process

• The types of collisions in which electrons may become attached to atoms or molecules to form negative ions are called attachment collisions. Electron attachment process depends on the energy of the electron and the nature of the gas and is a very important process from the engineering point of view.
• Atom +𝑒−^ → 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑖𝑜𝑛 + (𝐸𝑎 + 𝐾)
• The energy liberated as a result of this process is the kinetic energy K plus the electron affinity 𝐸𝑎.