Post Breakdown in Non-uniform Field and Applications (Electric Discharge)
The passage of current through a gas is usually termed as electrical discharge in gases. This phenomenon is observed when a gas usually a non-conductor carries charge carrier in addition to the general gas atoms on molecules and becomes electrically conducting.
The electric discharge in a gas can be divided into two.
- Non-self sustaining discharge
- Self sustaining discharge.
1. Non-Self Sustaining Discharge
- At a voltage, lower than the breakdown voltage (BDV), the current in the gas is due to presence of an external ionizer which continuously produces electrons and ions in the inter electrode space
- If this external ionizer is taken away, the current immediately ceases to flow in space and ionization under the action of applied electric field also ceases.
- This process is non-self sustaining one i.e. it cannot maintain itself only at the expense of internal resource of gaseous space.
- At voltage equal to the breakdown voltage the process becomes self sustaining i.e. it does not, any more depend upon help of an external ionizer Depending upon the electric field configuration power of source and gas pressure, it is possible to get several types of electric discharge.
Glow Discharge or Decay Discharge
This type of discharge is normally produced at low gas pressure. The gas cannot acquire a high conductivity even at high ionization because of insufficient number of gas molecules per unit volume. This type of discharge usually occupies all the space, between the electrodes and the current passing through the gas is not very large.
Application :
Most important application of these is in the cold cathode gaseous voltage stabilizer tubes. A fluorescent tube for lighting is another important application of glow discharge.
Spark Discharge
This type is characterized by a high voltage gradient and passage of la current for an extremely short duration of time (micro-second).
Example :
Lightning is one of the forms of spark discharge.
Arc Discharge
This type is characterized by the passage of a high current at relatively low voltage. This type of discharge is very luminous and noisy. When voltage is less the 20V and current density is in the range of 103 to 107 A/cm2 heavy arc is produced between the electrodes.
Application :
Used in various type of welding and other electro-thermal processes.
Corona Discharge
- This type of discharge is characteristic of sharply non-uniform electric field.
- In a highly non-uniform electric field, as for eg. point – plane pap or wire – cylinder gap, the local electric field near the point electrode ar wire electrode far exceeds the breakdown strength (BDS) of the medium (for air at atmospheric pressure and 20°C, it is approximately 30 kV/cm).
- The gas in the immediate vicinity of the high stressed electrode breaks down and a luminous glow appears.
- The colour of the glow is the characteristic of the gas and is generally observed as a bluish luminescence.
- This phenomenon of local breakdown is called Corona.
- Corona is always accompanied by a hissing noise The air surrounding the corona region is converted into ozone.
- Corona discharges for both polarities consists of current pulses each lasting for 5 brief time of the order of 10 μ sec.
- Corona is a form of high pressure glow discharge. Formation of corona does not mean full loss of insulating properties by the gaseous space, however formation of corona discharge in insulation construction is not desirable do to the following :
- Corona can occur in voids or occlusions of air in solid or liquid insulation. This causes progressive deterioration of insulation and its ultimate failure.
- Corona causes power loss which in the case of HV transmission network can assume serious proportions and in some cases can exceed the load losses.
- Due to pulse nature of corona, it causes electromagnetic radiations in the radio spectrum causing radio interference.
2. Self Sustaining Discharge
Secondary Ionization Process
The secondary Electrons are produced by secondary ionization. These initiate a sustained discharge . The sustained discharge is due to ionization by collision and photo ionization
Electron Emission Due to Positive Ion Impact
Ionization by collision of photo-ionization causes formation of positive ions. These travel towards the cathode. When a positive ion approaches a metallic cathode it causes emission of electrons from the cathode. In this process the positive ice gives away its kinetic energy. If the ionization energy is greater than twice the work function of the metal, then one electron will be ejected and a second electron will The probability of this process is measured as yi which is called the Townsend’s secondary ionization co-efficient due to positive ions.
yi is defined as the net yield of electrons per incident positive ions.
yi increases with ion velocity and depends on the type of gas and electrode material neutralize the ion used
Electron Emission Due to Photons
If electron is to be dislodged from the outer shell, it should be given enough energy so that it overcomes the surface potential barrier. The external energy can be supplied in the form of a photon of UV light of suitable frequency.
The electron emission from a metal surface takes place at a critical condition given by
h . v ≥ Φ
where Φ is the work function of metallic electrode. The frequency (v) is given by the relationship
v = Φ/h
is known as threshold frequency.
For a clean nickel surface with = 4.5 eV, the threshold frequency will be that correspond to a wavelength λ = 2755 A.
Since Φ is typically a few electrons volts, the threshold frequency lies in the far ultraviolet region of the electromagnetic radiation spectrum.
Electron Emission Due to Metastable and Neutral Atoms
A metastable atom or molecule is an excited particle.The lifetime of such metastable atom is very large (10-3 s) compared to the lifetime of an ordinary particle (10-8 s) .The electrons can be made to eject from the metal surface by means of an impact of an excited (meta-stable) atom. The total energy should be sufficient to overcome the work function. The process described is commonly observed metastable atoms since the life time of other excited states is very short so there cannot reach the cathode surface and thus cannot cause electron emission. The emission may take place in such cases only if these are very near to cathode. Neutral atoms in the ground state can also cause secondary electron emission. The kinetic energy required however should be high.