High Voltage Engineering

                  High voltage engineering

Breakdown of gaseous insulation

What is ionization?

Ionization means remove electrons from the atom. In this case some external energy is required to remove electrons from the atom. If the electron is located at very high distance from the nuclear it has less attraction from the neutral and can be easily removed (means it require less energy). The energy that required to move electron from its orbit to a far distance is known as first ionization potential. In the ionization process it leaves a positive ion.

Ionization process may happen in vice versa. It means electron can go to an inner orbit by releasing some energy. This energy released as photons. This is the LED operating principle.

Ionization of gases

Electrical insulation material include solid, liquid and gases. Each material can withstand up to some certain voltage. But after that voltage insulation will damage.

Normally in air there are no free electrons. But when applying an electric field it caused to create some free electrons. These free electrons are accelerated under the applied electric field from cathode (-) to anode (+). Then they collide with neutral air molecules between anode and cathode. In this case part of the kinetic energy of the electrons are transferred to the neutral molecules. That energy may be high enough to ionize the neutral molecule. So neutral molecule emit another electrons and they also start accelerate under the electric field. This incident is known as avalanche. After this avalanche is set up if we increase voltage further this ionization process increase rapidly until the breakdown occurs.

Kinetic energy of moving electrons are   but they are usually expressed in electron volts (eV). 1eV=1.6x10^-19J.

Ionization process in gases

For gas insulation breakdown it need ionization processes. It means neutral gas molecules should gain required ionization energy by some process. As an examples colliding neutral gas molecules with electrons, ions, photons. Another one is electron emissions near the cathode surface. Because at some higher value of cathode voltages cathode able to emit some electrons.

Gas ionization processes

Ionization by a simple collision

In this case a moving electron with kinetic energy is hit with a molecule. If that energy is sufficient to ionize the molecule (on the other hand to release an electron from the outer orbit) then it will result to create one positive ion with two electron. (Electron that hit with molecule and a new electron emitted by the molecule).

M + e^- -------> M⁺ + 2e^-

Ionization probability vs. electron energy curve is shown below. In here electron energy means kinetic energy of electron which hit with the molecule.

 

Let’s get some explanation about the above graph.

When electron energy is equal to the ionization energy probability is zero. But when the electron energy increases probability increases, at first linearly and the rapidly up to the maximum point. Why the probability is less for low electron energies? Because when the primarily electron (electron which hit with the molecule) has high power it release more electrons from the atom. But when that energy is low it release less number of electrons. It means the number of released electrons depend on the kinetic energy of the primarily electron.

So why probability is less for high electron energies? Because when that electron energy is high that electron will able to penetrate deeper in to the molecule. Then it will reduces the ionization probability.   

Excitation

In this case actually it is not occurred any ionization. A moving electron is hit with a molecule. Then molecule take some energy and become to intermediate state. Then molecule emit a photon which consist of energy that gain by previous process. In this case only happen is an electron moving one orbit to another orbit.

M + e^- -------> M* + e^-

M*       -------> M + hv

Ionization by double electron impact

In this case molecule with previous excited state collide with slow moving electron. Although that electron has no sufficient ionization energy it will cause to release another electron. But addition of previous energy plus energy in hit electron should be greater than the ionization energy.

M* + e^- -------> M + 2e^-

Photo ionization

One electron moving from one orbit to another orbit releasing a photon. If the energy in that photon is greater than the ionization energy it will cause to ionization.

M + hv-------> M⁺ + e^-

Electron attachment

In this case if the outer orbit has unoccupied electron, then the electron which hit with the molecule will occupied in the outer orbit making a negative ion. So this process reduces the number of free electrons.

M + e^- -------> M^-

Electron detachment

The previously attached electron is released from the atom.

M^- -------> M + e^-

Other processes

Except the above ionization processes there may be atom-atom collisions, ion-atom collisions, excited atom-molecule collisions etc. but they are rarely occurred. Because it take relatively large time to interact.

References

Prof. Lucas J.R., High Voltage Engineering. 2001, Colombo, university of moratuwa

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