Electrical insulation materials are the materials where the field can remain and this material also show high resistance to pass current. Due to the qualities, these type of electrical insulations materials is widely used for high voltage applications in electrical engineering. but there are some instances these materials unable to stand with an infinitely high voltage.When the certain voltage applied across these high insulating materials exceeds it die electrical value the insulation tends to get damage.
Most of the insulation materials used in electrical engineering are a solid form, but there are some instances that gaseous and liquid forms are used for the insulation purposes in applications.
Types of Breakdown of Gases
The Breakdown of Gas in dielectrics mainly happened due to the irradiation and field emission. There is a process occur as of this breakdown is mainly name as ionizations and there are several ways to occur this ionization in gaseous materials. There are seven major ionizations methods use in high voltage engineering such as.
- Ionization by simple collision
- Ionization by Excitation
- Ionization by Double electron impact
- Electron Attachment
- Electron detachment
- Other Processes
Process of Breakdown of Gas Materials
The breakdown process of the gases is also played important role in the breakdown of gases. The process of breakdown of gases can classify into two such as
- Avalanche Breakdown process (Townsend Breakdown Process)
- Streamer Breakdown process.
1. Avalanche Breakdown process (Townsend Breakdown Process)
In this method breaks down is initiated by free electrons and these free electrons are cause the ionizations of the atoms. Suppose a free electron exists in a gas due to a same external effect such as cosmic radiation effects, radioactivity. This will give a sufficient energy due to the applied electric field.
If this energy is really sufficient enough so it will cause ionization of the gas molecules will result from two free electrons. Then the two electrons will be able to cause further ionization by collision four electrons and four positive iron. This process will continue and the number of free electrons on increasing as they continuously movie under the electric field.
The creation of electrons and positive ions is called electron avalanche. This can also grow until it creates main million of an electron within a few mm space. If there are sufficient enough along the gas insulation that can ultimately break down the gas by following very high current.
Townsend coefficient is determined bin ionization chamber which is the first vacuum in order of 10-64 then the required gas is filled into the ionization chamber at a pressure of few torque. Then electrons separation will set a certain value and the desired voltage is applied to a lower value like 2kV for higher value 10kV.
Gradually the step leakage current is measured in each voltage steps and plot a graph will give a steady state leakage current range. So this leakage current is not corresponding to the given gap space. Something is repeated to a different type of gases and corresponding to leakage current are made. The graphs will be plotted between in current versus distance.
2. Streamer Mechanism Breakdown Process
The streamer is the type of breakdown mainly arrives due to added effect of the space charge field of avalanche and the photoelectric ionization in the gas vacuum. In the Townson mech it predicts the very diffuse form of discharge, but in actual practice, there are many discharges which are filamentary and irregular.
The stream of the array predicts the development of the spark changes forms an avalanche. According to the streamer theory as the electrons spark rapidly toward the anode. The positive iron left behind in relatively slow move trail. The field will enhance these positive ions and they will move towards the cathode.
Making some repulsion to the free electrons. So that these free electrons move away from the avalanche field and thus will create some elementary discharges in the side of the fil. This process is really fast and the space charges thus extend to the cathodes very rapidly forming a streamer. This process is known as streamer mechanism.