18-03-2014, 04:06 PM
Properties of gases
Properties of gases.pptx (Size: 74.12 KB / Downloads: 12)
Properties of atmospheric air (N2)
The most important, freely available and the cheapest gaseous dielectric is air. The atmospheric air is in fact a mixture of a number of gases. The detailed composition of earth's atmosphere is given as given by Goody and Walker in their book Atmospheres.
Properties of Sulphurhexafluoride, SF6 Gas
Sulphurhexafluoride was first produced in 1900 by French scientists Moissan and Lebeau by direct fluoronisation of sulphur. In the beginning it was mainly used as a dielectric in atomic physics. During late 1950s, it found application in high voltage circuit breakers. Ever since, its application in power systems has been continuously increasing.
Physical Properties
In a SF6 molecule, six fluorine atoms arrange themselves uniformly like an octahedron on a central sulphur atom. An excited sulphur atom can therefore form six stable covalence bonds with the strongly electronegative fluorine atoms by sharing the pair of electrons.
Amongst halogens, the fluorine element and the sulphur atom both have very high coefficients of electronegativity, of the order of 4 and 2.5, respectively. This coefficient is a measure of the tendency to attract electrons of other atoms to form dipole bondage.
Electrical Properties of Vacuum as High Voltage Insulation
In a vacuum better than 10-4 Torr (1.333 x 10-2 Pa), less than 3 x 1012 molecules per cm3 are estimated to be present and the length of the mean free path is of the order of meters. In such vacuum, an electron may cross a gap of a few cm between two electrodes without any collision.
Therefore, unlike in gases, in vacuum the initial stage of breakdown cannot be due to the formation of electron avalanche.
The process of multiplication of charged particles by collision in the space between the electrodes is far too insufficient to create avalanches. However, if a gas cloud forms in the vacuum, the usual kind of breakdown process can take place. Thus investigations of the breakdown mechanism in vacuum have been oriented to establish the way gas clouds could be created in a vacuum.
Mechanism, suggests that the prebreakdown currents that flow between vacuum insulated high voltage electrodes, frequently originate from nonmetallic emission mechanisms. These are associated with some kind of insulating/semiconducting surface oxides or impurity concentrations. From the technological point of view, the microscopic conditions of electrode surfaces continue to play an important role.
PREBREAKDOWN ELECTRON EMISSION IN VACUUM
When the voltage across a very small gap (a few mm) is sufficiently increased, a relatively steady current begins to flow.
A general observation made for small gaps is, that the prebreakdown current flow has been found to consist mainly of electrons.
For longer gap spacings (> 1 cm), small pulse currents of millisecond durations and charges of the order of micro-Coulombs (micro-discharges) are measured.
On raising the voltage further, the micro-discharge eventually give rise to a steady current .
Generation of charge carriers
The production of charge carriers from the neutral gas molecules is known as ionization process. The ionization process in a gas in long gap distances is the deciding factor leading to breakdown. One of the most significant feature of ionization process is the electron energy in different shells of a molecule. The total energy of an electron while still attached to the molecule, can be divided into two types of energies. First the kinetic energy WKE, which depends upon its mass and velocity, and second the potential energy Wpot depending upon its charge in the Coulomb field of the nucleus of a molecule.