Friday, August 28, 2009

Application of Gases in Power System

The gases find wide application in power system to provide insulation to various equipments and
substations. The gases are also used in circuit breakers for arc interruption besides providing insulation
between breaker contacts and from contact to the enclosure used for contacts. The various gases used
are (i) air (ii) oxygen (iii) hydrogen (iv) nitrogen (v) CO2 and (vi) electronegative gases like sulphur
hexafluoride, arcton etc.
The various properties required for providing insulation and arc interruption are:
(i) High dielectric strength.
(ii) Thermal and chemical stability.
(iii) Non-inflammability.
(iv) High thermal conductivity. This assists cooling of current carrying conductors immersed in
the gas and also assists the arc-extinction process.
(v) Arc extinguishing ability. It should have a low dissociation temperature, a short thermal
time constant (ratio of energy contained in an arc column at any instant to the rate of energy dissipation
at the same instant) and should not produce conducting products such as carbon during arcing.
(vi) Commercial availability at moderate cost. Of the simple gases air is the cheapest and most
widely used for circuit breaking. Hydrogen has better arc extinguishing property but it has lower di-
electric strength as compared with air. Also if hydrogen is contaminated with air, it forms an explosive
mixture. Nitrogen has similar properties as air, CO2 has almost the same dielectric strength as air but is
a better arc extinguishing medium at moderate currents. Oxygen is a good extinguishing medium but is
chemically active. SF6 has outstanding arc-quenching properties and good dielectric strength. Of all
these gases, SF6 and air are used in commercial gas blast circuit breakers.
Air at atmospheric pressure is ‘free’ but dry air costs a lot when stored at say 75 atmosphere.
The compressed air supply system is a vital part of an air blast C.B. Moisture from the air is removed by
refrigeration, by drying agents or by storing at several times the working pressure and then expanding
it to the working pressure for use in the C.B. The relative cost of storing the air reduces with increase in
pressure. If the air to be used by the breaker is at 35 kg/cm2 it is common to store it at 210 kg/cm2.
Air has an advantage over the electronegative gases in that air can be compressed to extremely
high pressures at room temperature and then its dielectric strength even exceeds that of these gases.
The SF6 gas is toxic and its release in the form of leakage causes environmental problems.
Therefore, the electrical industry has been looking for an alternative gas or a mixture of SF6 with some
other gas as an insulating and arc interrupting medium. It has been observed that a suitable mixture of
SF6 with N2 is a good replacement for SF6. This mixture is not only finding acceptability for providing
insulation e.g., gas insulated substation and other equipments, it is able to quench high current magni-
tude arcs. The mixture is not only cost effective, it is less sensitive to find non-uniformities present
within the equipment. Electric power industry is trying to find optimum SF6 to N2 mixture ratio for
various components of the system viz., GIS, C.B., capacitors, CT, PT and cables. A ratio 70% of SF6
and 30% of N2 is found to be optimum for circuit breaking. With this ratio, the C.B. has higher recovery
rate than pure SF6 at the same partial pressure. The future of using SF6 with N2 or He for providing
insulation and arc interruption is quite bright.

1 comment:

betitveno said...

nice and informative i like it very much!