Gas Insulated Substations(GIS) is a compact, multicomponent assembly enclosed in
a ground metallic housing which the primary insulating medium is compressed
sulphur hexaflouride(SF6) gas. It generally consists components Of
a) Bus
bars
b) Circuit Breakers
c) Disconnecting switches
d) Earthing
switches
e) Current transformers
f) Voltage transformers
g) Cable and
boxes
h) Gas supply and gas monitoring equipment
i) Densimeters and
j) Local
control
Why we need GIS
1) Gas insulated Substations have found a broad
range applications in power systems over the last three decades because of their
high reliability Easy maintenance, small ground space requirements etc.,.
2)
In our country also few GIS units have been in operation and a large number of
units are under various stages of installation.
3) Although GIS has been in
operation in several years, some of the problems are needful attention
4)
These problems include VFTO during switching operations or earth faults and
transient enclosure voltages and particle contamination.
Advantages of
GIS:
a) Because of the entire equipment being enclosed in enclosures,
filled with pressurized SF6 gas, installation is not subject to environmental
pollutions, as experienced along coastal areas or certain types of
industries.
b) Such installations are preferred in cosmopolitan cities,
industrial townships, etc., where cost of land is very high and higher cost of
SF6 insulated switchgear is justified by saving due to reduction in floor area
requirement. It is not necessary that high voltage or extra high voltage
switchgear to be installed out doors.
c) Since most of the construction is
modular and the assembly is done in the works, one site erection time both for
supporting structures and switchgear is greatly reduced.
Disadvantages
with Air Insulated Substations:
a) It requires huge amount of area
.
b) Each and every component of substation is exposed to air and
pollution.
c) Particularly in coastal area all the insulators are exposed to
air and saline contamination.
d) Frequent flashovers and breakdown
occurs.
e) Maintenance cost is more.
f) Installation time is also
more.
g) More concrete work is necessary.
A) The necessity of this
study :
Extremely high dielectric properties of SF6 have long been
recognized. Compressed SF6 has been used as an insulating medium as well as arc
quenching medium in electrical apparatus in a wide range of voltages.
Gas
Insulated Substations (GIS) can be used for longer times without any periodical
inspections. Conducting contamination (i.e. aluminum, copper and silver
particles) could, however, seriously reduce the dielectric strength of
gas-insulated system
B) The origin of these particles :
Metallic particles
in GIS have their origin mainly from the manufacturing process or they may
originate from moving parts of the system, such as breakers and
disconnectors.
Metallic particles can be either free to move in the GIS or
they may be stuck either to an energized electrode or to an insulator surface
(spacer, bushing etc.).
1) Different metallic contamination viz Cu, Al and Ag
have been considered for the above study. Typically a GIB of inner and outer dia
102/292mm have been considered for a 245KV system.
2) Wires of radii varying
from 0.1 to 0.5mm and length from 1mm to 10mm have been used for simulation.
Co-efficient of restitution and pressure have been varied from 0.1 to 0.9 and
0.1Mpa to 1Mpa respectively.
3) Typical results show that for a variation of
copper particles of radii from 0.2 to 5mm, the maximum radial movement reduces
from 6mm to 1mm.
4) Whereas in the case of variation of length from 1mm to
16mm the radial movement increases from 1mm to 9.8mm. With the change of
pressure the movement is found to be almost same for majority of particles
considered
If a metallic particle crosses the gap and comes into contact
with the inner electrode or if a metallic particle adheres to the inner
conductor, the particle will act as protrusion on the surface of the electrode,
and the voltage required for breakdown of the GIS will be dramatically
decreased.
A metallic particle stuck on an insulator surface in a GIS will
also cause a significant reduction of the breakdown voltage
Depending on the
shape of the particles, as well as the geometry and voltage levels of the
system, the particles get more or less influenced by the electric field which,
in turn, makes them hazardous to the electrical system, in terms of partial
discharges and breakdown.
1. The introduction of SF6 gas has
revolutionized not only the technology of circuit breakers but also the layout
of substations.
2. The dielectric strength of SF6 gas at atmospheric pressure
is approximately three times that of air.
3. It is incombustible, non toxic,
colourless and chemically inert.
4. It has arc-quenching properties 3 to 4
times better than air at equal pressure.
5. This enables the substation to be
laid in a much lesser space.
6. Space requirement is only 10 to 25 percent of
what is required is a conventional substation.
7. Thus where creation of a
substation becomes necessary but adequate space is not available, such as in big
cities or otherwise highly populated areas, GIS is the solution
