ABSTRUCT
A substation is an assembly of apparatus, which transform
the characteristics of electrical energy from one form to another say from one
voltage level to another level. Hence a substation is an intermediate link
between the generating station and the load units.
GIS is the substation which is kept enclosed in the SF6 gas.
The SF6 gas is the sulpherhexa fluoride gas this gas is used in the gss because
of the outstanding physical and chemical property. the SF6 gas make it an ideal
dielectric media for used in the power switch gears.
At normal temperature the SF6 is chemically inert
inflammable ,noncorrosive and non conductible at low temperature and it is not
toxic in nature as like the oil SF6 has the property that dissociated molecules
recombines rapidly after the source of arcing is removed when superior arc is
occur. That’s why we use the SF6 gas to encapsulate the equipments as well as
the live parts
Shakti singh
B.tech (EE)
08ELDEE055
Chapter 1.
Introduction:- 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 that are shown
below as-
Fig no.1 Common
Circuit diagram of GIS
·
Bus bars
·
Circuit Breakers
·
Disconnecting switches
·
Earthing switches
·
Current transformers
·
Voltage transformers
·
Cable and boxes
·
Gas supply and gas
monitoring equipment
·
Densimeters and
·
Local control
Fig no.2 Components of
GIS
The single line
diagram fo the gas insulated substation is shown in the figure above with its
components at the top SF6 bushing
connected with the fast acting Earthing
switch a dissconnector is placed to open and close the earthing
switchwhich is connected to the circuit breaker through the current transformer
the operating mechanism of circuit breaker open &close the circuit breaker.
Fig no.3 single line
diagram of GIS
Fig no.4 view of components
of GIS
Histry
of GIS particles
The first GIS’s
were put in operation in 1967 in Switzerland and Germany.
The GIS in
Germany is still in operation, whereas the GIS in Switzerland were recently
decommissioned after 35 years of operation without major fault or gas leak.
An assessment
made on SF6 gas leakage over the lifetime of the first GIS and concluded that
overall leakage rate was about 0.4% per year.
The best result
seemed to be obtainable from the use of mixed technology switchgear either in
the form of GIs modules with and AIs busbar or GIS modules using a GIs busbar
i.e outdoor GIS
a) 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.)
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. used for simulation. Co-efficient of restitution
and pressure have been varied from 0.1 to 0.9 and 0.1Mpa to 1Mpa
respectively.)
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.) 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 consider 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.
Fig no.5 view of
components of GIS
The necessity of this study
:
b) 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.
c) 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
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.
The introduction of SF6 gas has revolutionized
not only the technology of circuit breakers but also the layout of substations.
The dielectric strength of SF6 gas at atmospheric pressure is approximately
three times that of air. It is incombustible, non toxic, colourless and
chemically inert. It has arc-quenching properties 3 to 4 times better
than air at equal pressure. This enables the substation to be laid in a much
lesser space.
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, sucas
in big cities or otherwise highly populated areas.
Chapter 2
Fig no.6 Comparison
B/w GIS & AIS
Why we need GIS
GIS has small
ground space requirements.Gas insulated Substations have easy maintenance(
nearly zero Maintenance Less field erection time & less erection cost.For
underground powerhouse of Hydro electric power project where space constraint
is a major issue._For Fast Growing Major Cities where land availability is
costlier. Non-Flammability & Non-Explosive , Oil-free & Less Pollution.
Fig no.7 view of
Substation of GIS
Gas Insulated
Substations (GIS) is a compact, multicomponent assembly enclosed ina ground
metallic housing in which the primary insulating medium is compressedSulphur
hexafluoride (SF6) gas.SF6 acts as an insulation between live parts & the
earthed metal closure.The introduction of SF6 gas has revolutionized not only
the technology ofcircuit breakers but also the layout of substations.The
dielectric strength of SF6 gas at atmospheric pressure is approximately three
times that of air.It is incombustible, non toxic, colorless and chemically
inert.It has arc-quenching properties 3 to 4 times better than air at equal
pressure.Space requirement is only 10 to 25 percent of what is required is a conventional
substation.
Table no.1 types of
switchgear
Fig no.8 Diff. b/w
hybrid & conventional
Chapter 3
Specification of GIS
The weight and
size of the GIS equipment do not change appreciably with the voltageclass as
the bulk of the current – carrying components and enclosures have identical dimensions
for similar thermal and short time current.The additional insulation required
for the next voltage class is achieved by increased gas density.Owing to these
flexibilities, a few manufacturers offer the same equipment for twovoltage
classes (like 170/145 kV).Even when the GIS equipment is designed for an
individual voltage class, the dimensions and weights of the equipment differ
marginally.
Table no.2 types of
switchgear
Table no 3 showing
minimum clearance for different voltage level
Fig no.9 view OF
dimensions
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 In our country also few GIS units have been in operation and a
large number of units are under various stages of installation. Although
GIS has been in operation in several years, some of the problems are needful
attention. These problems include VFTO during switching operations or earth
faults and transient enclosure voltages and particle contamination
Fig no.10 Comparision
of dimension of GIS ,AIS & H-GIS
Components used:
1.Circuit
Breaker
2.Operating
mechanism
(CB)
3.Current
Transformer
4.Disconnector
5.Maintenance
earthing switch
6.Fast acting
earthing switch
7.Voltage
transformer
8.SF6 Bushing
Fig no.11 view of
circuit breaker of GIS
A circuit
breaker is an automatically-operated electrical switch designed to protectan
electrical from damage caused by overload or short circuit.Its basic function
is to detect a fault condition and, by interrupting continuity, toimmediately
discontinue electrical flow.Unlike a fuse, which operates once and then has to
be replaced, a circuit breaker canbe reset (either manually or automatically)
to resume normal operation
Fig no.12 advancement in circuit breaker
Current
interruption in a high-voltage circuit-breaker is obtained by separating two
contacts in a medium, such as SF6, having excellent dielectric and arc
quenching properties.After contact separation, current is carried through an
arc and the arc is interrupted & cooled by a gas blast of sufficient
intensity.
Fig no.13 internal
circuit of circuit breaker
Each CB
comprises three single-phase metal enclosed breaker poles. Each Pole consists
of operating mechanism, the interrupter column with 2 interrupting chambers in series
& the enclosure with the basic support structure. To guarantee simultaneous
interruption, the chambers are mechanically connected in series.One grading
capacitor guarantees an equalized voltage distribution.
Fig no.14 main circuit
of GIS
SELF BLAST TECHNIQUE
Making use of
arc energy to produce the pressure necessary to quench the arc and obtain current
interruption. Low current interruption, up to about 30% of rated short-circuit
current, is obtained by a puffer blast.A valve between the expansion and
compression volumes.When interrupting low currents the valve opens under the
effect of the overpressure generated in the compression volume. The blow-out of
the arc is made as in a puffer circuit breaker by compression of the gas
obtained by the piston action.
Fig no.15 self blast
technique
In the case of
high currents interruption, the arc energy produces a high overpressure in the expansion
volume, which leads to the closure of the valve and thus isolating the
expansion volume from the compression volume.The overpressure necessary for
breaking is obtained by the optimal use of the thermal effect
.Each Pole of
the CB is equipped with the Hydraulic spring operating mechanism.It combines
the advantages of both Hydraulic operating mechanism & Spring energy storage
type A Hydraulic pump moves oil from low
pressure reservoir to high pressure reservoir side of the energy piston.Opening
& Closing of CB is initiated by trip coil actuation.
Current Transformer
Current
transformer (CT) is used for measurement of electric currents. Current transformers
are also known as instrument transformers.Current transformers are commonly
used in metering and protective relays in the electrical power industry.When
current in a circuit is too high to directly
Fig no.16 current
& voltage Transformer
apply to
measuring instruments,a current transformer produces a reduced current
accurately proportional to the current in the circuit, which can be
conveniently connected to measuring and recording instruments.
Each CB comprises three
single-phase metal enclosed breaker poles. Each Pole consists of operating
mechanism, the interrupter column with 2 interrupting chambers in series &
the enclosure with the basic support structure. To guarantee simultaneous
interruption, the chambers are mechanically connected in series.One grading
capacitor guarantees an equalized voltage distribution.
Fig no.17 working of transformer
The single pole
inductive voltage transformer is connected to switch gear with the connecting flanges with a barrier insulator.The primary
winding is insulated with SF6 gas & connected to high voltage terminal.The
primary winding is wounded on the top of the core & secondary windings.The
secondary winding is connected to the terminals in the external terminal box through a gas tight multiple bushing.
Fig
no.18 Dissconnector & Earthing Switch
Disconnector and Earthing
switches are safety devices used to open or to close a circuit when there is no
current through them.They are used to isolate a part of a circuit, a machine, a
part of an overhead line oran underground line so that maintenance can be
safely conducted.The opening of the line isolator or busbar section isolator is
necessary for safety, but not sufficient. Grounding must be conducted at both
the upstream and downstream sections of the device under maintenance. This is
accomplished by earthing switches
Fig no.19 working of
the switch
Disconnect switches are designed
to continuously carry load currents and momentarily carry short circuit
currents for a specified duration.They are designed for no-load switching ,35 opening
, or closing circuits where negligible currents are made or interrupted (including
capacitive current and resistive or inductive current , or when there is no
significant voltage across the open terminals of the switch.
Fast acting Earthing Switch:-
Fast earth switch and maintenance
earth switch are the two types of earth switches used for gas insulated
sub-station systems. Fast earth switch is used to protect the circuit-connected
instrument voltage transformer from core saturation caused by direct current
flowing through its primary as a consequence of
charge stored online during isolation / switching off the line. Use of fast earth switch provides a parallel
(low resistance) path to drain the residual static charge quickly, thereby
protecting the instrument voltage transformer from the damages that may
otherwise be caused. The basic construction of these earth switches is
identical
Fig no.20 Fast
acting Earthing Switch
Each Pole of the CB is equipped
with the Hydraulic spring operating mechanism.It combines the advantages of
both Hydraulic operating mechanism & Spring energy storage type.A Hydraulic
pump moves oil from low pressure reservoir to high pressure reservoir side of
the energy piston. Opening & Closing of CB is initiated by trip coil
actuation.
Surge Arrestor /Wave Trap
Fig
no.21 Surge
Arrestor /Wave Trap
Capacitor
Voltage Transformers convert transmission class voltages to standardized low
and easily measurable values, used for metering, protection and control of the
high voltage system.Additionally, Capacitor Voltage Transformers serve as a
coupling capacitor for coupling high frequency power line carrier signals to
the transmission line.
GIS
Termination Element:-
Fig no. 22 GIS Termination
Element:-
SF6 – Air
Bushings are used for connecting to open terminal equipment & Overhead
transmission lines.SF outdoor bushings allow the enclosed switchgear to be
connected to overhead lines.
Cable termination
Fig no. 23 Cable termination
High-Voltage
cables of various types are connected to SF6 switchgear ia cable connection
assembly & also it enables the GIS & Cables to be tested separated Transformer
connection consists of Oil/SF6 bushing,
the enclosure, the main circuit end terminal & removable connection. For
Hi-Voltage test on GIS, transformer is isolated from switchgear by dismantling the
removable connection
The insulating
and interrupting capability of the SF6 gas depends on the density of the SF6
gas .The pressure of the SF6 gas varies with temperature, so a mechanical or
electronic temperature compensated pressure switch is used to monitor the
equivalent of gas density. Gas Density Monitor is directly mounted on the
enclosure. The gas pressure acts on metal bellows, with a reference volume for
compensation of the temperature. In case of gas leakage a micro-switch is
actuated. Thresholds for refilling (first stage) or lock-out alarm(second
stage) can be mechanically set. The response character is shown in the Molier
diagramme.
Inter Connecting Transformer
An
autotransformer is an electrical transformer with only one winding.The winding
has at least three electrical connection points called taps. The voltage source
and the load are each connected to two taps. One tap at the end of the winding
is a common connection to both circuits (source and load). Each tap corresponds
to a different source or load voltage.
An autotransformer for power applications is
typically lighter and less costly than a two-winding transformer, up to a
voltage ratio of about 3:1 beyond that range a two-winding transformer is
usually more economical.
In an
autotransformer a portion of the same winding acts as part of both the primary
and secondary winding.
Visual comparison between AIS & GIS Component
Disconnector and Earthing switches
Disconnector and
Earthing switches are safety devices used to open or to close a circuit when
there is no current through them.They are used to isolate a part of a circuit,
a machine, a part of an overhead line or an underground line so that
maintenance can be safely conducted.The opening of the line isolator or busbar
section isolator is necessary for safety, but not sufficient. 
Fig no .25 Disconnector and
Earthing switches
Voltage
transformers
Fig no. 26 Voltage
transformers
Voltage
transformers (VTs), also referred to as “Potential transformers" (PTs),
are used in high-voltage circuits.They are designed to present a negligible
load to the supply being measured, to allow protective relay equipment to be
operated at lower voltages, and to have a precise winding ratio for accurate
metering
Grounding must
be conducted at both the upstream and downstream sections of the device under
maintenance. This is accomplished by earthing switches.
Disconnect
switches are designed to continuously carry load currents and momentarily carry
short circuit currents for a specified duration.They are designed for no-load
switching opening , or closing circuits where negligible currents are made or
interrupted (includingcapacitive current and resistive or inductive current ,
or when there is no significant voltage across the open terminals of the switch.
Advancement In Gis Technology
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 isveryhighand 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.
1.
Gas Insulated Transformer (GIT) Instead of Oil
Immersed Transformer (OIT).
2.
_SMART GIS - Integration of Electronic CT’s
& PT’s
3.
_Combined Earthing Switch & Disconnector
Merits Of GIT Over OIT
1.
Nonflammability – Gas insulated transformers ,
using incombustible SF6 gas as insulation and cooling medium, enable to remove
a fire fighting equipment from transformer room.
2.
Non Tank – explosion - Pressure tank enables to
withstand the pressure rise in case of internal fault.
3.
Compactness – Since conservator or pressure
relief equipment is not necessary, height of transformer room can be reduced
approximately 2 – 2.5 meters.
4.
Easy installation – oil or liquid purifying
process is not necessary in case of gas-insulated transformer.
5.
Easy inspection and maintenance work -Only SF6
gas pressure shall be basically monitored
during periodically inspection.
in GIS.
Disadvantage of GIS
1.
Cost of GIS is high.
2.
The life of GIS is affected by certain factors
such as: conductive particles, partial discharges and contamination
(decomposition products, water, etc
3.
Gas Insulated Substations (GIS) can be used for
longer times without any periodical inspections.
4.
However, conducting contamination (i.e.
aluminum, copper and silver particles) could seriously reduce the dielectric
strength of gas-insulated system.
5.
SF6 has been identified as a greenhouse gas,
safety regulations are being introduced in order to prevent its release into
atmosphere.
World Scenario
The first GIS’s
were put in operation in 1967 in Switzerland and Germany.
The GIS in
Germany is still in operation, whereas the GIS in Switzerland were recently
decommissioned after 35 years of operation without major fault or gas leak.
An assessment
made on SF6 gas leakage over the lifetime of the first GIS and concluded that
overall leakage rate was about 0.4% per year.
The best result
seemed to be obtainable from the use of mixed technology switchgear either in
the form of GIs modules with and AIs busbar or GIS modules using a GIs busbar
i.e outdoor GIS
