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GAS INSULATED SUBSTATION
MARUDHAR ENGINEERING COLLEGE Page 1
INDEX PAGE NO.
I.INTRODUCTION 2
2 NEEDS OF GIS 5
3.SF6 CIRCUIT BREAKER 8
4.ELECTRICAL CONNECTION DIAGRAM 15
5.CURRENT TRANSFORMER 16
6.GAS INSULATED TRANSFORMER 19
7.ADVANTAGES OF GAS INSULATED TRANSFORMER 20
8.INTER-CONNECTION TRANSFORMER 21
9.DISCONNECTOR AND EARTHING SWITCHES 22
10.INTERNAL STRUCTURE OF GAS INSULATED TRANSFORMER 31
11.V-I SENSOR CURRENT &VOLTAGE MEASUREMENT 33
12.SURGE ARRESTER CVT –WAVE TRAP 34
13.ADVANTAGES OF GIS 35
14.DISADVANTAGES OF GIS 37
15.CONCLUSION 39
16.REFERENCE 40
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GAS INSULATED SUBSTATION
INTRODUCTION:
Gas Insulated Substations are high voltage Substations that are compact, requiring little
maintenance when compared to air-insulated conventional Substations. Compressed Gas
Insulated Substations (CGIS) consist basically a conductor supported on insulators inside an
enclosure which is filled with sulfur hexafluoride gas (SF6). The compactness is with the use
of SF6 gas, which has high dielectric strength. The voltage withstand capability of SF6
Busduct is strongly dependent on field perturbations, such as those caused by conductor
surface imperfections and by conducting particle contaminants. The contaminants can be
produced by abrasion between components during assembly or operations.
Electrical insulation performance of compressed gas insulated Substation is adversely
affected by metallic particle contaminants. Free conducting particles, depending upon their
shape, size and location, may lead to serious deterioration of the dielectric strength of the
system and also one of the major factors causing breakdown of the system and leading to
power disruption. These particles can either be free to move in the Gas Insulated Busduct
(GIB) or they may be stuck either to an energized electrode or to an enclosure surface. The
presence of contamination can therefore be a problem with gas insulated substations
operating at high fields. 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
a protrusion on the surface of the ii
electrode. Consequently, voltage required for breakdown of the GIS will be significantly
decreased. Several methods have been used to reduce the effect of conducting particles,
including electrostatic trapping, use of adhesive coatings, and discharging of conducting
particles through radiation. Dielectric coating of a metallic electrode surface affects the
particle charge mechanism.
The charge acquired by a particle, the equation of motion, the bounce and the drag are
discussed by several authors. The present work makes use of the equation proposed by H.
Anis, K.D.Srivastava and M.M.Morcos, it also includes the concept of random motion along
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axial direction. The random motion is due to the cross sectional irregularities of the metallic
particles.
Present work analyses the movement pattern of metallic particles in Gas Insulated Substation
(GIS) or Gas Insulated Busduct (GIB). In order to determine the particle trajectories in a GIB,
an inner diameter of 55 mm and outer enclosure diameter of 152 mm were considered.
Aluminum, Copper and Silver particles of 0.2 mm/12 mm (diameter/length) were considered
to be present on the enclosure surface. The motion of the metallic particle was simulated
using the charge acquired by the particle, the gravitational force on the particle, field intensity
at the particle location, drag force, gas pressure, restitution co-efficient and the Reynold‟s
number. The distance traveled by the particle, calculated using the appropriate equations, is
found to be in good agreement with the published work for a given set of parameters. The
results are also presented for other set of parameters.
In order to determine the random behavior of moving particles, the calculation of movement
in axial and radial directions was carried out by Monte-Carlo technique. Typically for
Aluminum particle for a given Busduct voltage of 100 kV RMS, the movement of the particle
(0.25 mm/12 mm) for 1.5 s was computed to be 30.839 mm in radial and 841.12 mm in axial
directions. Similar calculations are also extended for other types of voltages. Typical results
for aluminum, copper and silver particles are presented in this thesis.
The effect of various parameters like radii and length of particles, co-efficient of restitution,
pressure in the Busduct and the applied voltage has been examined and presented. Different
metallic contaminants viz., Al, Cu and Ag have been considered for the above study.
Typically a GIB of 55mm/152mm (inner conductor diameter is 55mm and outer enclosure
diameter is 152mm) has been considered for a 132 kV system.
The thesis presents the movement pattern of metallic particles at different operating voltages
in a Gas insulated Busduct (GIB) which has been simulated with and without enclosure
coating. The purpose of dielectric coating is to improve the insulation performance. Free
conducting particles situated inside the GIS enclosure decrease high local fields caused by
conductor roughness. The coating reduces the charge on the particle colliding with the coated
enclosure, which in turn reduces the risk of breakdown due to increase of the lift-off field of
particles. The movement of a particle has been carried out not only by its electric field effect
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on the particle but also considers electromagnetic field and image charge effects. The
simulated results are presented at Power frequency, lightning/switching impulse
superimposed on power frequency, triangular, square and asymmetric voltages. Particle
trajectories obtained for various voltages of aluminum, copper and silver particles are
presented and duly discussed.
Gas Insulated Substations (GIS) is a compact, multicomponent assembly enclosed in a
ground metallic housing which the primary insulating medium is compressed sulphur
hexafluoride (SF6) gas. GIS generally consists components Of
1. Circuit Breakers
2. Operating mechanism of circuit breaker
3. Current transformers
4. Disconnector
5. Maintenance Earthing switches
6. Fast acting Earthing switches
7. Voltage transformers
8. SF6 Bushing
9. Gas supply and gas monitoring equipment
10. Bus Bar
11. Voltage Transformer
12. Gas supply and Monitoring eqipment
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Why we need GIS:
Gas Insulated Substations are used where there is space for providing the substation is
expensive in large cities and towns. In normal substation the clearances between the phase
to phase and phase to ground is very large. Due to this, large space is required for the
normal or Air Insulated Substation (AIS). But the dielectric strength of SF6 gas is higher
compared to the air, the clearances required for phase to phase and phase to ground for all
equipments are quite lower. Hence, the overall size of each equipment and the complete
substation is reduced to about 10% of the conventional air insulated substation.
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.
A metallic particle stuck on an insulator surface in a GIS will also cause a significant
reduction of the breakdown voltage.
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...
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.
a) 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.
b) 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.
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Locations where Gas Insulated Substation is preferred:-
i. Large cities and towns
ii. Under ground stations
iii. Highly polluted and saline environment Indoor GIS occupies very little
space
iv. Substations and power stations located Off shore Mountains and valley
regions
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Explaination:
The SF6 Gas Insulated Substation (GIS) contains the same compartments as in the
conventional outdoor substations. All the live parts are enclosed in metal housings filled with
SF6 gas. The live parts are supported on cast resin insulators. Some of the insulators are
designed as barriers between neighboring modules such that the gas does not pass through
them. The entire installation is sub divided into compartments which are gas tight with
respect to each other. Thereby the gas monitoring system of each compartment can be
independent and simpler.
The enclosures are of non magnetic materials such as aluminum or stainless steel and are
earthed. The gas tightness is provided with static „O‟ seals placed between the machined
flanges. The „O‟- rings are placed in the grooves such that after assembly, the „O‟-rings are
get squeezed by about 20%. Quality of the materials, dimension of grooves and „O‟-seals are
important to ensure gas tight performance of Gas Insulated Substation.
Gas Insulated Substation has gas monitoring system. Gas inside each compartment should
have a pressure of about 3kg/cm2.The gas density in each compartment is monitored. If the
pressure drops slightly, the gas is automatically trapped up. With further gas leakage, the low
pressure alarm is sounded or automatic tripping or lock-out occurs.
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SF6 Circuit Breaker:
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Sulfur Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power
applications. It has been used extensively in high voltage circuit breakers and other
switchgears employed by the power industry. Applications for SF6 include gas insulated
transmission lines and'gas insulated power distributions. The combined electrical, physical,
chemical and thermal properties offer many advantages when used in power switchgears.
Some of the outstanding properties of SF6 making it desirable to use in power applications
are:
V High dielectric strength
V Unique arc-quenching ability
V Excellent thermal stability
V Good thermal conductivity
General Information:
Elimsan SF6 circuit breaker is equipped with separated poles each having its own gas. In all
types of the circuit breakers, gas pressure is 2 bars (absolute 3 bars). Even if the pressure
drops to I bar, there will not be any change in the breaking properties of the circuit breaker
due to the superior features of SF6 and Elimsan's high safety factor for the poles. During
arcing, the circuit breaker maintains a relatively low pressure (max 5-6 bars) inside the
chamber and there will be no danger of explosion and spilling of the gas around. Any leakage
from the chamber will not create a problem since SF6 can undergo considerable
decomposition, in which some of toxic products may stay inside the chamber in the form of
white dust. If the poles are dismantled for maintenance, it needs special attention during
removal of the parts of the pole. This type of maintenance should be carried out only by the
experts of the manufacturer. (According to ELIMSAN Arcing Products and Safety
Instruction for Working on SF6 Circuit Breakers)
Operation of Circuit Breaker:
In general, the circuit breakers consist of two main parts, the poles and the mechanism. The
poles consist of contact and arc-extinguishing devices. The mechanism is the part to open or
close the contacts in the poles at the same time instantaneously (with max. 5 milisec.
Tolerance). The closing and opening procedures are performed through springs which are
charged by a servomotor and a driving lever. In the system, the closing springs are first
charged. If "close" button is pressed the opening springs get charged while the contacts get
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closed. Thus, circuit breaker will be ready for opening. The mechanical operating cycle of the
circuit breaker is (OPEN-3 Min CLOSE/OPEN-3 Min- CLOSE/OPEN) or (OPEN-0.3 sec-
CLOSE/OPEN-3 Min CLOSE/OPEN). The second cycle is valid when the circuit breaker is
used with re-closing relay. In that case, after the closing operation, the closing springs are
charged by the driving lever or by driving motor (if equipped). Thus, the circuit breaker will
be ready for opening and re-closing.
Mechanical Life and Maintenance of The Mechanism:
Elimsan breaker mechanism can perform 10.000 opening-closing operations without
changing any component. The mechanical life of the circuit breaker is minimum lO'.OOO
operations. However, it needs a periodical maintenance depending on its environment. In
ideal working conditions, lubrication once a year or after every 1000 operations is sufficient.
In dusty and damp environment, the mechanism should be lubricated once every 3 - 6 months
or after every 250 - 500 operations.
Thin machine oil and grease with molybdenum must be used for lubricating. Owing to
mechanism's capability of operating between -5°C and + 40 °C, it does not require a heater.
Auxiliary Switch:
The auxiliary switch mounted on the circuit breaker has 12 contacts. One of them is for
antipumping circuit, four of them are allocated for opening and closing coils. The remaining
7 contacts are spare. Three of them are normally opened and four are normally closed. When
it is necessary, the number of the contacts can be increased.
Rapid Automatic Reclosing:
The circuit breaker which opens due to a short circuit failure, can be re-closed automatically
after a pre selected time by arc closing relay, assuming the fault is temporary. Thus, we avoid
long time power loss in case of temporary short circuits. But, if the fault lasts after re-closure,
the protection relay will trip to open the circuit breaker again.
What to Specify on The Order:
1- Rated voltage of the circuit breaker
2- Rated current of the circuit breaker
3- Rated short circuit breaking current
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4- Voltages of opening and closing coils
5- Motor supply voltage (if equipped)
Closing and Opening Operation Of the Circuit Breaker:
When manual or motor-drive is used, the circuit breaker will be ready to close. The closure
can be actuated pressing the closing button located on the circuit breaker. It is recommended
to close it using remote control system for secure operations. The opening can be performed
either by opening button or remote controlled opening coil. In case of a fault, the relay signal
actuates the opening coil and circuit breaker opens. (This is mechanically a primary
protection system). In addition, there is an anti-pumping relay for preventing the re-closing
and opening of the circuit breaker more than one cycle (O - C - O) and for preventing
possible troubles created by remote closing button.
Commissioning:
The outer surfaces of epoxy insulating tubes of the poles are to be wiped out with a clean and
dry cloth. The wiring and connections of the auxiliary circuit are to be carefully examined.
DC voltage should be checked to see whether it is suitable for coil and motor or not (if
equipped). The opening-closing coils are to be operated 15-20 times and the accuracy of the
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relay circuit is to be checked before energizing the circuit breaker. The circuit breaker is to be
mounted with two MI2 bolts through its anchoring shoes. It should not move during
operation. No excessive load should be exerted to the poles and if possible flexible cables
should be used. The incoming and outgoing contacts must have clean surfaces and their
contact resistance should be as low as possible. When connecting the circuit
breaker to protection system and auxiliary supply, the cable cross sections should be
according to the table given. The circuit breaker must be grounded through at least 16 mm
steel tape (by cable shoe). After all, the following procedure must be performed:
1. Open the isolator of circuit breaker,
2. Prepare the circuit breaker for closing operation by driving mechanism,
3. Close the isolator of circuit breaker firmly,
4. Send the closing signal to the circuit breaker,
The Maintenance Of Circuit Breaker During Operation:
Normally, at least once a year or after every 500 operations, the circuit breaker must be
maintained. During maintenance, the moving parts of the mechanism must be lubricated
carefully. The insulating parts are to be wiped out by a clean and dry cloth. When
maintaining, the circuit breaker should be open and high voltage sides must be grounded.
Auxiliary power supply should also be disconnected. On saline areas near seaside, the
insulating parts of the circuit breaker must be carefully cleaned, at least once every two
months. If not, the microscopic salt particles drawn by wind from the sea will create
conductive layers on the insulating surfaces and may cause surface flashover. Before
maintenance, first circuit breaker, then isolator should be opened and grounded carefully. The
maintenance of circuit breaker must be done after checking the open position of isolator
contacts by eye.
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MAIN DIMENSIONS (IN mm):
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ELECTRICAL CONNECTION DIAGRAM:
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Current tansformer:
Current transformers are used in electrical grids for measurement and protective applications
to provide signals to equipment such as meters and protective relays by stepping down the
current of that system to measurable values. Their role in electrical systems is of primary
importance because the data sent by current transformers represent the heartbeat of the entire
system.
RHM International‟s proprietary dry type Current Transformers are unique as they provide a
rugged, reliable option for high voltage metering and protection operations up to 550 kV.
Our high quality Current Transformers are based on a unique U-shaped bushing design for
the primary winding. The bushing is a very fine capacitance graded insulator. In a new