Gas Insulated Switchgear? Gas-Insulated High-Voltage Switchgear (GIS)

Gas Insulated Switchgear?
Gas-Insulated
High-Voltage
Switchgear
(GIS)
What is

It is very much required to establish an electrical substation at
load center. Since, establishing a substation at load center is
quite economical and profitable in many aspects. As it reduces
length of feeders and due to short length feeders, the quality of
voltage regulation improves.
But the main obstruction of establishing a substation at load
center is space. Generally main load center of any place is
situated at very congested place where, sufficient land for
establishing conventional electrical substation is very hardly
available.

This problem can be solved by using Gas Insulated Switchgear
Technology. In this Type of Switchgear, all the necessary
components of switchgear can be assembled in very limited space.
GIS is a kind of Metal enclosed Switchgear.
That means, all the equipments of the Electrical Switchgear are
enclosed by gas tight metal enclosure and SF6 Gas is used as
insulation between live parts of the equipments and earthed metal
enclosure. This type of switchgear, means, and gas insulated
switchgear is available from 12 KV systems to 800 KV system.
For establishing electrical substation in very limited place this type
of SF6 insulated electrical switchgear plays the major role.

Gas insulated components of substation are generally,
Electrical bus bars:
 Electrical Isolators or Disconnectors.
 Circuit Breakers.
 Current Transformers.
 Voltage Transformers.
 Earth Switches.
 Surge Arrestors or Lightning Arresters.

The substation, assembled by gas insulated switchgear, is popularly
known as gas insulated metal enclosed substation (GIMES). GIMES
technology is not very recent invention, it is successfully running
for over thirty years.
In gas insulated medium voltage switchgear, vacuum technology is
used as interrupting purpose and SF6 gas is used as insulation
material. Although for both interruption and insulation, SF6 gas is
used in many medium voltage GIS system. But for such equipments
rated SF6 gas pressures are different for interruption and insulation.
SF6 gas pressure for insulating purpose is generally kept below 2.5
bar whereas SF6 gas pressure for interrupting purpose is ranged
from 5 bar to 7 bar.

As vacuum technology is not available for high voltage, so for GIS or
gas insulated switchgear system above 72.5 KV, only SF6 is used
both for interruption medium and insulation.
There are different types of gas insulated metal enclosed
switchgears available depending upon their constructional feature.
Isolated Phase GIS
In this configuration, each phase of the bay is assembled
separately. That is, for each phase, one pole of circuit breaker, a
single pole of electrical isolator, one phase assembly of current
transformer are assembled together. This type of GIS requires
larger bay width as compared to other gas insulated switchgear
system.

Integrated 3 Phase GIS
In this configuration all three phase of circuit breaker, 3 phases of
disconnectors and three phase current transformer are encapsulated in
an individual metal enclosure. The arrangement forms a three phase
module for the element. The size of this type of module is one third of
the isolated phase GIS.
Hybrid GIS System
It is a suitable combination of isolated phase and three phase common
elements. Here three phase common bus bar system simplifies the
connection from the bus bar. The isolated phase equipment prevents
phase to phase faults. This is an optimum design considering, both facts
in mind, i.e. space requirement and maintenance facility.

Compact GIS
In this GIS or gas insulated switchgear system than one functional
element are encapsulate in a single metal enclosure. For example, in
some design, a three phase circuit breaker, current transformer, earth
switches, even other feeder elements are covered together in a single
metal capsule.
Highly Integrated System
This design was introduced in the year of 2000, where, total substation
equipments are encapsulated together in single enclosure housing.
This single unit gas insulated substation has gained user appreciation as
it is a complete solution for an outdoor substation, in a single unit. As
such, only equipment (HIS) is substitute of a total outdoor switch yard.

◼ Introduction to GIS
◼ Advantages Of SF6-Insulated Installations
◼ Switchgear Characteristic
◼ Surge Arresters
◼ Disconnectors
◼ Earthing Switches
◼ Circuit Breakers
◼ Instrument Transformers for GIS Substations
◼ Tests for GIS

Introduction
◼ At normal atmospheric conditions, the insulation distances
determine the main sizes of the classic distribution substation.
◼ For a long time, the development in substation construction
concentrated simply in combining existing devices to obtain the
arrangement more adequate for exploitation and supply security.
◼ On the other hand, the increasing needs to convey electric energy,
at higher voltages, towards the regions with grand population
density and the industrial centres, causes great difficulties due to
the size of the involved installations.
◼ Official prescriptions and the town-planning requirements
complicate the construction.

Introduction
 Due to the troubles caused by pollution in insulators, the tendency is to build
indoors installations.
◼ However, the cost increasing in construction leads to reduce, as much as
possible, the dimensions of the installation.
◼ Hence, to solve this problem, smaller installations are needed, which
should also comply the following requirements:
◼ They shall be insensible to climatic influences.
◼ They can be raised outdoors, inside a building or underground.
◼ They require reduced maintenance.
◼ They are silent.
◼ They should not generate radioelectric disturbances.
◼ They shall not imply danger for the nearby populated zones.

Introduction
As a solution for this problem, metal-enclosed installations with
SF6 insulation are used.

Introduction
 In this system, the parts of the main current circuit (such as the circuit breaker, the
disconnector, the buses, etc.) are mechanically integrated in a set that can be called
coupling group.
◼ The compact manufacturing and the highly developed wiring technique allow the free
determination of the site, and ensure the independence regarding climatic conditions.
◼ Some possible applications of this very high voltage metal-enclosed stations:
◼ Main distribution stations inside cities.
◼ Main distribution stations for important customers.
◼ Main distribution stations in zones with pollution, salt, or risk of explosion.
 Maindistribution stations with special characteristics(underground
stations, shelters of reinforced concrete, etc.).
◼Classic installation expansion, in case of reduced space.
◼Mobile transformation stations.

Introduction
 The gas-insulated substations utilize the same switchgear of conventional substations,
but with design and characteristics slightly different.
◼ The whole station is integrated inside a grounded aluminium enclosure filled with
SF6, which ensures the insulation to ground.
◼ Diagram of a 220 kV gas-insulated substation.

Introduction
 Fig. presents some basic layouts of the assorted types of gas- insulated
substations, which will vary according to the requirements of each
installation

◼ Introduction to GIS
◼ Advantages Of SF6 - Insulated Installations
◼ Switchgear Characteristic
◼ Surge Arresters
◼ Disconnectors
◼ Earthing Switches
◼ Circuit Breakers
◼ Instrument Transformers for GIS Substations
◼ Tests

Advantages of SF6-Insulated Installations
 Reduced required space
◼ Outlook compatible with surroundings without altering architectural or
natural beauty.
◼ Reduced erection and assembly times.
◼ The installations are dielectrically and totally tested in-site (unlike
conventional substations).
◼ Reduced maintenance and consequently, lower costs.
◼ From 30 kV to 500 kV they might result cheaper than conventional units.
◼ Up to 170 kV, tripolar design is used (three phases in the same casing).
For upper voltages, each phase is separately insulated, enclosed and
compartmentalized.

Surge Arresters
 When the connection with the network is performed through an overhead
line, the surge arrester is located at the entrance of the gas-insulated
substation.
◼ The surge arrester can be conventional or SF6-insulated.
◼ In this last case, if distances in the substation are higher than the
protection distance of the surge arrester, it is convenient to install,
besides, a surge arrester inside the metal-enclosed substation.

 When the substation is connected to the network through a
high voltage cable, it is advisable to integrate the surge arrester
inside the substation casing.
◼ This advice becomes mandatory if the conductor is longer
than 50 m, or if the substation occupies a very extended surface.
◼ The surge arresters are also encapsulated and their discharge
voltage is lower than in conventional arresters, due to the lack of
pollution and to the direct coupling to the breaking device (not
necessarily connected to the line).
Surge Arresters

Surge Arresters Metallic Casing
 The cylindrical metallic casing is comprised of concentric sheets of
aluminium, welded as a tube.
◼ The clamps are also aluminium pieces.
◼ The rated values of filling pressure (Pats) for the SF6 are 420 540 kPa
at 20 º C.
◼ Ifthe surge arrester needs accessories,such as gas valves, densimeter
ormanometer, discharge counter or breaking disks (pressure lever), they will
be mounted in its top.
◼ As mentioned before, up to 170 kV the three phases are located in the
same casing. For upper voltages, each phase is mounted in individual
compartments.

Surge Arresters Metal-Oxide Disks
 The surge arresters insulated in SF6,
alike conventional arresters, are
constituted by columns with several piled
up metal-oxide disks of variable
resistance.
◼ These columns are concentrically
mounted in the metallic casing.

 The flat contact surfaces are manufactured with a
conductive
metallic layer and the cylindrical hermetic sides are elaborated
with a layerof passivated crystal (neutral metal that prevents
corrosion).
◼This design allows that the disks can be in direct contact with
the SF6, which increases the capability of heat evacuation
during, for example, repetitive strong discharges.
◼ To avoid the excessive size of surge arresters for voltages
upper than 170 kV, the disks are piled up in three columns,
forming a triangle, electrically connected in series.
◼ Each column consists of fibreglass axles and springs that
compress the disks.

 A device that evenly distributes the voltage among all disks is located
in the top of the columns.
◼Fig.compares arresters with and without such device.

Disconnectors
 Slide-in disconnectors are used in gas-insulated
substations, capable to break the capacitive currents that
take place during the coupling manoeuvres in the
installation, and to break commutation currents caused by
changes in the buses configuration.
◼ To optimise the operation of the disconnectors in different
points of the distribution station, the active parts are
encased separately, reducing to minimum the number of
flanged joints.

Disconnectors
 The disconnector consist of the main contact (6) and the casing of the
sliding contact (9), in which interior is located the coupling of the sliding
contact (12).
◼ The conductor is connected to both contacts by sheet contacts.
◼ The sliding contact (12)
is tripolar-operated by the
drive (10), which comprises
lever, pitman and insulating
rotary bar (8) with motorised
drive.

Disconnectors
 Each disconnector can include one or two earthing switches (4).
◼ The receiving earthing contacts (5) are mounted in the casings of the
main and sliding contacts (6, 9).
◼ The gas compartment of the disconnector, usually jointed to other parts of
the installation, is enclosed in the casing (3) and surveyed by means of
densimeters (7).
◼ The discharging plate
(1) protects the casing against
excessive overpressure, and the
absorber (2) keeps the gas dry.

Disconnectors
 Each disconnector has a spyhole to observe the position of the contacts
and allows checking their condition.
◼ The disconnectors shall be opened solely for inspection works.
◼ For maintenance or expansion of the installation, the disconnectors can
be mechanically interlocked in the desired ended position, and locked by
means of a padlock or any other
locking device.
◼ The interlocking with circuit
breakers, other disconnectors or earthing
switches is electric.

1. Casing
2. Insulator
3. Fixed contact
4. Mobile contact
5. Mobile contact casing
6. Fixed contact
7. Insulating rotary bar
8. Drive
9. Earthing switch (optional)
1. Fixed contact 2. Fixed contact casing
3. Mobile contact 4. Insulating rotary bar
5. Support of the contact 6. Drive
7. Insulator 8. Casing
Disconnectors

Earthing Switches
• ◼ The maintenance earthing switches endure short-circuit currents in the closed position.
• ◼ They comprise the casing of the mechanism with sliding contact incorporated and contact
bar by lever and pitman.
• ◼ The switching is either unipolar by hand or
• tripolar by motor.
◼ The earthing switches can be adapted to diverse
components, and according to the layout and the
buyer specifications, can be mounted in any point of
the installation, as maintenance earthing switches or
as fast- closing earthing switches.
◼ Locking bolts provided with padlocks or similar
devices ensure the locking in the desired
position.

Earthing Switches
 The fast-closing earthing switches are used to ground parts of the
installation under supply normal conditions.
◼Their tripolar switching mechanism is capable to make short-circuit
currents.
◼ During the closing manoeuvre, the closing spring (10) is tightened by the
linkage (6) and the manoeuvring lever (5), through a motorised drive.

Earthing Switches
 Once tightened the spring, the trigger (11) releases the manoeuvring
lever (9), and the manoeuvring bar (2) closes rapidly by means of the
coupling bar (8), the axle and the pitman.
◼ The receiving contact (1) is a standardised piece of these devices, as
well as in the maintenance earthing switches in the corresponding
device.

Circuit Breakers
 Likewise in conventional installations, the automatic circuit breakers
ensure the break of short-circuit currents.
◼ The gas in the breaking chamber is injected by means of a piston
mechanically coupled to the mobile contact, which compress the gas at a
pressure two or three times higher than the supply pressure, in order to
generate the blowing, enlargement and cooling of the arc, and its
extinction at the zero crossing of the current wave.
◼ They can be arranged horizontally or vertically to minimize the size of
the substation.
◼ The number of chambers depends on the rated voltage and the
breaking capacity of the circuit breaker.
◼ The drive can be mechanical or hydromechanical.

Circuit Breakers
 Fig.shows a circuit breaker in horizontalposition for 245
kV, with mechanical drive.
◼ In the pole enclosure (2), the truncated-cone insulator (1) supports the
main fixed contact (5), which is jointed to the expansion casing (3).
◼ The other truncated-cone insulator (1) carries, in its vertical axis, a
clamp contact (8) that links the breaking device to the fixed part of the
installation.

Circuit Breakers
 This allows, with inspection and revision purposes, to unplug and extract
the active part fixed to the lid by means of two cylindrical insulators (9).
◼ The energy required to generate the extinction flux in the blowing cylinder
(7) and nozzle (6) is conveyed by the gear (14) and the insulating pitman of
traction (11).
◼ The densimeter (4) surveys the gas compartment of the pole, protected
against excessive overpressure by a discharging plate (12).

Circuit Breakers
 The following figure shows a
circuit breaker in vertical position
for
245 kV with
hydromechanical drive.
1. Hydromechanical drive,
2. SF6, 3. Breaking chamber
4. Casing, 5. Input feeder connection,
6. Output feeder connection

Circuit Breakers ; Hydromechanical Drive
Highpressure
Low pressure
Circuit Breakers; Breaking Chamber

Instrument Transformers for GIS
 The measurement and protection transformers for SF6-insulated
substations are different from conventional transformers in their especial
constructive characteristics.
◼Fig. shows the shape and location of voltage and current transformers in an
SF6-insulated substation.

Current Transformers
 The one-phase current transformers are bushing type, with ring-
shaped core and toroidal secondary winding.
◼ The cores are externally stick on the metallic enclosure, outside
the SF6 container, separated from the high voltage region by a
cylindrical shield.
1. Insulator
2. Terminal box
3. Support of co 4.

Current Transformers
 The secondary winding is located above the core and connected to the terminal
box. The commutation of the transformation ratio is possible through the
secondary winding.
◼ The stresses caused by the inner overpressure of the gas and for the current
circulating through the casing are conveyed by the traction bars.
◼ The maximum number of cores that can be placed inside the casing depends on
the transformation ratio and on the characteristics of the cores.
1. Insulator
2. Terminal box
3. Support of co 4.

Voltage Transformers
 The voltage transformer is one-
phase (phase-to-ground connection)
and can be inductive or capacitive.
◼ In the Inductive voltage
transformer, the
Active parts are enclosed in a
casing of melted aluminium.
◼ The stratified core supports the
primary and secondary windings.
1. Terminal box
2. Primary winding
3. Secondary winding
4. Core
5. High voltage terminal

 The insulation among the layers
of the primary winding is
elaborated with plastic sheets, and the
insulation between the primary
winding covered by a shielding
electrode, and the external casing is
SF6.
◼ The voltage transformer is
accommodated in an independent gas
compartment, separated form the others
by a conical fastening insulator.

 The high voltage connection is performed
through an interconnection bolt.
◼ The opposite extreme of the primary
winding, connected to ground, as well as
the extremes of the secondary windings,
are carried in a gastight fashion out of the
transformer casing, and jointed to the
terminals for the external connections of
the connection box.

 In the capacitive voltage transformer, the core and the windings
are replaced by a capacitive divider, created between the
metallic casing and the conductor.
◼ The capacitive divider is coupled to an operational amplifier
that provides the signalling to the protection and measurement
devices.

Tests
 The type tests performed in the gas-insulated switchgear
(disconnectors, circuit breakers, surge arresters and protection and
measurement transformers) follow the same standards and guidelines
than conventional switchgear.
◼ However, additional tests shall be carried out, based on the standard
IEC 62271-203 “Gas-insulated metal-enclosed switchgear for rated
voltages above 52 kV”, such as
◼ the gas tightness test and
◼ the test of internal failure arc.
◼ Besides, every manufacturer has its own testing protocol no
prescribed in the standards

Gas Tightness Test
 This test is performed to verify that the metal-enclosed substation complies
with the permitted value of gas losses, which shall be lower than 1%.
◼ The test is divided in several steps:
◼ Individually, the porosity of the aluminium casing is verified, filling it
with helium.
◼ With the substation totally assembled, the air is extracted until a pressure
inferior to 100 Pa is reached. Then, the possible pressure rises are
observed.
◼ Finally, the substation is filled with SF6, in the nominal conditions of the
installation, and tests are performed in order to verify the pressure
specifications of every gasket and welding.

Test of Internal Failure ARC
 The test consist in verifying that the metal-enclosed installation complies
the demands of endurance to arc, even in the unlikely cases of a failure
with the highest short-circuit current possible.
◼ Under these conditions, the following requirements shall be satisfied:
◼ No metal-enclosed part shall explode.
◼ The damages shall be exclusively restricted to the gas compartment.
◼ The arc should not puncture the shielding during the first step of
protection.

Gas Insulated Switchgear? Gas-Insulated High-Voltage Switchgear (GIS)

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