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Gas insulated Switchgear (GIS):

Gas insulated Switchgear (GIS) General

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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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.

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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.

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

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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. Main distribution stations with special characteristics (underground stations, shelters of reinforced concrete, etc.). Classic installation expansion, in case of reduced space. Mobile transformation stations.

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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.

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

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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Advantages of SF 6 -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.

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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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.

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Surge arresters 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).

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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. If the surge arrester needs accessories, such as gas valves, densimeter or manometer, 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.

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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.

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Surge arresters. Metal-Oxide Disks The flat contact surfaces are manufactured with a conductive metallic layer and the cylindrical sides are elaborated with a hermetic layer of 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.

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Surge arresters. Metal-Oxide 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.

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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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.

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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.

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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.

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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.

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Disconnectors There might be variations in the design, linear or angular, according to the requirements in the installation configuration. Fig. shows different possibilities.

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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Earthing switches 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. 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.

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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.

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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.

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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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.

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Circuit breakers Fig. shows a circuit breaker in horizontal position 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.

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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).

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Circuit breakers The following figure shows a circuit breaker in vertical position for 245 kV with hydromechanical drive.

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Circuit breakers. Breaking chamber

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Circuit breakers. Hydromechanical drive

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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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.

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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.

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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.

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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.

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Voltage transformers 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.

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Voltage transformers 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.

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Voltage transformers 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.

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AGENDA INTRODUCTION ADVANTAGES OF SF 6 -INSULATED INSTALLATIONS SWITCHGEAR CHARACTERISTICS Surge arresters Disconnectors Earthing switches Circuit breakers Instrument transformers for gis substations TESTS

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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

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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.

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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.

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