TOPIC-GAS INSULATED SUBSTATION By : Arupananda Pattanaik 2nd Year,B.TECH(EEE) MODERN INST. OF TECH. AND MGMT. BHUBANESWAR UNDER GUIDENCE OF Er.Nibedita Das(HOD EEE)

Introduction It is a one type of switchgear. The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects. There are 3 types of switchgear.i.e AIS HYBRIT GIS GIS Gas Insulated Substations (GIS) is a compact,multicomponent assembly enclosed in a ground metallic housing in which the primary insulating medium is compressedSulphur hexafluoride (SF6)gas. SF6acts as an insulation between live parts & the earthed metal closure. Gas insulated switchgear is used in industrial areas to fulfil high-energy demands by space saving design with a minimum of cost SF6 switchgear installed in Canada in a 550 kV substation with 100 kA 

Comparison between AIS &GIS Air-insulated substation:- An electric power substation that has the busbars and equipment terminations generally open to air. It utilizes insulation properties of ambient air for insulation to ground. Very fast electromagnetic transients (VFT) in air insulated substations (AIS) Air Insulated Substation (AIS) makes it possible to install High Voltage substations that take-up less space These are more cost-efficient than conventionally designed outdoor infrastructures.

Air Insulated Substations Image of AIS Air Insulated Substations

Air Insulated Substations

245 kV circuit breaker in air insulated substation

Hybrid Gas Insulated Switchgear Small equipment cost. It takes less area. High reliability similar to GIS substation. Easy extension. It is a combination of AIS and GIS.

Hybrid Gas Insulated Switchgear

Hybrid Gas Insulated Switchgear

Hybrid Gas Insulated Switchgear

GIS Gas Insulated Substations (GIS) is a compact,multicomponent assembly enclosed in a ground metallic housing in which the primary insulating medium is compressedSulphur hexafluoride (SF6)gas. SF6acts as an insulation between live parts & the earthed metal closure.  Gas insulated substations are particularly suitable for underground construction beneath public buildings. Gas insulated substation mainly used for a power transmission system or a sub-station system, of which outgoing bus bar is shortened to reduce consumption of the outgoing bus bar

Sulfur Hexafluoride A colorless, odorless, non-toxic, liquified gas. Shipped as a liquid under its own vapor pressure. Sulfur Hexafluoride gas, which is itself inert. It disassociates in the presence of an RF field to form reactive fluorine ions. SF6acts as an insulation between live parts & the earthed metal closure. The dielectric strength of SF6 gas at atmospheric pressure is approximately three times that of air. pace requirement is only 10 to 25 percent of what is required is a conventional substation.

SF6 can be prepared from the elements through exposure of S8 to F2 SF6 can be prepared from the elements through exposure of S8 to F2. This is also the method used by the discoverers Henri Moissan and Paul Lebeau in 1901 It does not react with molten sodium, but reacts exothermically with lithium. Of the 8,000 tons of SF6 produced per year, most (6,000 tons) is used as a gaseous dielectric medium in the electrical industry,  it has a much higher dielectric strength than air or dry nitrogen. This property makes it possible to significantly reduce the size of electrical gear.

Properties Molecular formula SF6 Molar mass 146.06 g/mol, Density 6.164 g/L (gas, 1 bar) 1.329 g/ml (liquid, 25 °C) 2.510 g/cm3 (solid, −50.8 °C) Melting point- 50.7 °C (triple point) Boiling point− 64 °C (209 K) (subl.) decomp. at ca. 500 °C (773 K) Solubility in  waterslightly soluble Solubility in  ethanolsoluble

GEOMETRY OF SULFUR HEXAFLUORIDE (Sulfur-hexafluoride-2D) (Sulfur hexafluoride-3D)

Specification of GIS The weight and size of the GIS equipment do not change appreciably with the voltage class. 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 two voltage 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.

Component of GIS Bus bars. Circuit Breakers. Disconnecting switches. Earthing switches. Current transformer. Voltage transformers. Cable and boxes. Gas supply and gas monitoring equipment.

Circuit Breakers A circuit breaker is an automatically-operated electrical switch designed to protect from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. 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.

Circuit Breakers

Circuit Breakers with handles

Circuit Breakers

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.

Current transformer

Current transformer

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

The primary winding is insulated with SF6 gas & connected to high voltage terminal. 32 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.

Voltage transformers

Disconnecting switches Disconnector switches are safety devices used to open or to close a circuit when there is no current through them. These 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.

Disconnecting switches

Disconnecting switches

Life cycle cost AIS GIS Planning and engineering 100% 80% Real estate 100% 40% Primary equipment 100% 120% Secondary equipment 100% 100% Earthwork, civil work 100% 60% Electrical assembly 100% 70% Maintenance 100% 50% Outage 100% 50% Life cycle costs after 10 years 100% 70%

Advantages 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

Disadvantages Cost of GIS is high. The life of GIS is affected by certain factors such as: conductive particles, partial discharges and contamination (decomposition products, water, etc Gas Insulated Substations (GIS) can be used for longer times without any periodical inspection. However, conducting contamination (i.e. aluminum, copper and silver particles) could seriously reduce the dielectric strength of gas-insulated system. SF6 has been identified as a greenhouse gas, safety regulations are being introduced in order to prevent its release into atmosphere.

Conclusion From the life cost table we conclude that GIS has advantages over AIS. But it contributes minorlly to the global warming potential. Instead of this we are using GIS,as it gives us much more power. Lastly we should try our best to built this as environment friendly.

QUESTION

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