gas insulated substations

Gas Insulated Substations

(G.I.S) Definition :

A gas insulated substation (GIS) is a substation that uses a superior dielectric gas, sulfur hexafluoride (SF6), high-voltage.

The basic principle of gas-insulated equipment :

Is that the high-voltage current- carrying parts are within a metal enclosure and are held in a concentric configuration by cast epoxy spacer insulators.

The space between the conductor and the enclosure is filled with sulfur hexafluoride gas under moderate pressure.


(G.I.S) Usage :

Gas insulated substation mainly used for a power transmission system or a substation system, of which outgoing bus-bar is shortened to reduce consumption of the outgoing bus-bar.

The equipment can be designed for any bus scheme.


(G.I.S) Components : 1 → Circuit breaker.

2 → Disconnector (Isolator).

3 → Ground switches.

4 → Bus-bar.

5 → Transformer.

8 → Outgoing bus-bar for electr-ically connecting the gas insul-ated substation with bus-bar.

9 → Outgoing portion of the transformer being arranged in accor-dance with extendingdirection of outgoing bus-bar.


(G.I.S) (SF6) gas :

The dielectric medium is the sulfur hexafluoride (SF6) gas,

SF6 has been used as an insulating medium inelectronic devices, power apparatus, and HVDC converter stations.

Its excellent properties make it ideally suited both as an insulating and as an arc-quenching agent.

SF6 gas is colorless, odorless, chemically inert, nontoxic, nonflammable, and non-corrosive.

Its dielectric strength is greatly superior to that of air, and it is close to 100

times as effective as air in quenching an electric arc.


(G.I.S)Safety :

The live parts and supported on at resin insulators. Some of the insulators are designed as barriers between neighboring modules such that the gas does not pass through them.

The GIS has gas-monitoring system. The gas density in eachcompartment is monitored.

If pressure drops slightly, the gas is automatically tapped up with further gas leakage, the low-pressure alarm is sounded or automatic trippingor lock-out occurs.


(G.I.S)GIS advantages and disadvantages :

GIS Disadvantages :

1. Excessive damage in case of internal fault. Long outage periods as the repair of damaged part at site may be difficult.

2. Requirement of cleanliness is very stringent. Dust or moisture can cause internal flashovers.

3. Such substations are generally indoor, so they need a separate building. This is generally not required for conventional outdoor substations.



GIS Disadvantages :

4. Procurement of gas and supply of gas to site is problematic, adequate stock of gas must be maintained.

5. Project needs almost total imports including SF6 Gas. Spares conventional substation is totally indigenous up to 400 kV.



GIS Advantages :

1. Reduced required space: The space occupied by SF6 installation is only about 8% to 10 % of that a conventional outdoor substation. High cost is partly compensated by saving in cost of space. A typical (420/525) kV SF6 GIS requires only 920m2 site area against 30000 m2 for a conventional air insulated substation.

2. Reliable: The complete enclosure of all live parts guards against any impairment of the insulation system.



GIS Advantages :

3. Reduced erection and assembly times: The principle of building block construction reduces the installationtime to a few weeks. Each conventional substation requires several months for installation. In SF6 substations, The time-consuming is highly cost for galvanized steel structures and it is eliminated. Heavy foundations for galvanized steel structures, equipment support structures ...etc are alsoeliminated. The results are economy and reduced project execution time. Modules are factory assembled, tested and dispatched with nominal SF6 gas. Site erection time is reduced to final assembly of modules.

4. The installations are dielectrically and totally tested in-site (unlike conventional substations).



GIS Advantages :

5. Reduced maintenance and consequently, lower costs.

6. The safety is increased: As the enclosures are at earth potential there is no possibility of accidental contact by service personnel to live parts.

7. From 30 kV to 500 kV they might result cheaper than conventional units. High flexibility and application versatility provide unique, and economic overall concepts.



GIS Advantages :

8. 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.

9. Provide a protection against the pollution. The external moisture. Atmospheric Pollution, snow, dust … etc. have little influence on SF6 insulated substation. Due to the troubles caused by pollution in insulators, the tendency is to build indoors installations. However, the cost is increasing in construction.


(G.I.S) Hence, to solve this problem, smaller installations are

needed, which should also comply the following requirements:

1. They shall be insensible to climatic influences.

2. They can be raised outdoors, inside a building or underground.

3. They require reduced maintenance.

4. They are silent.

5. They should not generate radio-electric disturbances.

6. They shall not imply danger for the nearby populated.


(G.I.S)GIS Requirements :

The following requirement are important to satisfy, the requirement of GIS can be summarized as :

1. Conductors which conduct the main circuit current and transfer power these are of copper or aluminum tubes.

2. Conductors need insulation above grounded enclosures, conductors also need phase to phase insulation, in SF6 requirements are met by cast resin insulators and SF6 gas insulation. GIS these insulation

3. Gas filled modules have nonmagnetic enclosures, enclosures are of aluminum alloy or stainless steel, aadjacent modules are joined bymeans of multi-bolts tightened on flanges.


(G.I.S)GIS Requirements :

4. Various circuit components in main circuit are: CB, Isolator, Earthingswitches for conductors, CTs, VTs, cable-ends, Bushing-ends and Bus-Bars, each of these main components has its own gas -filled metal enclosed module.

5. Gas filling, monitoring system.

6. Auxiliary low voltage DC and low voltage AC supply system, control, protection and Monitoring systems.

7. The bus-bars are conducting bars to which various incoming andoutgoing bays are connected. In SF6 GIS the bus-bars are laidlongitudinally in GIS hall. The bays are connected to bus-bars cross- wise, bus-bars are either with a three-phase enclosure or single phase enclosure.

gas insulated substations

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