IN-PLANT TRAINING REPORT

Gas insulated substationsubstationA substation is a part of an electrical generation, transmission, and distribution system.

Substations transform voltage from high to low, or the reverse, or perform any of several other important functions such as switching operations etc.

Classification of substationsClassification based on voltage levels: EHV,HV,LV,HVDC

Classification based on configuration: Conventional AIS, Non-conventional GIS Composite substationAir insulated substationAir used as dielectric.Normally used for outdoor substations.Easy to expand provided no issue of space.Simple handling and easy access.

LIMITATIONS OF AIR INSULATED SUBSTATIONS

Large dimensions due to statutory clearances and poor dielectric strength of air. Susceptible to pollutants.Wastage of space.Large planning and execution time.Regular maintenance of substation required.THE NEED FOR GISNon availability of sufficient space.Difficult climatic and seismic conditions at site.Urban site(high rise building).High altitudes.Limitations of AIS.Gas insulated substationsA gas insulated substation (GIS) is a high voltage substation in which the major structures are contained in a sealed environment with sulfur hexafluoride gas as the insulating medium.The total space required for a GIS is 10% of that needed for a conventional substation. Gas insulated substations offer other advantages in addition to the reduced space requirements. It can be used in very harsh environments.Because the substation is enclosed in a building, a GIS is less sensitive to pollution, as well as salt, sand or large amounts of snow. The various models of GIS produced here at alstom are :

F35 - upto 145kV B105 - 170 to 300kV T155 - upto 420kVProperties of sf6Non-toxic, very stable chemical.Insulating properties : 3 times that of air.Colorless & heavier than air.Non inflammable.Superior cooling characteristics.Greater dielectric strength.Good arc quenching property.It is inert , non corrosive and non reactive. COMPONENTS PRESENT:

Circuit breaker.Combined circuit breaker/ disconnector.Combined disconnector/ earthing switch.Make proof earthing switch.High voltage interfaces.Customized instrument transformers.

ADVANTAGES:

It is light weight thanks to the aluminium alloy enclosure.It is portable.It can be used to set up a mobile power station.Simple transport and on site erection.

DISADVATAGES:

Can be used only upto 145kV.

F35 gas insulated substationFEATURES:

It is a three phase gas insulated , high voltage metal enclosed switch gear.

It is light weight and can be used upto 145 kV.

It is designed for both indoor and outdoor installations.

It is very compact and upto 3 bays can be combined in one transport unit.

COMPONENTS PRESENT:Circuit breakerHigh performance operating mechanismDisconnectorEarthing switchesInstrument transformersStandard terminationsInterconnecting components

ADVANTAGES:

Minimum floor space requirements.Long service life expectancy.High operational reliability.Modular design.

DISADVATAGES:

Can be used only upto 300kV.

B105 GAS INSULATED SUBSTATIONFEATURES:

It is a single phase, gas insulated, high voltage, metal enclosed switchgear.

It is used for operation between 170 & 300kV .

Almost unlimited extension and modification possibilities.

The electromagnetic field strength in the switchgear room is harmless.

COMPONENTS PRESENT:Aluminium enclosure.Sf6 sealing.High quality conductor.Safe insulator.Circuit breakerSpring mechanism.Disconnector.High power earthing switch.Busducts.Instrument transformers.Surge arrestors.Interconnecting components.High voltage terminations.

Advantages:

Maximum safety & easy to expand.Low total cost of ownership.Total adaptability.It is smart grid ready.

Disadvantages:

Can be used only upto 420kV

T155 GAS INSULATED SUBSTATIONFEATURES:

Can be used up to 420kV, 5000A.

It makes use of gas insulated lines to raise reliability.

It is suitable for all kinds of applications indoor, outdoor, underground , mobile etc.

Clear and simple layouts.

It has best seismic resistance in its class.

COMPONENTS OF GIScircuit breakerDisconnecting switch or isolatorEarthing switchMake proof earthing switchCurrent transformerVoltage transformerInterconnecting componentsStandard terminationsAluminium enclosureSurge arrestorSf6 sealingGIL(gas insulated lines)Failure & protective measures :

Failure of circuit breaker is usually caused due to improper operating mechanisms.

In order to avoid such failures timers are used .

Local breaker failure protection and remote breaker scheme are also used to avoid such circuit breaker failures.

Local breaker detects weather the circuit breaker trips within 0.5 seconds else sends a pulse to trip all the circuit breakers in the system.

Types :

According to the operating mechanism of circuit breaker they can be divided as: 1) Spring operated circuit breaker 2) Pneumatic circuit breaker 3) Hydraulic circuit breaker

According to their arc quenching (rapid cooling) media the circuit breaker can be divided as: 1) Air circuit breaker 2) SF6 circuit breaker

Circuit breakerA circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.

A circuit breaker is usually operated under load condition.

It is operated in GIS using a spring mechanism.Disconnecting switch or isolatorA disconnector, disconnect switch or isolator switch is used to ensure that an electrical circuit is completely de - energized for service or maintenance.

It is always operated in open condition.

Usually disconnector are connected in both load side and supply side.

Disconnectors are manually operated unlike that of circuit breakers.

Usually when the disconnector switch is opened the earthing switch is closed to remove trapped\residual charges and vice verse.

Earthing switchAn earthing switch is a mechanical switching device used to remove the trapped charges present in the system by grounding of conducting parts of during maintenance.

They can be installed anywhere according to users protection concept.

They are generally slow acting devices that are operated during open state of the GIS equipment.

Make proof earthing switches are used for safe earthing of primary circuits including closing on short circuit & to switch induced current.

The moving contacts are activated by a charged spring for opening and closing.

It has reduced maintenance.

Make proof Earthing switchPrinciple:

The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary winding circuit.

An essential objective of current transformer design is to ensure the primary and secondary circuits are efficiently coupled, so the secondary current is linearly proportional to the primary current. current transformerA current transformer (CT) is used for measurement of alternating electric currents.

When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.

Principle:

A transformer basically is very simple static (or stationary) electro-magnetic passive electrical device that works on the principle of Faradays law of induction by converting electrical energy from one value to another.Voltage transformerA voltage transformer is used for measurement of the voltage.

When voltage in a circuit is too high to apply directly to measuring instruments, a voltage transformer produces a reduced voltage accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.

INTERCONNECTING COMPONENTSThe various components of the GIS are assembled together by means of standardized bolted flanges which are referred to as interconnecting components.

Interconnecting components are comprised of aluminium enclosures with insulated supported conductors.

Telescopic couplings are used to connect adjacent switchgear sections.

Expansion bellows are provided to compensate for thermal expansion movements and tolerances.

Cables, overhead lines and transformers can all be connected to their respective GIS interface components.

These include sf6 to air bushings, cable terminations and direct transformer connections.

Standard terminations aluminium enclosureThe enclosures used for making the substation bays are made of aluminium alloys, cast or welded due to the following reasons:

Low electrical resistance and reduces resistive losses.

Non magnetic behavior so, no magnetic losses.

High resistance to corrosion .

Light weight.

Function of surge arrestor :

To protect a unit of equipment from transients occurring on an attached conductor, a surge arrester is connected to the conductor just before it enters the equipment.

The surge arrester is also connected to ground and functions by routing energy from an over-voltage transient to ground if one occurs, while isolating the conductor from ground at normal operating voltages.

This is usually achieved through use of a varistor, which has substantially different resistances at different voltages.Surge arrestorA surge arrester is a device to protect electrical equipment from over-voltage transients caused by external (lightning) or internal (switching) events.

Surge arrestors are employed to protect GIS units, the connecting cables, transformers etc against all types of over voltages .

Sf6 sealingGas insulated lines(GIL)

It provides three concentric sealings.

It reduces the gas leakage rate of substation by less than 0.1% per year.

It also maintains constant pressure inside the GIS and prevents the entry of moisture into the GIS.

GIL are connections of very long runs in shafts, underground galleries or at ground level to connect the GIS to overhead lines with the minimum number of interfaces.

Main features and characteristics

Open air (on supports, in trenches ) Tunnels (includes with slope or vertical shafts)Buried (to reduce visual impact)

ADVANTAGES OF GISLow space requirement.Increased safety and reliability.Suitable for both indoor & outdoor applications.Possesses modularity and adaptability.Suitable for harsh environment conditions.Long service life. The complete substation is reduced to about 10% of the conventional air insulated substation.

Locations where Gas Insulated Substation is preferred:

Large cities and townsUnder ground stationsHighly polluted and saline environment Indoor GIS occupies very little spaceSubstations and power stations located Off shoreMountains and valley regions

Demerits Of Gas Insulated Substation

Cost is higher compared to Air Insulated Substation or conventional substation.

Procurement of SF6 gas and supply of gas to the site is problematic.

Normally this type of substations are indoor type and requires separate building.

Maintaining Cleanliness is very important. Dust or moisture inside the compartment causes the flash overs.

When fault occurs internally, the outage period will be very long. The damage effect will also be severe.

Gas leakage & dielectric failure occurs.

FUTURE TRENDS IN GISPD detection Partial discharge (PD) is an electrical discharge that does not completely bridge the gap between two electrodes. PD happens all the time in power systems, but usually occurs in components that can withstand it (such as switchgear). You may be aware of PD activity near you (that buzz you hear from medium or high voltage substation equipment or overhead power lines). Modern cable and cable accessories cannot withstand partial discharge. In these cases, PD can slowly erode the insulating material until failure.

Green Gas for Grid In August 2014, Alstom became the first company in the world to offer the electrical industry an alternative to SF6 for high voltage applications.

Alstom g3 green gas for grid is an environmentally friendly, non-toxic, non-flammable, SF6-free gas mixture that can replace SF6 for high voltage air-insulated switchgear (AIS) or gas-insulated switchgear (GIS) applications. This gas mixture was jointly developed with 3MTM, a leader in environmentally friendly solutions.

Alstom g3 has an extremely low global warming potential (GWP); in fact, its GWP is 98% lower than the GWP of SF6. Alstom g3 is safe to handle and has no impact on ozone depletion.

features of g3

Use at extremely low temperatures (25C and -30), Compatibility with switchgear materials, Stability versus temperature and time, Easy handling for filling and topping-up, Similar design compactness for equipment as of today.