gis-construction.docx

Electrical SubstationsElectrical Network comprises the   following regions: 1 - Generating Stations.   2 - Transmission Systems. 3 - Distribution Systems.    4 - Load Points. www.sayedsaad.com

Functions of a Substation1 - Supply of required electrical power. 2 - Maximum possible coverage of the supply network. 3 - Maximum security of supply. 4 - Shortest possible fault-duration. 5 - Optimum efficiency of plants and the network. 6 - Supply of electrical power within targeted frequency limits, (49.5 Hz and 50.5 Hz). 7 - Supply of electrical power within specified voltage limits. 8 - Supply of electrical energy to the consumers at the lowest cost. www.sayedsaad.com

Substation Layouts 1. Switching requirements for normal operation. 2. Switching requirements during abnormal operations,      such as short circuits and overloads. 3. Degree of flexibility in operations, simplicity. 4. Freedom from total shutdown and permissible period of shutdown. 5. Maintenance requirements, space for approaching various 6. Safety of personnel.7. Protective zones, main protection, back-up protection8. Bypass facilities.9. Technical requirements such as ratings, clearances,     Earthing lightning protection, Noise, radio  interference, etc.10. Provision for extensions, space requirement. 11. Economic considerations, availability, foreign        exchange involvement, cost of the equipment. 12. Requirements of network monitoring, power line     communication, data collection, Data transmission etc. 13. Compatibility with ambient conditions. 14. Environmental aspects, audible noise, RI, TI etc. 15. Long service life, Quality, Reliability, and Aesthetics. www.sayedsaad.com

Essential Features for substation 1 - Outdoor Switchyard having any one of the above.2 - Bus-Bar schemes.3 - High voltage Switchgear. Medium voltage Switchgear, Low voltage    Switchgear and control room.  4 - Office building. 

5 - Roads and rail track for transporting equipment.6 - Incoming line towers and outgoing line towers/cables. 7 - Store. 8 - Maintenance workshop (if required). 9 - Auxiliary power supply Low voltage AC. 10 - Battery room and low voltage DC. Supply system.11 - Fire fighting system. 12 - Cooling water system; drinking water system, etc.13 - Station Earthing system.14 - Lighting protection system, overhead shielding. 15 - Drainage system.16 - Substation lighting system etc. 17 - Fence and gates, Security system etc. www.sayedsaad.com

SF6 Gas Insulated Substations (GIS)

1. IntroductionSF6 Gas Insulated Substations (GIS) are preferred for voltage ratings of 72.5 kV, 145 kV, 300 kV and 420 kV and above. In such a substation, the various equipments like Circuit Breakers, Bus-Bars. Isolators, Load Break Switches, Current Transformers, Voltage Transformers Earthing Switches, etc. are housed in metal enclosed modules filled with SF6 gas.The SF6 gas provides the phase to ground insulation. As the dielectric strength of SF6 gas provides the phase to ground insulation. As the dielectric strength of SF6 gas is higher than air, the clearances required are smaller. Hence, the overall size of each equipment and the complete substation is reduced to about 10 % of conventional Air-insulated substations.As a rule GIS are installed indoor. However outdoor GIS have also been installed earlier.

High voltage Gas Insulated Switch gear Type B95 Double Bus-Bar (make Alostom)

                                                      Single line diagramHigh voltage Gas Insulated Switch gear Type B95 Double Bus-Bar (make Alostom)   1 – Circuit Breaker .               2 – Spring Mechanism .   3 – Disconnected .                  4 – Slow Earthing Switch   5 – Make Proof Earthing Switch.   6 – Current Transformer.       7 – Voltage Transformer.       8 – HV cable connection.   

The various modules of GIS are factory assembled and are filled with SF6 gas at a pressure of about 3 kg/cm2. Thereafter, they a taken to site for final assembly. Such substations are compact and can be installed conveniently on any floor of a multi-storied building or in an underground substation.As the units are factory assembled, the installation time is substantially reduced. Such installations are preferred in cosmopolitan cities, industrial townships, etc., where cost of land

is very high and higher cost of SF6 insulated Switchgear (GIs) is justified by saving due to reduction in floor area requirement.They are also preferred in heavily polluted areas where dust, chemical fumes and salt layers can cause frequent flashovers in conventional outdoor air-insulated substations

GIS bay single Bus-Bar Make Mitsubishi

            1-     Circuit Breaker2-     Disconnector Switch (GL-Type)3-     Disconnector Switch (GR-Type)4-     Earthing Switch (GRE-Type)5-     3-ph. Bus-Bar.6-     Current Transformer.7-     Base. 8-     Voltage Transformer.

The SF6 Gas Insulated Substations (GIs) contains the same Components as in the conventional outdoor substations. All the live parts are enclosed in metal housings filled with SF6 gas. 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.  www.sayedsaad.com The entire installation is sub-divided into compartments which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simpler. The enclosures are of non-magnetic material such as aluminum or stainless steel and are earthed. Static O-seals placed between machined flanges provide the gas tightness. The O-rings are placed in the grooves' such that after assembly, the O-rings are squeezed by about 20 %. Quality of material and dimension of grooves and O-seals are important to ensure

gas-tight performance.The GIs has gas-monitoring system. The gas density in each compartment is monitored. If pressure drops slightly, the gas is automatically tapped up with further gas leakage, the low-pressure alarm is sounded or automatic tripping or lock-out occurs www.sayedsaad.com Advantages of GIs and Application Aspects:

    1- Compactness. The space occupied by SF6 installation is only about  8 to 10 % of that a conventional outdoorsubstation. High cost is partly compensated by saving in cost of space. A typical 420/525 kV SF6 GIs requires only 920 m2 site area against 30.000 m2 for a conventional air insulated substation.    2 - Choice of Mounting Site. Modular SF6 GIS can be tailor made to Suit the particular site requirements. This  results is saving of otherwise Expensive civil-foundation work. SF6 GIS can be suitably mounted indooron any floor or basement and SF6 Insulated Cables (GIC) can be taken through walls and terminated through SF6 bushing or power cables.    3 - Reduced Installation Time.The principle of building block construction (modular construction) reduces the installation time to a few weeks. Each conventional substation requires several months for installation.In SF6 substations, the time-consuming high cost galvanized steel structures are eliminated. Heavy foundations for galvanized steel structures, Equipment support structures etc are eliminated. This results in 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 - Protection from pollution. The external moisture. Atmospheric Pollution, snow dust etc. have little influence on SF6 insulated substation. However, to facilitate installation and maintenance, the substations are generally housed inside a small building.     5- Increased Safety.  As the enclosures are at earth potential there is no possibility of accidental contact by service personnel to live parts.6 - Explosion-proof and Fire-proof installation. Oil Circuit Breakers and oil filled equipment are prone to explosion. SF6 breakers and SF6 filled equipment are explosion proof and fire-proof.. 

Summary of Merits of SF6 GIS

SafeOperating personnel are protected by the earthed metal enclosures

ReliableThe complete enclosure of all live parts guards against any Impairment of the insulation system.

Space savingSF6 Switchgear installations take up only 1/10 of the space Required for conventional installations.

EconomicalHigh flexibility and application versatility provide novel, and economic overall concepts.

Maintenance free

An extremely careful selection of materials. an expedient design and a high standard of manufacturing quality assure Long service life with practically no maintenance requirement.

Low weight

Low weight due to aluminum enclosure, correspondinglyLow cost foundations and buildings.

Shop assembledQuick site assembly ensured by extensive preassembly and Testing of complete feeders or large units in the factory.

Disadvantages of GIS: 1- High cost compared to conventional outdoor substation.2 - Excessive damage in case of internal fault. Long outage periods as      Repair of damaged part at site may be difficult.3 - Requirement of cleanliness is very stringent. Dust or moisture can cause       internal flashovers. 4 - Such substations are generally in door. They need a separate building.      This is generally not required for conventional outdoor substations.5 - Procurement of gas and supply of gas to site is problematic.     Adequate stock of gas must be maintained.6 - Project needs almost total imports including SF6 Gas. Spares conventional       substation is totally indigenous up to 400 kV.Configuration of GIS: The GIS installations are assembled from a variety of standard modules.Which are joined together by flange connections and plug contacts on theConductors. So as to easily permit subsequent disassembly of individual  components. Gas-tight barrier insulators in the Switchgear sections prevent neighboring Switchgear parts from being affected by overhauls.Any maintenance and overhaul work on Switch contacts can be done without removing the enclosure.With GIS installations, all basic substation Bus-Bar schemes used, inconventional plant constructions can be realized. Installations with single ormultiple Bus-Bar-also alternatively with a bypass bus-can be made with thestandard modules, including Bus-Bar sectionalizing with disconnects and Breakers, and Bus-Bar coupling. The two-breaker. One and-a-half circuitbreaker and ring-bus systems can also be realized economically.     The essential parts of a GIS are: -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 GIS these insulation requirements      are met by cast resin insulators and SF6 gas insulation.3 - Gas filled modules have nonmagnetic enclosures. Enclosures are of aluminum       alloy or stainless steel. Adjacent modules are joined by means of multi-bolts       tightened on flanges. Suitable neoprene rubber “O” ring gaskets are provided      for ensuring Gas-tight sealing joints.   4 - Various circuit components in main circuit are: CB, Isolator, Earthing Switches        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 LV DC and LV AC supply system, control, protection and Monitoring      system. This is air-insulated like in conventional sub-station.  The Bus-Bars are conducting bars to which various incoming and   outgoing bays are connected. In SF6 GIS the Bus-Bars are laid l  longitudinally 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.

Alternatives of Enclosures, Single three phaseand three single enclosures

Three phase Single Enclosures

     Three phase and three single enclosures

The following alternatives are available to the designers for configuration of GIs.1. Separate enclosure for each phase. This alternative was used for Components     and Bus-Bars in early GIs. Now it is used only for EHV and UHV, GIS.     The GIS above 420 kV are generally with separate enclosure for each phase.

2. Separate enclosure for components and a common single enclosure    For three phase enclosure for Bus-Bars.    This alternative is more widely used now for all GIS3. Common single enclosure for all three phases for components and    For Bus-Bars. The per cent trend is to use single three phase modules for    components and Bus-Bars for all GIS. The GIS developed during 1980’s are with    this philosophy. 

Design Aspects

The SF6 insulated Switchgear contains the same components as a conventional outdoor substation. Fig (1) illustrates the construction of typical bay.

Fig (1) Section of a 145 KV SF6 GIS with duplicate bus-bar

1 – 3- phase Bus enclosure. 2 – Isolator.3 – Earthing Switch.4 – C.B puffer type.5 – CT's 

6 –  Line Isolator.7 –  VT.  8 –  High Speed Earthing Switch.9 –  Cable sealing End.10 – Operating mechanism (cabinet).     11 – Conductor tube.     12 – Epoxy partition fig. (2).

All the live parts are enclosed in metal housing filled with SF6 gas. Live parts are supported on cast resin insulators. Some of the insulators are designed as barriers between neighboring modules such that the gas doesnot pass through them. The entire installation is sub-divided into compartments, which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simple

The enclosures are of nonmagnetic material such as aluminum or stainless steel and are earthed. The gas tightness is provided by static O-seals placed between machined flanges.The O-rings are placed in the grooves such that after assembly, theO-rings get squeezed by about 20 %. Quality of material and dimension of groove are important. Aluminum or stainless steel enclosures surround all live parts. Enclosures are earthed.Pressurized SF6 gas provides internal insulation between conductors and metallic enclosures. Fig (2) below.High grade insulators of Epoxy partition resin give support to active parts inside the enclosures and are also used as barriers between adjacent gas filled compartments.

Fig(2) Epoxy partition resin

Individual compartments (modules) are connected by silver platedPlug contacts for current conduction. Flanges of enclosures are bolted.Control cabinet installed near the bay contains instruments, relays, auxiliary Switches, control wiring etc. for local control, indication, alarm etc. GIS is installed on self supporting steel structures fixed on t he floor.

Expansion bellows(Bellows compensators)

Expansion Bellows

Expansion Bellows

Variations in length due to temperature changes and dimensional differences due to assembly tolerances are resolved by making use of the wide range of bellows,which take up axial or lateral tolerances.These bellows are self compensated or compensated in compression by tie-rodsBellow compensators permit absorption of manufacturing tolerances in Bellow Compensators also permit absorption of vibrations caused bylength of enclosures Bus-Bars, Transformers, reactors.Conductors are usually of aluminum alloy tubes. The conductors areplugged to silver plated finger contact assembly mounted on support insulators.These sliding contacts permit tubular conductors to expand axially with temperature rise without any additional stress on support insulators. The enclosures are of welded aluminum or stainless steel plates to whichcast aluminum or stainless steel flanges are welded. Metallic connections between adjacent enclosures are ensured to permit circulation of full return current. The induced currents circulate in enclosures and provide magnetic field of [heir own such that.       - Outside the enclosures the magnetic field of enclosures opposes the magnetic field of conductor currents.      - Inside the enclosures, the magnetic field of enclosure

currents addsto that of conductor current resulting in centralizing force on conductor.The conductor tends to remain along the central axis of enclosure. 

Cable connection

All cables, irrespective of their type of insulation (oil impregnated paper or XLPE) and section, can be connected.The cable sealing end is fixed inside the SF6 gas Filled compartment, in accordance with the IEC 859 standard commonly used. Isolation of the Switchgear from the high voltage cables during dielectric testing is achieved by removing the contact (1) and the conductor (2).Safety is fully ensured by earthing of the cable Side through access (3), in parallel with closing of the cable earth Switch.

                    Connection to Transformer                          1 – Removable contact's                      2 – Removable conductor.                      3 – Expansion bellows.                       4 – Bushing.

           Cable connection box             1 - Removable contact's            2 - Removable conductor.            3 - Access for Earthing rod.            4 - Gas tight bushing.            5 - High voltage.

Earthing Switch

 Earthing Switch is necessary to earth the conducting parts before maintenance and also to provide deliberate short-current while testing. There can be three  types of  Earthing witches in metal-clad Switches manually operated automatic high speed Earthing Switch, protective Earthing Switch for Earthing the installation.

There are several versions of Earthing Switches for following applications1 - Maintenance Earthing Switches. These are single pole or three pole units; manually operating mechanism with a provision of filling motor mechanism.

   2 - High Speed Earthing Switches. These are operated by spring energy. Spring is charged by motor-mechanism

Fig (1)

Fig (2) the one pole Earthing Switch

Earthing Switch:1- Moving contact2- Operating lever3- Position indicator

Closed position                                                    

Open position

The earth Switch is mounted direct on the enclosure Fig. (1) Earthing Switch has to satisfy various requirements. For Earthing isolated sections of Switchgear for protection of personal during maintenance and over-hauls or erection, the maintenance Earthing Switches are employed. For Earthing higher capacitances (cables, overhead line etc.) high speed Earthing Switch are employed. Depending on the substation scheme, the Bus-Bars may be earthed either by maintenance                                            or high-speed Earthing Switches.

 Special high speed Earthing Switches with interrupting

capability are also available. These are suitable for interrupting capacitive and inductive currentsfrom parallel overhead lines. In certain cases, Earthing Switches are fitted tothe enclosure with interposed insulation.

 This enables various tests to be performed on the Switchgear

or item of equipment, such as testing the current Transformer of measuring the operating time of breakers, without having to open the enclosure. During normal operation the insulation is bypassed by a short-circuit-proof link.

To check whether a point to be earthed really is dead, the Earthing Switch can be equipped with a capacitive tap for connecting a voltage test unit. This additional safety device reduces the risk of closing onto a live conductor.