POWERGRID CORPORATION OF INDIA LIMITEDA government of India enterprise

“Gas Insulated Substation:

An overview”

Project Under the supervision of: Report Submitted by:

Sh. Alok Saurabh kumar (DGM) 7 EL-062 & LIMAT Sh.R.G.Agrawal

(Manager)

GIS, Maharani Bagh

1

------- Acknowledgement -----

I would like to express my gratitude to Sh. Alok, DGM,

GIS, Maharani Bagh, Sh. R.G. Aggrawal, Manager, GIS

Maharani Bagh and Sh. Ashish, Senior Engineer, GIS,

Maharani Bagh for the opportunity they provided and

without whose valuable cooperation and guidance this

training programme and project work would have

become a difficult task.

I am deeply indebted to all the staff of GIS, Maharani

Bagh, especially Mr. Subrat, who helped me throughout

my stay here.

-------- INDEX --------

2

1. Gas Insulated substation: Introduction

2. Properties of SF6 which makes it suitable for GIS

3. Construction and service life of GIS

GIS Maharani Bagh

4. Components of GISCircuit breakerOperating mechanismCurrent transformerDisconnectorMaintenance earthing switchFast acting earthing switchVoltage transformerSurge arrester

5. Versatile connecting elements

6. SF6/Air termination Cable termination Transformer connections

7. Reliable gas compartments

8. Maintenance practices at GIS

9. Reference

1. Gas insulated substation: An introduction

3

Gas insulated substation uses a superior dielectric gas, SF6, at moderate pressure for phase to

phase and for phase to ground insulation. The high voltage conductors, circuit breakers

interrupter switches current transformers and voltage transformers are in SF6 gas inside

grounded metal enclosures. The atmospheric insulation used in conventional air insulated

substations (AIS) requires meters of air insulation to do what SF6 can do in centimeters. GIS can

therefore be smaller up to a factor of 10. It is mainly used where space is expensive or not

available. In a GIS the active parts are protected from the deterioration from exposure to

atmospheric air moisture contamination etc. As a result GIS is more reliable and require less

maintenance than AIS.

GIS was first developed in various countries between 1968 and 1972. After about 5 years of

experience, the use rate increased to about 20%of new substations in countries where space is

limited. IN other countries with space easily available, the higher cost of GIS relative to AIS has

limited use to special cases. For example, in USA, only about 2% of new substations are GIS.

1.1 The versatility of GIS can be recapitulated as:

. Its a switching station with rigorous safety requirements

. Indoor substation to occupy a minimum of space in a densely populated urban area

. Protection installations exposed to unusually dirty, polluted or corrosive environment.

. Underground substation for pumped storage and other high electric power station

. Generating plants where the switchgear can be very close to the power transformer to give an

ideal overall lay out.

. The extension of conventional out door installations where space is constraint

. Requiring existing s/s and upgrading the voltage level without taking extra space

1.2 Main design features:

. Light weight enclosure aluminum with good conductivity, no eddy current losses and a

4

high resistance to corrosion.

. Single phase encapsulated design, assuming minimum dielectric and dynamic stress

. Proven metal to metal flange connections, ensuring high gas tightness and return

Current conductivity, avoiding cross bending.

. Increased personnel safety and electromagnetic compatibility

. Horizontal circuit breaker design, saving building elevation and cost, ensuring convenient

personnel access.

. Minimal outage concept with segregated, individually monitored gas compartments

. Digital control, online condition monitoring and synchronized switching options

. Short erection time and high reliability, by providing large transport units assembled and tested

in a factory.

2. Properties of SF6 which make it suitable for GIS

Sulfur hexafluoride is an inert, nontoxic colorless odorless tasteless and non-flammable gas

consisting of a sulfur atom surrounded by and tightly bounded to six fluorine atoms.

5

It is about five times as dense as air. It is used in GIS at pressure from 400 to 600 Kpa absolute.

The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperature

the equipment experiences .SF6 has two to three times the insulating ability of air at the same

pressure.

It is about 100 times better than air for interrupting arcs. It is the universally used interrupting

medium for high voltage circuit breakers replacing the older medium of oil and air. SF6

decomposes in the high temperature of an electric arc but the decomposed gas recombines back

into SF6 So that it is not to be replenished in GIS.

There are some reactive byproducts formed because of the trace presence of moisture air and

other contaminants. The quantity formed is very small.

Molecular sieve absorbents inside the GIS enclosures eliminate these reactive byproducts. SF6 is

supplied in 50 Kg gas cylinders in liquid state at a pressure of about 6000Kpa for convenient

storage and transport.

The SF6 in the equipment must be dry enough to avoid condensation of moisture as a liquid on

the surface of the solid epoxy support insulators because liquid water on the surface can cause a

dielectric breakdown. However if the moisture condenses as ice the breakdown voltage is not

affected.

So due point of the gas in the equipment need to be below about –10 deg c. For additional

margin level of less than 1000ppmv of moisture are usually specified and easy to obtain with

careful gas handling. Absorbents inside the GIS enclosures help keep the moisture level in the

gas low even though over time moisture will evolve from internal surfaces and out of the solid

dielectric materials.

Small conducting particles of mm size significantly reduce the dielectric strength of SF6 gas.

This effect becomes greater as the pressure is raised par about 600 Kpa absolute. The particles

are moved by the electric field possibly to the higher region inside the equipment or deposited

along the surface of solid epoxy support insulators, leading to dielectric breakdown at operating

voltage levels.

6

Cleanliness in assembly is therefore very important for GIS. Fortunately, during the field power

frequencies high voltage test contaminating particles can be detected as they move and cause

small electric discharge and acoustic signals so they can be removed by opening the equipment.

Some GIS equipments are provided with internal ‘particle trap’ that capture the particles before

they move to a location where they might cause the breakdown. Most GIS assembly is of a shape

that provides some ‘natural’ low electric field regions where particles can rest without causing

any problems.

SF6 is a strong green house gas that could contribute to global warming. At an international

treaty conference in Kyoto in 1997, SF6 was listed as one of the six green house gases due to

human activity, but it has a very long life in the atmosphere ( half life is estimated at about 3200

years), so the effect of SF6 released to the atmosphere is cumulative and permanent.

The major use of SF6 is in electric equipments. Fortunately in GIS it can be contained and can be

recycled. By following the present international guidelines for use of SF6 in electric equipments,

the contribution of SF6 to global warming can be kept to les than 0.1% over a 100-year horizon.

The emission rate from use in electric equipment has been reduced over the last three years.

Most of this effect has been due to simply adopting better handling and recycling practices.

Standards now require GIS to leak less than 1 % per year. The leakage rate is normally much

lower. Field check of GIS in service for many years indicates that the leak rate objective can be

as low as 0.1% per year when GIS standards are revised

2.1 Effects of contamination on the body due to SF6

1. As with other chemicals used everyday in industry, SF6 gas and its decomposition products

will not cause injury or illness if handled properly. Health hazards may exist if exposure or

handling is careless or improper.

7

2. In the presence of moist air, noxious decomposition gases will have the characteristic odor of

rotten eggs. Inhaling such gas should be avoided. The results could cause nausea, drowsiness and

if prolonged and extensive, serious breathing difficulty and damage to the respiratory system.

3. Solid decomposition products are normally metal fluorides in the form of white powder. Skin

contact should be avoided as it may, in some people, cause rashes and severe irritation. Inhaling

airborne dust should be avoided since, as with gaseous products, it may cause breathing

difficulty and damage to the respiratory system. Airborne dust can cause eye irritation and

should be avoided.

3. Construction and service life of Gas Insulated Substation

GIS is assembled of standard equipment modules (circuit breakers, voltage transformers, voltage

transformers, disconnect and ground switches, interconnecting bus, surge arresters, and

connections to the rest of the power systems) to match the electrical one line diagram of the

8

substation. The modules are joined using bolted flanges using ‘o’ ring seal system for the

enclosure and a sliding plug in contact for the conductor.

The cast epoxy insulators support internal parts of GIS. These support insulators provides a gas

barrier between parts of GIS, or are cast with holes in the epoxy to allow gas to pass from one

side to the other.

Up to about 170Kv system voltage all three phases are often in one enclosure. Above 170 Kv,

the size of the enclosure for “ 3 ph enclosure” GIS becomes too large to be practical. So a single-

phase enclosure design is used. There are no established performance differences between the

two types. Some manufacturers use the Single enclosure for all voltage levels.

Enclosures today are mostly cast or welded aluminum but steel is also used. Steel enclosures are

painted inside to prevent rusting. Aluminum enclosure does not need to be painted but may be

painted for ease of cleaning and better appearance.

The pressure vessel requirements for GIS enclosures are set by GIS standards, with the actual

design, manufacture and test following an established pressure vessel standard of the country of

manufacturer. Because of the moderate pressure involved, and the classification of GIS as

electric equipment, third party inspection and code stamping of the GIS enclosure are not

required.

Conductors today are mostly aluminum. Copper is sometimes used. It is usual to silver plate

surfaces that transfer current. Bolted joints and sliding electric contacts are used to join

conductor sections. There are many designs for the sliding contact element. In general, sliding

contacts have many individually sprung copper contact fingers working in parallel. Usually the

contact fingers are silver-plated.

A contract lubricant is used to ensure that the sliding contact surface do not generate particles or

wear out over time. The sliding conductor contacts make assembly of the modules easy and also

allow for conductor movement to accommodate the differential thermal expansion of the

conductor relative to the enclosure. Sliding contact assemblies are also used in circuit breakers

and switches to transfer current from the moving contact to the stationary contacts.

9

Support insulators are made of highly filled epoxy resin cast very carefully to prevent formation

of voids and /or cracks during curing. Each GIS manufacturer’s material formulation and

insulator shape has been developed to optimize the support insulator in terms of electric field

distribution, mechanical strength, resistance to surface electric discharges , and convenience of

manufacture and assembly. Post, Disc and cone type support insulators are used. Quality

assurance programs for supporting insulators include a high voltage power frequency withstand

test with sensitive partial discharge monitoring. Experience has shown that the electric field

stress inside the cast epoxy insulator should be below a certain level to avoid aging of the solid

dielectric material. The electric stress limit for the cast epoxy support insulator is not a severe

design constraint because the dimensions of the GIS are mainly set by the lightning impulse

withstand level and the need for the conductor to have a fairly large diameter to carry to load

current of several thousand amperes. The result is space between the conductor and enclosure for

support insulators having low electric stress.

Service life of GIS using the construction described above has been shown by experience to be

more than 30 years. The condition of GIS examined after many years in service does not indicate

any approaching limit in service life. Experience also shows no need for periodic inspection or

maintenance. Inside the enclosure is a dry, inert that itself into subject to aging. There is no

exposure of any of the internal material to sunlight. Even the ‘o’ring seals are found to be in

excellent condition because there is almost always a double seal system.

So far we have been discussing about the general features incorporated in GIS technology.

Power Grid has used the expertise of ABB to construct its own GIS which is first of its type in

India This Substation was commissioned on Aug 31, 2007 and is located at Maharani Bagh. By

looking at the real time execution of this technology it will be a lot easier to grasp the

aforementioned concepts. It is another gateway for Delhi to avail 630MW of secure power from

central Gri

3.1 400/220KV Gas Insulated Substation Maharani Bagh

400/220Kv Gas Insulated Maharanibagh S/S has been constructed under East- North high

capacity TALA Transmission system for meeting out the additional power demand of Delhi.

10

This substation will further increase the security and reliability of the Delhi Grid in addition to

meet the additional power demand. The station is situated on the right bank of river Yamuna near

Sarai Kale Khan ISBT in Delhi. The sub station has been commissioned on 31/8/07. This is the

first 400KV Gas Insulated sub station of POWERGRID in India. It is also the first sub station of

POWERGRID in the state of Delhi.

Maharanibagh S/S is having five 400KV bays, seven 220KV bays (details of which are shown

ahead), two 315MVA, 400/220KV ICT’s. The entire station has been constructed on 5 acre land

and has a provision of extension of as many bays.

The details of bays are as:

400 KV Bays (Two Bus scheme) 220KV Bays (Two Bus scheme)

1) Dadri 1) Lodhi Road I

2) Ballabgarh 2) Lodhi Road II

3) ICT-I 3) Sarita Vihar

4) ICT-II 4) IP Extension

5) Bus Coupler 5) ICT- I

6) ICT II

7) Bus Coupler

3.2 Land Requirement

As we have discussed earlier that the area requirement in case of GIS is about 8 to 10 % of what

is required by AIS, this will be clear by learning that GIS maharani Bagh has been constructed

in 2 Hectare land and is also having provision of further extension of as many bays. For a similar

size of conventional air insulated S/S the land requirement is approximately 14-15 Hectare. 05

bays of 400KV are installed in a hall of size 27mx12mx12m where as 07 bays of 220KV have

been installed in a hall size of 24mx10mx8m.

3.3 Requirement of maintenance

11

As compared to Air Insulated S/S the maintenance requirement of GIS is very less or more or

less we can say that GIS Substation is maintenance free. The maintenance staff has to be very

vigil about the gas leakage as this could be a possible threat to normal functioning of each

module.

3.4 Cost criteria

The cost of GIS Substation is approximately double the Air insulated substation. However if the

cost of land is also considered GIS Substation will be more cost effective. The expensive

technology is compensated by the cost cutting in land procurement.

4. COMPONENTS OF GAS INSULATED SUBSTATION

12

Figure 1: Parts of GIS

The Picture above shows the layout of a typical Gas insulated sub station, which depicts how the conductor is carried through various SF6-gas filled enclosures/modules to connect them with the bus. Now in the sections to come, the above shown components will be discussed in detail including their operating mechanism and working principle.

4.1 Circuit breaker

13

Circuit breaker

GIS uses essentially the same dead tank SF6 puffer circuit breaker used in AIS. Instead of SF6-

to-air as connection into the substation as a whole, the nozzle on the circuit breaker enclosures

are directly connected to the adjacent GIS mode. Some salient features of circuit breaker used in

GIS are :

. Reliable making and breaking capacity for heavy loads and short circuit currents

. Easy access to active parts and overhaul

. Low noise level

. Maintenance free design

. Separate contact system for continuous current and current interruption

. High dielectric withstand in open and closed position

. Single phase auto-enclosing

. Compact hydraulic spring operating system

.Continuous self supervision of hydraulic system

. No external hydraulic piping

. Type tested according to IEC and ANSI standards

4.1.1 Breaker Design

Each breaker comprise three single phase metal enclosed breaker poles each pole consist of the

operating mechanism, the interrupter column with two interrupting chambers in series and the

14

enclosure with the basic support structure. To guarantee simultaneous interruption, the chambers

are mechanically connected in series. One grading capacitor for each chamber guarantees an

equalized voltage distribution across the interrupting chamber. In case of an overhaul, the

interrupter column can easily be removed from the enclosure. The circuit breaker is of the single

pressure type and works on the well proven puffer principle. During an interruption, a

compression piston in chamber generates the Sf6 gas pressure required to extinguish the arc

between the contacts.

4.1.2 Operating mechanism

Each pole of the CB is equipped with the hydraulic spring operating mechanism. It combines the

advantages of the hydraulic operating mechanism with those of the spring energy storage type.

Its compact, molecular, design consist of

.The housing with position indicator

. Power pack for energy storage with out ant kind of external hydraulic pipe

. Monitoring module for control

15

It guarantees easy access to all components inside the drive for overhaul and repair. Sealing of

the pressure operated hydraulic circuit against the atmosphere is achieved entirely by highly

reliable static seals

4.1.3Working principle

Hydraulic pump moves oil from the low pressure reservoir to the high pressure side of the energy

storage piston, connected to the disc springs. The output piston which is connected to the

operating rod of the circuit breaker column is controlled by a change over valve. For opening, it

switches hydraulically to the open position after the trip coil is actuated for opening the breaker

and connects the bottom side of the output piston with the low pressure reservoir. The CB then

moves to open position where it will be retained due to hydraulic pressure. For closing the circuit

breaker, the change over valve connects the bottom side of the output piston to the high pressure

reservoir after the actuation of the closing coil. Now, both the sides out put piston are connected

to the high pressure and the CB is moving to its closed position due to the differential breaker

principle

4.2 Current transformer

16

CTs are inductive ring type installed either inside the GIS enclosure or out side it. The GIS

conductor is the single turn primary for the CT. CTs inside the enclosure must be shielded from

the electric field produced by the high voltage conductor or high transient voltage can appear on

the secondary through capacitive coupling. Fir CTs out side the enclosure, the enclosure itself

must be provided with an insulating joint, and enclosure current shunted around the CT. Both

type of construction are in wild use

4.3 Disconnector

Disconnect switches have a moving contact that opens or close a gap between stationary contact

when activated by an insulating operating rod that is itself moved by a shaft coming through the

17

enclosure wall. The stationary contacts have shields that provide the appropriate electric field

distribution to avoid too high a surface stress. The moving contact velocity is relatively low

(compared to a circuit breaker moving contact) and the disconnect switch can interrupt only low

levels of capacitive current (for example, disconnecting a section of GIS bus) or small inductive

currents (For example transformer magnetizing current). Load break disconnect switches have

furnished in the past, but with improvement and cost reductions of circuit breakers, it is not

Practical to continue to furnish load break disconnect switches and a circuit breaker should be

used under. The operating mechanism is located outside the gas chamber on one phase. The two

other phases are operated by immediate gears. The same mechanism is also employed for

maintenance earth switch. Manual operation is possible. In this case electrical control connection

is automatically disconnected. The mechanism is padlock able in both, open or close position.

18

4.4. Maintenance earthing switch

Safe earthing of sections of main circuit Maintenance Fast acting

Earthing switching

. Designed for full short circuit in closed position earthing switch

. Motor operated mechanism slow closing or high speed spring loaded

. Full short circuit making and induced current switching capability

Earthing switches are mounted directly on enclosure (Shown on left in Figure). Slow motion

maintenance earthing switches are used for earthing isolated sections of switchgear to protect

personnel during maintenance and overhauls or erection. The operating mechanism is the same

as used for disconnector switches. It is mounted on the same. The other two are operated by

immediate gear.

4.5 Fast acting earthing switch

19

It is (shown on right) mainly used to discharge the static charge on overhead lines, HV cables or

long buses. It is designed to close on and carry full short circuit currents and to safely break

capacitive and inductive induced from long energized parallel lines. Each phase is provided with

its own operating mechanism. For the closing operation a spring in the drive is loaded by the

motor. At the end of the charging operation the spring is automatically released and the switch is

closed. The opening process is slow, similar to that of the maintenance earthing switch. In

certain case an interposed insulation is fitted between the earthing switch and the enclosure. This

provides access to the active parts for measuring purposes through the closed contacts, without

opening the enclosure. During normal operation the insulation is by passed. All the earthing

switches have a mechanically coupled position indicator. The gap between the contacts can also

be verified through a view port by means of an endoscope. To check whether a point to be

earthed really is dead, the earthing switch is equipped with a capacitive tap for connecting a

voltage test unit. This additional safety device reduces the risk of closing on to a live conductor.

4.6. Voltage transformer

20

VTs are inductive type with iron core. The primary winding is supported on insulating plastic

film immersed in Sf6. The VTs should have an electric field between the primary and secondary

winding to prevent capacitive coupling of transient voltages. The VT is usually a sealed unit with

a gas barrier insulator. The VT is either easily removable so the gas can be high voltage tested

without damaging the equipment, or the VT is provided with a disconnect switch or removable

link.

Highlights of VT incorporated in GIS

. Inductive type, with SF6-gas insulated high voltage winding

. Rectangular type core of low loss magnetic sheets

. High cable or line discharging capability

. Efficient damping of ferro-resonance and fast transient

It is connected to the switch gear with standardized connecting flange with a barrier insulator.

The primary winding is insulated with SF6-gas and connected to the high voltage terminal. The

primary winding is wound on the top of the core and the secondary windings which in turn are

connected to the terminals in the external terminal box through a gas tight multiple bushing. The

transformer may be equipped with two metering windings and one tertiary winding for earth

fault protection

4.7 Surge arrester

21

. Metal oxide resistor housed by SF6-gas insulated enclosure

. Without spark gaps

. Low protection level

. High energy input capacity

. Stable characteristics. Ageing proof

GIS and connected HV cables, Power transformers, etc. often require over voltage protection by

surge arresters. Gas insulated arresters are a reliable and space saving alternative to conventional

type, avoiding the need of aerial links. The active parts mainly non-linear metal oxide resistors,

series connected and assembled to stack, are housed in a pressurized SF6-gas enclosure, which is

connected to the GIS by a standard flange connection with barrier insulator. The resistors are

manufactured as disks with a conductive, metallic coating on the flat contact surfaces and a gas

tight coating on the cylindrical surfaces. Arrestors with metal oxide resistors have a high energy

absorbing capacity. A grading electrode surrounds the resistors on top and ensures an even

voltage distribution along the resistor. An advantage of this design is that the SF6-gas flows

around the resistors, ensuring efficient cooling when require, i.e. after repeated heavy discharges.

22

5. VERSATILE CONNECTING ELEMENTS:

A modular range of straight bus bar, trees, and elbows is available, enabling the different major components to be arranged in a space saving way, coupling with any customer requirements. The bus bar enclosure is dimensioned for full return current. The metallic connection of the flanges is also assured at points where supporting or barrier insulators are interposed. Tubular conductors are supported at each end by an insulator. The conductors are plugged to silver plated spiral spring contact on the supporting insulator. These sliding contacts permit the tubular contacts to expand axially on a temperature rise without imposing any mechanical stress on the supporting insulator where required. Longitudinal expansion is taken up by metal bellows or parallel compensators.

Lateral dismantling unit:-Where required, lateral dismantling units are inserted. They enable sections of switchgear to be removed and reinserted, without interfering with the adjacent parts. This is achieved by sliding sections in the enclosure and in the tubular conductor.

23

6. SF6/ AIR TERMINATION

Sf6-air bushings used for connections to open terminal equipment and overhead lines are

available with composite or porcelain insulators. The internal insulation consists of pressurized

SF6-gas or resin impregnated paper winding (RIP).

Nowadays composite insulators are used more and more for high voltage applications because of

their outstanding mechanical and dielectric properties; the latter due to hydrophobic capability of

silicon rubber sheds. Bushings with composite insulators are absolutely explosion proof.

24

6.1 CABLE TERMINATION

High voltage cables of various types are connected to the SF6 switchgear via cable connection

assembly. It consists of the cable sealing with connecting flanges, the main circuit end terminal

and the surrounding enclosure. An isolating link between the switchgear and the cable

termination enable the GIS and the cable to be tested separately.

6.2 TRANSFORMER CONNECTION

The transformer connection consists of the oil/SF6-bushing. The enclosure, the main circuit the

end terminal and the removable connection. The pressure-tight bushing separates the gas-filled

compartment from the insulating oil of the transformer. The lateral dismantling unit takes up

axial assembly tolerance and enables the transformer to be easily connected, or disconnected

from the switchgear. For the high voltage tests on the GIS, the transformer is isolated from the

switchgear by dismantling the removable conductor connection.

25

7. RELIABLE GAS COMPARTMENTS

A prerequisite for safe and reliable is the segregation of the sections and phases of the equipment

into individual and independent gas compartment. This is also required to keep parts of the

system in operation while other parts are out of service. The segregation is achieved by barrier

insulators. To avoid any impact from each other, all gas compartments are fitted with their

individual non-returning filling valve and density monitor. Any pipe connection to common

sampling points is therefore strictly avoided. Each compartment is also fitted with its moisture

absorber and pressure relief.

Safe gas sealing system: Reliability of SF6 gas-insulated Switchgear depends to a large extent on

the low rate of gas leakage. Thus special attention is given to the

sealing system. Single sealing rings (‘O’ rings) affect the proven static sealing

System at the flange point. High quality finish limits the gas losses to much less than 0.5% per

annum.

26

7.1 GAS DENSITY CHAMBER

Gas density chamber

The dielectric strength of the GIS depends on the gad density, which is therefore directly

monitored. The proven density monitor is directly mounted on the enclosure. The gas pressure

acts on metal bellows, with a reference value for compensation of the temperature. In case of gas

leakage a micro-switch is actuated. Threshold for refilling (first stage) or lockout alarm (second

stage) can be mechanically set. The response characteristic is shown ahead in mollier

diagramme. Density monitors can be easily removed for testing with a separate calibration

device.

Mollier Diagram

27

TECHNICAL DATA OF GIS

28

8. MAINTENANCE PRACTICES AT GAS INSULATED SUBSTATION

8.1 General handling practice of SF6 gas

1.SF6 gas is supplied in pressurized cylinders, which like any gas cylinders, are capable of being

damaged or ruptured by careless handling. Gas cylinders should never be left standing

unsecured. Cylinders can develop accidental leaks. They should not be stored in an unventilated

area where escaping SF6 gas can build-up.

2. SF6 gas handling, in filling or retrieval from SF6 insulated equipment, should only be done

outdoors or, if indoors, with ventilating equipment in operation.

3. Personnel will refrain from smoking. No exposed heaters, flames, or arc-producing equipment

will be used in the area while the gas is being handled.

4. If, when handling used SF6, leakage results in the odor of rotten eggs, personnel should

evacuate the area unless equipped with respiratory equipment.

5. Significant leakage should not be tolerated at hose fittings, etc. due to the possibility of a

build-up of gas concentrations in local or low areas and emissions to the atmosphere.

8.2 Routine adding of SF6 gas

29

SINGLE PRESSURE CIRCUIT BREAKERS SHOULD BE REMOVED FROM SERVICE, IF

SYSTEM CONDITIONS PERMIT, WHEN ADDING SF6 GAS.

1. Attach filling hose and pressure regulating device to cylinder. If possible, try to ensure the

hose is moisture free and there are no particles, dirt or oil inside the hose or fittings.

2. Attach fill hose, loosely, to the SF6 fill port on the equipment.

3. Flush filling hose with a minimal amount of SF6 to displace the air or contaminates in the

hose. Secure the fill hose to the fill port.

4. Open valve on fill port and add to desired pressure. Shut off valve on fill port and shut off

valve on cylinder. Remove fill hose from equipment and recap fill port.

5. Return equipment to service if removed for adding.

8.3 INSPECTION PROCEDURE PRIOR TO SERVICE

THE INTENTIONAL RELEASE OF SF6 TO THE ATMOSPHERE IS PROHIBITED. A GAS

RECLAIMER MUST BE USED TO EVACUATE AND STORE SF6 FROM EQUIPMENT.

1. Evacuate the SF6 gas from the equipment into the gas reclaimer. Follow the manufacturer

instructions for the proper operation of the gas reclaimer. The use of a pre-filter is required when

evacuating gas from equipment that has internally failed and contaminated the SF6 gas.

2. Pull a vacuum on the equipment to 2mm then break vacuum with dry air or atmosphere. Dry

air should be used if atmospheric conditions could contaminate the equipment.

3. When opening equipment, personnel shall be properly suited up with coveralls, gloves, and a

full face respirator equipped for organic compounds and acidic gases.

IF ARC PRODUCTS ARE FOUND, FOLLOW THE TROUBLE MAINTENANCE PRACTICE AS OUTLINED IN SECTION AHEAD:

30

8.4 Normal Maintenance Practice

.Purge the equipment with air for a minimum of five minutes. The atmosphere inside the equipment shall be monitored with an oxygen monitor and have a reading of 19.5% oxygen content or greater. An air blower shall be utilized whenever personnel are working inside the tank..Protective clothing is not necessary for this procedure except to maintain cleanliness of the internal parts of the equipment.

8.5 Trouble Maintenance Practice

IF SIGNIFICANT ARC POWDERS OR NOXIOUS ODORS ARE PRESENT WHEN

OPENING APPARATUS, A RESTRICTED SAFETY ZONE INTO WHICH PERSONNEL

ARE NOT ALLOWED WITHOUT PROTECTIVE GEAR SHOULD IMMEDIATELY BE

ESTABLISHED AROUND THE EQUIPMENT.

1. Personnel shall wear full protective clothing and a supplied air full-face respirator when

entering and cleaning the equipment.

2. Since particles may become airborne, DO NOT PURGE EQUIPMENT WITH AIR FROM A

BLOWER.

3. Powdered arc products should be removed by use of a dedicated vacuum cleaner equipped

with a special in-line filter.

4. Powdered arc products that remain after vacuuming should be wiped up with rags using

approved cleaning solvents.

5. After arc products are removed, proceed as per Normal Maintenance Practice.

8.6 Filling of SF6 Equipment after Servicing

31

1. Connect gas reclaimer to equipment to be filled.

2. Pull vacuum on lines connected to equipment, shut down, check for leaks.

Repair leaks if found.

3. Check manufacture instructions for length of vacuum and desired level.

4. Start vacuum pump and open equipment fill valve. Pull vacuum until specifications are

achieved.

5. Fill the equipment according to the manufacture instructions for the gas reclaimer you are

using. Fill equipment to the desired level as indicated by the SF6 Pressure Curve Chart.

6. Allow a temperature equalization period, check again and correct SF6 pressure if necessary.

8.7 Handling Contaminated SF6 Gas :

1. Follow manufacturer instructions and remove contaminated SF6 gas from equipment to gas

reclaimer.

2. Follow the manufacturer instructions for the gas reclaimer and cycle the SF6 gas through the

purification loop of the gas reclaimer, if equipped.

3. After adequate recycling of the SF6 gas, obtain samples and send them to the lab for analysis.

Results of the analysis will determine if the SF6 acceptable for reuse or will need to be properly

disposed.

4. If the SF6 gas is acceptable for reuse, follow the steps listed in section 9.0.

5. If the SF6 gas is unacceptable for reuse, the gas will need to pumped into cylinders and

disposed of through an approved contractor.

8.8 Disposal of Arc Products

32

1. Disposal of solid arc products should be done outdoors since corrosive or toxic gases may

evolve from the solid arc products or from the absorption filters. Full protective equipment

should be worn during disposal.

2. Place materials used and retrieved in clean-up operations in a 55-gallon drum.

3. Fill the drum with water 1.5 times that above the contents of the container. Fill quickly for the

first half of the water and slowly thereafter. Chemical reactions will produce some heat and

foam.

4. Measure a quantity of soda ash (sodium carbonate) or lime (calcium oxide) equal to one-

quarter the volume of the refuse material. Add this to the refuse container and mix thoroughly.

Add to this and mix in all other contaminated disposal material including:

♦ cleaning rags

♦ filters from respirators

♦ Vacuum filter elements

♦ Disposable protective clothing

5. Allow the refuse container to stand uncovered for 24 hours.

6. Test the pH of the refuse mixture. The mixture should be at least slightly alkaline (>7). If the

pH is between 7 and 10, the refuse is now rendered harmless and may be disposed of in a normal

manner.

7. If the solution is less than 7, it is still acidic and harmful; add additional soda ash or lime to

neutralize the refuse mixture to a pH greater than 7 but less than 10.

8.9 Removal of SF6 gas residue from tools and Equipment

33

1. Prepare a mild (10%) lime or soda ash solution and use as the neutralizing solution.

2 .Wash hand tool, non-disposable protective gear, and equipment employed in the clean-up

operation.

3. Vacuum some of this neutralizing solution through the hose and into the vacuum cleaner to

neutralize particles adhering to this equipment.

4. Rinse all washed tools and equipment thoroughly with water.

5. Wash rubber boots and Neoprene gloves in water.

-------- References --------

34

Books and journals

1. McDonald, John d., Electric power substations, CRC Press, published in 2003

2. Rao, s., Substation engineering

3. ABB Substation Manuals

4. IEEE journals

Websites

1. www.google.co.in2. en.wikipedia.org/wiki/Electrical_substation3. www.answers.com/topic/gas-insulated-substation4. www.abb.com/industries

35