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Collection Technique

Cahier technique no. 188

SF6properties, and use

in MV and HV switchgear

D. Koch


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"Cahiers Techniques" is a collection of documents intended for engineersand technicians, people in the industry who are looking for more in-depthinformation in order to complement that given in product catalogues.

Furthermore, these "Cahiers Techniques" are often considered as helpful"tools" for training courses.They provide knowledge on new technical and technological developmentsin the electrotechnical field and electronics. They also provide betterunderstanding of various phenomena observed in electrical installations,systems and equipments.Each "Cahier Technique" provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

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Cahier Technique Schneider Electric nO.188/ p.1

n188

SF6properties, and usein MV and HV switchgear

E/CT 188updated February 2003

D. KOCH

Graduate Engineer IEG Grenoble in 1979.

He joined the same year Merlin Gerin Group, in HV transmission: firstas technical manager of MV circuit breakers and, later, as productmanager.Since 1995, within the T & D organisation, he is in charge, inside thestrategic marketing team, of standardisation, technology andenvironment related with T & D offer.He is also in charge of geographic development in Eastern Europe.


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SF6properties, and use

in MV and HV switchgear

Contents

1 Introduction 1.1 A brief history of use of SF6 p.4

1.2 SF6 manufacture p.5

1.3 Other uses of SF6 p.5

2 Physical and chemical properties of SF6 2.1 Physical properties p.6

2.2 Chemical properties p.8

3 Overview on SF6switchgear 3.1 MV and HV switchgear p.11

3.2 SF6consumption and switchgear quantities p.12

3.3 EDF experience: 20 years of SF6in MV p.13

3.4 Future trends p.13

4 Use and handling of SF6in switchgear 4.1 Filling with new SF6 p.14

4.2 Leakage from SF6- filled equipment p.15

4.3 Maintenance of SF6- filled equipment p.15

4.4 End of life of SF6 - filled equipment p.16

4.5 Abnormal situations p.16

4.6 SF6and the global environment p.19

5 Conclusions p.21

Appendix 1: bibliography p.22

Thegeneral properties of SF6gas and its SF6by-products are presented.

A brief history of the use of SF6 in switchgear is given. The effects of SF6on the environment are discussed. Guidance is given for working with SF6gas and SF6 filled equipment under normal and abnormal conditions of

service.

The content of the present document is based on the technical report

IEC 1634, entitled the use and handling of SF6 HV Switchgear andcontrolgear.


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1 Introduction

1.1 A brief history of use of SF6

Sulphur hexafluoride was first synthesised in thelaboratories of the Facultde Pharmacie deParis in 1900 by Moissan and Lebeau. Fluorine,obtained by electrolysis, was allowed to reactwith sulphur and a strongly exothermic reaction,giving rise to a remarkably stable gas.

Gradually thereafter the physical and chemicalproperties of the gas were established, withpublications by Pridaux (1906), Schlumb andGamble (1930), Klemm and Henkel (1932-35)and Yest and Clausson (1933) concerning

particularly the chemical and dielectric properties.The first research into industrial applications wasby the General Electric Company in 1937 whorealised that the gas could be used for insulationin electrical plant. In 1939 Thomson-Houstonpatented the principle of using SF6 for insulatingcables and capacitors.

Immediately after the 2nd world war, publicationsand applications began to appear in rapidsuccession:

c towards 1947, work on transformer insulation,

c development of industrial manufacture of SF6in the USA in 1948 by Allied Chemical

Corporation and Pennsalt,c large scale commercialisation of SF6manu-facture for use in electrical plant construction

towards 1960 in the USA and in Europe,coinciding with the appearance of the first SF6circuit-breakers and switches at High Voltage HVand Extra High Voltage EHV.At Merlin Gerin, the research work, concerning theuse of SF6 gas for insulation and circuit-breakingwas initiated around 1955. This concides withthe appearence of the first industrial products inthe U.S.

The first industrial applications from Merlin Gerinwere at EHV, then in Medium Voltage MV

applications following:c 1964: the first SF6 - insulated substation,ordered by EDF and put into service in the Parisregion in 1966.

c 1967: the FA circuit-breaker was launchedand progressively replaced the compressed airequipment which had established its position inFrance and elsewhere during the previous25 years.

c 1971: changes in the needs of the industry ledMerlin Gerin to launch the Fluarc SF6 MV circuit-breaker.

c More recently, SF6 has been adopted for use

in MV switches, ring main units, contactors andcircuit-breakers, GIS, covering all the needs ofthe elecrical distribution industry.

fig. 1 : process of SF6production by direct combination. The purification chain is necessary to obtain high purity

gas. The quality of SF6 for delivery is defined by IEC publication 376 which specifies the admissible concentrations

of impurities.

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flow

pump

alcaline

solution

storing

gazometer

storing

bottle

compressor

drying towers

combustion

reactormelted

suffer


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1.2 SF6 manufacture

The only industrial process currently in use isthe synthesis of sulphur hexafluoride byallowing fluorine obtained by electrolysis toreact with sulphur according to the exothermicreaction:

S + 3F2 ! SF6+ 262 kcal

During this reaction, a certain number of otherfluorides of sulphur are formed, such as SF4,SF2, S2F2, S2F10, as well as impurities due to thepresence of moisture, air and the carbon anodesused for the fluorine electrolysis. These by-products are removed by various purificationprocess (see fig. 1 ).

1.3 Other uses of SF6

The unique properties of SF6have led to itsadoption for a number of industrial and scientificapplications including, for example:

c medical applications: electrical insulation inmedical equipment (eg X-ray machines), orsurgery,

c electrical insulation in scientific equipment(electron microscopes, particle accelerators suchas Van der Graf generators),

c acoustic insulation in double glazed windows,

c as a tracer gas for studying airflow inventilation systems (for instance in mines) or inthe high atmosphere,

c as a tracer for leak detection in pressurisedsystems,

c to provide a special atmosphere formetallurgical processing (aluminium andmagnesium) or for military purpose.


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2 Physical and chemical properties of SF6

2.1 Physical properties

fig. 2 : main physical characteristics of SF6 at

atmospheric pressure and a temperature of 25C.

SF6is one of the heaviest known gases (seefig. 2 ). Its density at 20C and 0,1 Mpa (that isone atmosphere) is 6.139 kg/m3, almost fivetimes higher than that of air. Its molecular weight

fig. 3 : vapour pressure curve and lines of equivalent gas density of SF6.

0.13

0.12

0.11

0.10

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.010.2

50

Temperature in C

Pressure in MPa Density in kg/l

30 10 +10 +30 +50 +70 +90 +110 +130

0

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

is 146.06. It is colourless and odourless. SF6does not exist in a liquid state unless pressurised.

Equation of state

Sulphur hexafluoride gas having an equation ofstate of the Beattie-Bridgeman type, up totemperature of about 1200 K, behaves as aperfect gas:

pv2 = R T (v + b) - a where:

p = pressure (Pa)v = volume (m3.mol-1)

R = ideal gas constant (8.3143 J.mol-1.K-1)T = absolute temperature (K)a = 15.78 x 10-6(1 - 0.1062 x 10-3v-1)b = 0.366 x 10-3(1 - 0.1236 x 10-3v-1)

Pressure/temperature relation

The variation of pressure with temperature islinear and relatively small in the range of servicetemperatures (-25 to + 50C), (see fig. 3 ).

Density 6.14 kg m-3

Thermal conductivity 0.0136 W m-1K-1

Critical point:

cTemperature 45.55C

cDensity 730 kg m-3cPressure 3.78 MPa

Sound velocity 136 m s-1

Refractive index 1.000783

Formation heat -1221.66 kJ mol-1

Specific heat 96.6 J mol-1 K-1


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Specific heat

The volumetric specific heat of SF6is 3.7 timesthat of air. This has important consequences forreducing the effects of heating within electricalequipment.

Thermal conductivity

The thermal conductivity of SF6is below that ofair but its overall heat transfer capability, inparticular when convection is taken into account,is excellent, being similar to that of gases such ashydrogen and helium and higher than that of air.

At high temperatures, the thermal conductivitycurve of SF6(see fig. 4) reveals one of the excep-tional qualities of the gas, which allows it to beused for extinguishing arcs by thermal transport.The peak of the thermal conductivity correspondsto the dissociation temperature of the SF6mole-cule at 2100 to 2500 K. The dissociationprocess

absorbs a considerable amount of heat which isreleased when the molecules reform at theperiphery of the arc, facilitating a rapid exchangeof heat between the hot and cooler regions.

009

Thermal conductivity

(W cm-1K-1)

Temperature 103 K

SF6

N2

008

007

006

005

004

003

002

001

0 2 4 6 8 10 12 14 16

fig. 4 : thermal conductivity of SF6 and nitrogen.

Electrical Properties

The excellent dielectric properties of SF6are dueto the electronegative character of its molecule.It has a pronounced tendency to capture freeelectrons forming heavy ions with low mobilitymaking the development of electron avalanchesvery difficult.

The dielectric strength of SF6is about 2.5 timeshigher than that of air under the same conditions.The advantage of SF6over nitrogen as a dielectricis clearly illustrated by the curve (see fig. 5 ).

fig. 5 : breakdown voltage as a function of the pd

product between two spheres of 5 cm diameter.

300

250

200

150

100

Breakdown voltage

(kV)

pd

pd product:

p: pressure (MPa)

d: distance between the electrodes (10-3m)

SF6

N2

50

1 2 3 4 5

For non-uniform fields (see fig. 6 ) a maximumbreakdown voltage is obtained at a pressure ofabout 0,2 MPa.

fig. 6 : breakdown voltage as a function of the

pressure for a non-uniform electrical field.

200

160

120

80

Breakdown voltage

(kV)

p

(MPa)

SF6

N240

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7


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Because of its low dissociation temperature andhigh dissociation energy, SF6is an excellent arcquenching gas.

When an electric arc cools in SF6, it remainsconductive to a relatively low temperature, thus

minimising current chopping before current zero,and thereby avoiding high overvoltages.

For more information, consult Merlin GerinCahier Technique n145.

Figure 7lists the main electrical characteristicsof SF6.

Sonic characteristics

The speed of sound in SF6is one third of that inair, making SF6 a good phonic insulator.

fig.7 : main electrical characteristics of SF6.

2.2 Chemical properties

Sulphur hexafluoride fully satisfies the valencyrequirements of the sulphur molecule. Itsmolecular structure is octahedral with a fluorinemolecule at each apex.

The effective collision diameter of the SF6molecule is 4.77 . The six bonds are covalentwhich accounts for the exceptional stability ofthis compound.

c SF6can be heated without decompositionto 500C in the absence of catalytic metals.

c SF6is non-flammable.

c Hydrogen, chlorine and oxygen have no action

on it.c SF6is insoluble in water.

c It is not attacked by acids.

In its pure state SF6has no toxicity and this isregularly confirmed on new gas prior to delivery,by placing mice in an atmosphere of 80% SF6and 20% oxygen for a period of 24 hours(biological essay recommended by IEC 376).

Arc decomposition products

In an electric arc, the temperature can reach15,000 K and a small proportion of SF6isdecomposed. The decomposition products areformed in the presence of:

can electric arc formed by the opening ofcontacts normally comprising alloys based ontungsten, copper and nickel, containing residualquantities of oxygen and hydrogen,

c impurities in the SF6such as air, CF4 andwater vapour,

c insulating components comprising plasticmaterials based on carbon, hydrogen and silica.

c other metallic or non-metallic materials fromwhich the equipment is constructed.

The above explains why the solid and gaseousdecomposition products contain, in addition tofluorine and sulphur, elements such as carbon,silicon, oxygen, hydrogen, tungsten, copper etc.

The principle gaseous by-products, identified bylaboratories which have worked on this subject,combining gas-phase chromatography withmass spectroscopy, are:

v Hydrofluoric acid HF

v Carbon dioxide CO2v Sulphur dioxide SO2v Carbon tetrafluoride CF4v

Silicon tetrafluoride SiF4v Thionyl fluoride SOF2v Sulphuryl fluoride SO2F2v Sulphur tetrafluoride SF4v Disulphur decafluoride S2F10.

Certain of these by-products can be toxic, but itis very easy to adsorb most of them usingmaterials such as activated alumina or molecularsieves.Certain also are formed in extremelyminutequantities (S2F10).

If the adsorbent (molecular sieve or activatedalumina) is present in the equipment in sufficientquantity, then the level of corrosion due to SF6

decomposition products (HF in particular) is veryslight if not negligible. This is due to the fact thatthe adsorbents have a very rapid and effectiveaction such that the corrosive gases do nothave sufficient time to react with other materialspresent.

Nevertheless, in order to avoid any risk, MerlinGerin has prohibited the use of certain materialsand constituents which have shown signs ofdegradation after long tests at high pollutionlevels, without adsorbents present.

Breakdown field relative to pressure 89 V m-1Pa-1

Relative dielectric constant 1.00204

at 25C and 1 bar absolute

Loss factor (tan") < 2 x10

-7

at 25C and 1 bar absolute

Ionisation coefficient #=A p E/(p - B)

#: (m-1)

E: (V m-1)

p: (Pa)

A=2.8 x10-2 V2

B=89 V m-1Pa-1


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Injection

CF4

SiF4

SF6

SOF2

CO2

SO2F2

SOF4

H2O

air SO2

fig. 8 : analysis of gases sampled from equipment.

a- Chromatogram without absorbent

b- Chromatogram with absorbent (molecular sieve)

Injection

CF4CO2

SF6

SO2

air

SOF2

Analysis of gas taken from equipment

Numerous aspects can be studied throughanalysis of the gas and its decompositionproducts. Here we will consider only the influenceof adsorbents, specifically a molecular sieve.

Chromatogram ainfigure 8shows the results ofanalysis of gas taken from a prototype polewithout any adsorbent.Chromatogram bin figure 8shows the results ofanalysis of gas from an identical pole, subjectedto the same electrical stresses, but with amolecular sieve fitted.

Whilst it is possible therefore for the inhaledatmosphere to contain a high proportion of SF6without harmful effects, a maximumconcentration of 1,000 ppmv (6,000 mg/m3)has been established for places of work inwhich personnel spend up to eight hours perday, five days per week.

This TLV (Threshold Limit Value) is thatcommonly used for harmless gases not normallypresent in the atmosphere.

Pure SF6has no ecotoxic, mutagenic, orcarcinogenic (neither genotoxic nor epigenetic)

effects on health.When handling new SF6, therefore, it isnecessary only to adopt procedures whichensure that the specified maximumconcentration is not exceeded.

Owing to the manufacturing process,commercially available SF6is not perfectly pure.The permitted levels of impurities are laiddown in IEC Standard 376. These are shownin figure 10 .

fig. 10 : maximum permitted impurity levels in

new SF6.

fig. 9: results of analysis of SF6 from circuit-breakers

with and without molecular sieves.

The table infigure 9 allows the quantities ofgaseous decomposition products in the twocases to be compared. The effectiveness of theadsorbent is clearly apparent.

Health characteristics of pure SF6

Pure SF6is non toxic and biologically inert. Testsperformed with animals have shown that whenpresent in a concentration of up to 80% SF6to20% O2, no adverse effects are experienced.

Gas No adsorbent With absorbent

(%) (molecular sieve)

(%)

Air 0.17 0.03CF4 2.83 2.80

SiF4 2.88 0.25

CO2 0.24

SF6 remainder remainder

SO2F2 0.12

SOF2 3.95 trace

H2O + HF 0.20 0.05

SO2 2.90 trace

Impurity Maximum permitted

CF4 500 ppm weight

O2, N2 500 ppm weight

Water 15 ppm weight

HF 0.3 ppm weight

Hydrolysable fluorides 1.0 ppm weight

expressed as HF


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Assessment of risk to health posed byarced SF6

The level of risk to health due to exposure toused SF6depends on a number of factors:c the degree of decomposition of the SF6and

the types of decomposition products present,c the of dilution of used SF6in the localatmosphere,c the time during which an individual is exposedto the atmosphere containing used SF6.

Definition of TLV Threshold Limit Value

Potentially toxic gases are assigned a valueknown as TLV, which is expressed as a concen-tration in the air, normally in parts per million byvolume (ppmv). The TLV is a time-weightedaverage concentration at which no adversehealth effects are expected, for exposure during8 hours per day, for up to 40 hours per week.

Assessment of toxicity using SOF2concentration

Although used SF6contains a multi-componentmixture of chemical agents, one particularconstituent has been shown to dominate indetermining the toxicity. This is the gaseousdecomposition product thionyl fluoride SOF2.The dominance of this component results fromits high production rate (formed volume in l perarc energy in kJ) relative to those of the otherdecomposition products, combined with itstoxicity rate. The TLV for SOF2is 1.6 ppmv.

SOF2may further react with water, leading tothe generation of sulphur dioxide SO2 andhydrofluoric acid HF: however due to similarconcentration and TLV values, the overalltoxicity effect is similar, for SOF2or the productsof its hydrolisis.

Table of figure 11compares the threedecomposition products:

c Thionyl fluoride SOF2

c Sulphuryl fluoride SO2F2

c Disulphur decafluoride S2F10

The first two are the most abundant decompo-sition products of arcing in SF6whereas thelatter is estimated to be the most toxic.

To have a toxic effect, a chemical agent must bepresent in sufficient quantity relative to its TLV.The risk indexin the table gives an indicationof the relative contributions of the three decom-

position products to overall toxicity. In a typicalsample of arced SF6 , the contribution to toxicitydue to SOF2outweighs that due SO2F2by about200 times, and that due to S2F10by about10,000 times. S2F10 can clearly be neglected, ascan SO2F2.

In section 4, the quantities of SOF2 producedunder various circumstances will be calculatedand used to assess the levels of risk topersonnel, taking into account the degree ofdilution of the used SF6 in the local atmosphereand the likely exposure time.

fig. 11 : comparison of three SF6decomposition products from power arcing.

Thionyl Fluoride Sulphuryl Fluoride Disulphur decafluoride

SOF2 SO2F2 S2F10

Production rate (l/kJ) 3.7 x 10-3 0.06 x 10-3 2.4 x 10-9

TLV (ppmv) 1.6 5 0.01

Production rate 1 0.016 0.65 x 10-6

relative to SOF2: Pr

Toxicity 1 0.32 160

relative to SOF2: Tr

Risk index: Pr x Tr 1 5.12 x 10-3 0.104 x 10-3


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3 Overview on SF6switchgear

3.1 MV and HV switchgear

As mentionned above, switchgear manufacturersuse the unique dielectric or/and breakingproperties to design their equipment. The mainuse of SF6 is in MV and HV switchgear: the placeof SF6 can be globally summarized as detailedin figure 12 .

fig. 12 : the place of SF6in switchgear.

In HV field, SF6 will be soon, on a worldwide

basis, the single technology used for GIS(see fig. 13) and CB design (see fig. 14) ; theold oil or compressed air technologies aredisappearing, due to the numerous advantagesrelated to SF6.

fig. 13: GIS (Merlin Gerin)

fig. 14 : circuit-breaker for HV substation equipment

(SB6 circuit-breaker - Merlin Gerin).

In MV field, when compact switchgear isrequired, SF6is the single proposed solution(GIS, RMU) (see fig. 15 and 16 ). For loosecomponents however, SF6technology is sharing

the market with air for LBS Load BreakSwitches: but air market share is rapidlydecreasing to the benefit of SF6, and withvacuum for CBs. Vacuum and SF6 CBs aremodern solutions, and will continue to expanddue to the decreasing influence of minimum oiltechnology.

Purpose Switchgear MV HV

($52 kV) (> 52 kV)

Insulation GIS + + + + + +RMU + + + NA

Breaking CB + + + + +

LBS + + + + +

Worldwide market share:

Low + GIS Gas Insulated Switchgear

Medium ++ RMU Ring Main Unit

High +++ CB Circuit Breaker

LBS Load Break Switch


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To be complete in terms of application, we have

to mention SF6 insulated transformers, mainlypopular in Japan, and HV gas insulated cables,which are quite similar to HV GIS technology.

fig. 16: SM6 (Merlin Gerin)

3.2 SF6consumption and switchgear quantities

Worldwide SF6consumption is shared betweenswitchgear and non electrical applications:IEC estimates a total annual consumption of5-8000 tons, divided into 2 more or lessequivalent parts between these two applications.

In order to understand the relative proportionof SF6 in MV and HV field, it is usefull toestimate the worldwide total installed parc andthe quantities annually put into service (see tableoffigure 17).

fig. 17 : size of MV and HV SF6markets.

b- HV switchgear

a- MV switchgear

fig. 15: equipment for MV distribution for ring main

distribution system ; the output, protected by the circuit

breaker is placed in the centre (RM6 - Merlin Gerin).

SF6mass Total installed parc Annual additional

per unit installed switchgear

(kg) quantity SF6mass quantity SF6mass

(tons) (tons)

GIS 500 20,000 10,000 3,000 1,500

Open type CBs 50 100,000 5,000 8,000 400

Gas insulated 30,000 m 1,000 3,000 m 100

cables

Total 2,0000 %2,000

SF6mass Total installed parc Annual additional

per unit installed switchgear

(kg) quantity SF6mass quantity SF6mass

(tons) (tons)

RMU/ 0.6 3,000,000 1,850 240,000 RMU 140

switches + 70,000 switches 8

GIS 6 50,000 300 7,000 42

CB 0.3 500,000 150 40,000 17

Total 2,000 - 2,500 %200


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3.3 EDF experience: 20 years of SF6in MV

EDF is probably the only electricity distributioncompany having such an experience:20 years of SF6 experience in MV CBs andswitches,with aMerlin Gerin installed parc ofmore than 20,000 CBs and 200,000 switches(of modular or compact type) in 1995.

EDF has recently carried out a complete checkup of some of the oldest apparatus, having thehighest number of operations: tests such as shortcircuit tests, dielectric tests, temperature risetests, tightness measurements and mechanicalendurance, contact wear mesurements, gas

analysishave been made, in order to estimatethe probable remaining life.

The results of these tests have been publishedin IEE 1994 and CIREC 1995 conference. More

precisely concerning CB wear estimation, contactwear measurements showed a maximum valueof 25%; gas analysis has been used in order toevaluate the potential toxicity in two differentcases: standard leakage and abnormal suddenrelease due to a fault:

c In case of a standard leakage, concentration ofby-products in the switchgear are extremely low(of the order of 10,000 times lower than the TLV).

c In the the second case, which occurs excep-tionnaly rarely, the by-products concentrationsremain far below any danger limit.

As a consequence, it has been demonstrated,through actual service conditions on site,that the prospective life duration of SF6switchgear is at least 30 years.

3.4 Future trends

SF6 switchgear fully complies with customerrequirements, in terms of compactness, reliability,reduced maintenance, personnel safety, lifedurationWe may expect that in the future, themechanical energy will continue to decrease due

to the mastering of new breaking principles suchas combination of rotating arc and self expansion(see fig. 18) leading to higher reliability (seeCahier Technique n171). Filling pressures willalso decrease, contributing to optimumpersonnel safety. Concerning maintenance, wecan foresee diagnostic features, being able tomonitor, in real time, the status of the apparatusand being able to give the information to theuser, when maintenance has to be provided.

As far as tightness is concerned, a probabledevelopment of HV switchgear can be expectedwith lower leakage rates such as 0,1 to 0,5% peryear. CIGRE WG 23.10 is working on a SF6recycling guide, covering purity definition for SF6to berensed in power equipment. By this way,we are considering the use of SF6in a closedcycle, which will contribute to minimise itsemission in the atmosphere.

fig. 18: Merlin Gerin LF circuit breaker.

It clearly shown in these tables that the use ofSF6is predominant in HV: 90 % of thetotal SF6 volume in switchgear is for HV use,

and only 10 % for MV. In MV, SF6consumptionconcerns mainly RMU/switches and GIS, thepart of MV CBs being almost negligeable.


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4 Use and handling of SF6in switchgear

This section has been included to respondto users concerns regarding the possibleeffect with SF6gas and its decomposition

products to personnel safety or environment.For more details, please refer to the IEC 1634technical report.

4.1 Filling with new SF6

New SF6is supplied in cylinders as a liquid.The pressure of SF6 above the liquid is about22 bar gauge. New SF6 should comply withIEC 376 which defines limits for impurityconcentrations.

New SF6 may be handled outdoors without anyspecial provisions. When working indoors withnew SF6, the following should be considered:

c The TLV for new SF6 is 1,000 ppmv whichmeans that workers may be exposed toconcentrations up to this level during eight hoursper day for five days a week. The TLV is notbased on potential toxicity, rather it is a valuewhich is assigned to gases which are not alreadypresent in the atmosphere.

c Temperatures above 500C, or the presenceof certain metals above 200C, provoke thedecomposition of SF6. At threshold temperaturesthis decomposition can be extremely slow. It is

therefore advised that there be no smoking,

naked flames, welding (except in a neutralatmosphere) or other sources of heat that mayapproach these temperatures, in areas whereSF6may be present in the atmosphere.

c The normal precautions associated with pres-

surised cylinders of gas should be exercised. Forexample, a sudden release of compressed gaswill give rise to low temperatures which can causerapid freezing. Workers manipulating equipmentwhere this could occur should wear thermallyinsulating gloves.

c When equipment is to be filled with new SF6 ,manufacturersinstructions should be followed inorder:

v to be sure that the quality of the SF6 inside theequipment is adequate,

v to eliminate any risk of over-pressurising theenclosure which is being filled.Furthermore spillage of SF6to the atmosphere

during filling should be avoided.

fig. 19: leakage of SF6 in a high voltage (145 kV) indoor substation.

HV case studied, conditions and calculations

A Volume of room (m3) 700

B Volume of CB enclosure (m3) 0.5

C Filing pressure (Mpa) 0.5

D Volume of SF6/CB (m3) B x C 0.25

E No of CBs 7

F CB fault rating (kA) 31.5

G Arc voltage (V) 500

H Arc duration (s) 0.015

I Arc energy (1 break/1 pole) (kJ) F x G x H 236.25

J Arc energy (3 breaks/3 poles) (kJ) 9 x I 2,126

K SOF2production rate (I/kJ) 3.7 x 10-3

L Quantity of SOF2produced in CB (I) K x J 7.87

M Total quantity of SOF2(I) E x L 55.07

N Typical leak rage (% year) 1

P Volume of SOF2 released in 1 year (I) M x N/100 0.55

Q Concentration after 1 year (ppmv) P/A x 103 0.79


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4.2 Leakage from SF6- filled equipment

This section examines the implications ofleakage of SF6 gas and gaseous decompositionproducts into an indoor environment. The

potential toxicity of the atmosphere iscalculated using the concentration of thionylfluoride SOF2.

Two case studies (one high voltage and onemedium voltage) are presented. In both cases,the following worst case assumptions are made:

c the switchgear room is sealed from theexternal atmosphere, ie there is no ventilation,

c the switchgear room containsrespectively 7 and 15 circuit breakers,

c the effects of adsorbents in switchingenclosures are neglected,

fig. 20: leakage of SF6 in a medium voltage (12 kV) indoor substation.

c the circuit-breakers (CBs) have each interruptedtheir rated fault current three times,

c SOF2is produced at a rate of 3.7x10-3litres

per kJ of arc energy, (value most frequentlycited by the scientific press).

Tables offigures 19 and 20summarise the dataand the calculations in each case.

The results show that in both cases, the TLV forSOF2(1.6 ppmv) is not exceeded after 1 year ofleakage at the maximum leak rates into a sealedvolume. In reality, normal ventilation would furtherdilute the SOF2leading to a negligible SOF2concentration.

There is hence no health risk associated withnormal leakage of used SF6from switchgear.

4.3 Maintenance of SF6- filled equipment

Sealed-for-life MV equipment requires nomaintenance to parts inside SF6enclosures:they are hence not concerned by this section.

Certains designs of MV GIS may require main-tenance and most types of HV circuit breakersare designed to be be maintained periodically.Extending a GIS switchboard, both MV and HV,may require the removal of the SF6.

Many local codes of practice and manufacturersrecommendations exist to advise safe workingpractises under these circumstances. The fol-

lowing guidelines are common to many of these:

c Care should be taken when removing SF6which may contain decomposition products. Itshould not be released in an indoor environmentand should be collected in a storage vessel. Careshould be taken not to breathe SF6 removed froman item of equipment. If this possibility cannot beavoided, a respirator should be worn. This shouldbe fitted with a filter capable of removing the gasesdescribed in arc decomposition products.

c Enclosures should be evacuated such as toremove as much as possible of the residual SF6.

In some cases, flushing enclosures with dry air

MV case studied, conditions and calculations


B Volume of CB enclosure (m3) 45 x 10-3


D Volume of SF6/CB (m3) B x C 13.5 x 10-3

E No of CBs 15





J Arc energy (3 breaks/3 poles) (kJ) 9 x I 850.5



M Total quantity of SOF2(I) E x L 47.2

N Typical leak rage (% year) 0.1

P Volume of SOF2 released in 1 year (I) M x N/100 0.0472

Q Concentration after 1 year (ppmv) P/A x 103 0.39


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or nitrogen is recommended prior to openingthem. In any case, workers should be aware ofthe presence of residual SF6when enclosuresare first opened and should wear respirators atthat time. Ventilation of the work area should beadequate to remove rapidly any gas releasedinto it.

c Metal fluoride powders are more chemicallyactive in the presence of moisture so effortsshould be made to keep these in a dry condition

prior to and during their removal. Fine powderscan remain suspended in the air for long periodsof time ; where powders are present respiratorsfitted with sub-micron range powder filtersshould be used. Particular attention should bepaid to protecting the eyes.

c Components, metal fluoride powders andadsorbents removed from equipment in serviceshould be packed in sealed containers forsubsequent neutralisation.

4.4 End of life of SF6 - filled equipment

SF6- filled equipment which has been removedfrom service may require treatment to neutraliseany decomposition products remaining after theSF6has been removed. For environmentreasons. SF6has to be removed and not

released to the atmosphere, (see SF6and theglobal environment). The need for neutralisationis dependent on the level of decomposition ;expected decomposition levels are shown in thetable of figure 21.

Prior to treatment, the SF6should be removedand stored for recycling. The equipment shouldthen be treated according to the expecteddecomposition level. After treatment, theequipment can be disposed of as normal waste(respecting local regulations), for example in alandfill site, or subjected to reclamationprocesses to recover the metals. Solutions usedin the neutralisation process can be disposed of

as normal waste.

fig. 21 : expected levels of SF6 decomposition for various equipment types.

Low decomposition:

No special action is required.

Medium and high decomposition:

The internal surfaces of gas enclosures should beneutralised using a solution of lime (calciumhydroxide) prepared by diluting 1 kg of limein 100 litres of water. The enclosure to be treatedshould, if possible, be filled with the lime solutionfor a period of 8 hours before emptying. If it issubsequently necessary to work on the enclosure,it should first be rinsed with clean water.

For larger enclosures where filling is impractical,powders should be removed, using a vacuumcleaner equipped with a suitable filter, forneutralisation separately. The internal surfacesshould then be washed with the lime solutionwhich should be left in place for at least one hour

followed by rinsing with clean water.

4.5 Abnormal situations

This section deals with the assessment of risk topersonnel in the case of an abnormal situationleading to an uncontrolled release of SF6gasinto the atmosphere. Such situations, whichoccur only very infrequently, are:cabnormal leakage, due to a failure of theSF6 enclosure seals to contain the gas,

cinternal fault, resulting from uncontrolled arcinginside the SF6enclosure,

cexternal fire, giving rise to abnormal leakage.

Abnormal leakage

The method of risk assessment is similar to thatused in section leakage from SF6- filled

Application Expected degree of SF6 decomposition

GIS disconnector Medium voltage load-break switch cfrom zero to a few tenths of a percent

and ring-main unit cno visible powder deposit

Medium voltage and high voltage circuit-breaker medium:

cup to a few percent

clight powder deposits

Any enclosure in which abnormal arcing has occured high:

ccould exceed 10%

cmedium to heavy powder deposits


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equipmentwhich deals with normal leakage.The same case study data will be used for highand medium voltage situations. In the calculationsthat follow it is assumed that all of the SF6in onecircuit-breaker escapes suddenly and it is againassumed that the switchroom is sealed and thatventilation is inoperative. (see fig. 22).

Thus if all of the SF6were to escape into theswitchroom, the concentration of SOF2 wouldreach 9.0 ppmv, or about 6 times the TLV.

In practice, attention would be drawn to anabnormal leak by underpressure detectors fitted

fig. 22: abnormal leakage of SF6in a high voltage (145 kV) indoor substation.

to the CBs. These normally operate around 80%of normal filling pressure, and at this point in time,only 20% of the available SF6 would be in theatmosphere, leading to an SOF2 concentrationof 1,8 ppmv.

In the case of abnormal leakage of SF6in amedium voltage indoor substation (see fig. 23),the SOF2 related to complete escape of SF6fromone CB would reach 17,5 ppmv.

The concentration of SOF2 in the switchroom at20% relative pressure loss (commonly the alarmlevel) would be 3.5 ppmv.

HV Case studied, conditions and calculations

fig. 23 : abnormal leakage of SF6in a medium voltage (12 kV) indoor substation.

MV Case studied, conditions and calculations


B Volume of CB enclosure (m3) 0.5


D Volume of SF6/CB (m3) B x C 0.25

E No of CBs 7





J Arc energy (3 breaks/3 poles) (kJ) 9 x I 2,126



M Filling pressure relative to atmospheric (Mpa) C - 0,1 0.4

N Amount of SOF2which could leak (I) L x M/C 6.3

P Switchroom SOF2 concentration (ppmv) N / A 9.0


B Volume of CB enclosure (m3) 45 x 10-3


D Volume of SF6/CB (m3) B x C 13.5 x 10-3

E No of CBs 15





J Arc energy (3 breaks/3 poles) (kJ) 9 x I 850.5



M Filling pressure relative to atmospheric (Mpa) C - 0,1 0.2

N Amount of SOF2which could leak (I) L x M/C 2,098

P Switchroom SOF2 concentration (ppmv) N / A 17.5


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In both cases above, the TLV for SOF2(1.6 ppmv)may be exceeded, but by a relatively smallfactor. Under these circumstances exposure fora short period would present negligible risk. Thepungent, unpleasant odour of SOF2is noticeablefrom concentrations of around 1 ppmv and thiswould mean that, for most people, attentionwould be drawn immediately to concentrationsapproaching the TLV. Smell is however notrecommended as a detection method.

Internal fault

An internal fault can develop when an arc isformed abnormally between the main conductorsof an item of switchgear, or between a mainconductor and an earthed conducting part. Suchfaults occur very rarely. Abnormal arcing givesrise to a rapid increase in pressure which cancause hot gases and other materials to beexpelled. Whilst an internal fault could develop in

any high-voltage enclosure, this section isconcerned with internal faults within enclosuresfilled with SF6. There are three possibilities forsuch faults:

1 -Internal fault which does not lead to anabnormal release of SF6. This can occur whenthe energy delivered to the fault is insufficient tolead to burn-through or pressure relief of theenclosure.

2 -Internal fault where heat from the arc causesthe enclosure wall (usually metallic and formingone arc electrode) to melt or vapourise such thata hole is formed. This type of fault is associatedmainly with high-voltage GIS equipment.

3 -Internal fault where the pressure-rise withinthe enclosure is sufficient to lead to operation ofpressure relief devices. This is controlled by apressure-relief valve or by a well-defined, stressreleasing zone of the enclosure, allowing hotexhaust to be directed.

The risks to personnel associated with faults oftypes 2 and 3 will be considered here. The risksassociated with the use of SF6are evaluatedbased on the quantity of SOF2released into theatmoshere. The potentially harmful effects ofother toxic vapours not related to the use of SF6are also considered ; it will be seen that theseother by-products, which are also present duringan internal fault in any type of equipment, can bethe dominant contributors to toxicity.

The following assumptions are made:

cFor medium-voltage equipment containing smallvolumes of SF6, it is assumed that the majority ofthe gas is expelled from the enclosure within 50ms. This assumption is supported by pressuremeasurements made during internal fault tests.The quantity of SOF2formed is thereforecalculated using a 50 ms production period.

cFor high-voltage equipment a SOF2production

period of 100 ms will be used, because fault-times in high-voltage systems are usually limitedto around 100 ms.

cIt is assumed that the switchroom is closed tothe external environment.

cThe effects of adsorbents are likely to benegligible within the time period of interest.

cAll of the SOF2generated during the fault isreleased into the switchroom. (see fig. 24 & 25).

The results indicate that significant concentrationsof SOF2can be generated within a switchroom.Detailed toxicological data for SOF2areunfortunately not available but it is known thatlarger mammals (rabbits) can withstand exposurefor one hour at concentrations of up to 500 ppmv(see section 4.2.2. of IEC 1634 -1995).

Other potentially toxic substances are producedduring an internal fault including metal and plasticvapours and it can be shown that these inevitable

fig. 24 : internal fault in a high voltage (145 kV) indoor GIS substation.


B CB fault rating (kA) 31.5

C Arc voltage (V) 1,000

D Arc duration (s) 0.1

E Arc energy (2 arcs, phase-phase) (kJ) B x C x D 6,300

F SOF2Production rate (AI electrodes) (I/kJ) 10 x 15-3

G Quantity of SOF2produced in CB (l) E x F 94.5

H Switchroom SOF2 concentration (ppmv) G/A 135

HV Case studied, conditions and calculations


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fig. 25 : internal fault in a medium voltage (12 kV) indoor substation.

products, which are not related to the use of SF6,can dominate when the overall toxicity of theatmosphere is considered. This applies to anytype of switchgear, SF6 filled or otherwise (seesection C 4.7.2. and C 4.7.3 of IEC 1634 -1995).

If, for example during a busbar fault, only10 grammes of copper were evapourated intothe example medium voltage switchroomatmosphere, the concentration (neglecting theeffects of oxidation) would be (mass of Cu / roomvolume) = 83 mg/m3. The TLV for copper vapouris 0.2 mg/m3. This means that the copper vapourconcentration could reach 400 times the TLV.

Similarly, the full vaporisation of only 32 grammesof PVC (equivalent to the insulation from 1.2 mof standard 1 mm2wire) could give rise to anatmospheric concentration of 100 times the TLV(2.6 mg/m3) of vinyl chloride.

It can be therefore concluded that in anyinternal fault situation, corrosive and/or toxicfumes are produced whether or not SF6ispresent. In cases where these fumes enterthe switchroom atmosphere, it has beenshown that non SF6related products arelikely to be the dominant contributors tooverall toxicity. This further strengthens the

view that the use of SF6in switchgear doesnot significantly add to the risks associatedwith an internal fault.

External fire

Fires in outdoor installations rarely causeproblems because of the relative absence offlammable material in the vicinity of theswitchgear. In indoor installations, particularly inthe case of medium voltage consumersubstations, there is a greater risk of fire in theproximity of the switchgear.

Research has shown that fire temperaturesrarely exceed 800C and the temperatures in theregion of SF6 enclosures which are protected bymetal cladding, are likely to be much lower thanthis. A release of SF6 is very unlikely to beprovoked by a fire; if it should occur, averagetemperatures are likely to be too low (becauseSF6 will be rapidly dissipated by convection tolower temperature regions) to lead to significantdecomposition, which requires at least 500C.SF6 is non-flammable and will have anextinguishing effect.

Personnel engaged in fighting a fire will beadequately protected by precautions normallyused against vapours from burning plastics.

4.6 SF6and the global environment

Atmospheric pollutants produced by humanactivity are divided into two major categoriesaccording to their effects:

cstratospheric ozone depletion (hole in theozone layer),

cglobal warming (greenhouse effect).

SF6does not contribute significantly to strato-spheric ozone depletion because it contains no

chlorine which is the main agent in ozonecatalysis, or to the greenhouse effect becausequantities present in the atmosphere are verysmall (see annexe D of IEC 1634 (1995) andElectra n164 (2), 1996).

Schneider Group Policy

Schneider group has set up an environmentalpolicy in order to provide systems and products to


B CB fault rating (kA) 31.5

C Arc voltage (V) 1,000

D Arc duration (s) 0.5

E Arc energy (2 arcs, phase-phase) (kJ) B x C x D 1,575

F SOF2Production rate (Cu electrodes) (I/kJ) 3.7 x 10-3

G Quantity of SOF2produced in CB (l) E x F 5.83

H Switchroom SOF2 concentration (ppmv) G/A 48.6

MV Case studied, conditions and calculations


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help people use electricity safely: actions aretaken in order to protect the environment andpromote its care, not only inside the group, butalso outside, by providing appropriate informationto our customers, suppliers and partners.

As far as SF6handling is concerned, regulationsand qualified procedures have been set up forinternal and customer purpose:

cInternal procedures specify the quality of SF6necessary to guarantee the switchgearperformances, either for dielectric or breakingapplication.

Criteria here are based on IEC 376 standards forpersonnel and environment safety, adequateprocedures and qualified equipment (coupling,pumping) have been chosen to minimize SF6release at each stage (production, site,maintenance, end of life).

cFor the user (customer) relevant information is

displayed in the switchgear leaflet, for differentsituations (normal use, maintenance).However, for personnel safety and environmentalcare reasons, maintenance specifically heavymaintenance, can be carried out by Schneidergroup or under his supervision, in our facilities orspecially designed workshops.

In these procedures, special operations aredescribed, in terms of gas treatment ; A gas whichis not in accordance with the specified valuescan be locally on site treated, by using adequategas cart equipment ; Gas treatment for arcdecomposed gas may also be carried out bySchneider, a patented procedure using lime

solution for neutralisation purpose has beendevelopped in order to answer to customersrequirements, for instance for end of life disposal.This service is supported by Schneider Services.

The existing procedures are applied:

vin our facilities (filling, recyling),

vduring maintenance (rincing, refilling),

vfor end of life disposal.

Ozone depletion

The international community has recognised therisks, due to destruction of the ozone layer, tohealth and the environment.

The ozone (O3) destruction mechanism in thecase of CFCs (chlorinated fluorocarbons) requiresthe presence of free chlorine atoms which arereleased when CFC molecules are exposed toultraviolet radiation. The reactions are:

CFC UV& !&

Cl + CFC remainder (reaction 1)

Cl + O3 UV& !&

ClO + O2 (reaction 2)

ClO + O UV& !&

Cl + O2 (reaction 3)

O + O3UV

& !&2O2 (reaction 4)

CFC molecular bonds are broken by UV irradiationand free chlorine atoms are released (1). Thesereact with ozone to producing ClO, which in turn

reacts with free oxygen, liberating the chlorineatom which can go once more through reaction2.This is termed the catalytic cycle. A singleatom of chlorine can participate in this cycle tenthousand times before it is neutralised by areaction not involving ozone.

SF6however is not photo-decomposed at ozonelayer altitudes (32-44 km), so very little atomicfluorine is released. Any free fluorine which isreleased has a strong tendency to combine withfree hydrogen rather than with ozone. Moreover,the concentration of SF6is 1,000 times less ofthat of the CFCs.

Greenhouse effect

The temperature at the surface of the earth risesduring the day due to solar radiation and fallsduring the night as heat is lost due to infra-redradiation. Some of the infra-red radiation,

particularly in the range of wavelengths between7 and 13 m, is reflected back to the surface ofthe earth and does not escape. The reflectivity ofthe atmosphere at these wavelengths isenhanced by the presence of greenhousegases such as CO2, H2O and O3 and is particu-larly increased by man-made gases such as CO2from the burning of fossil fuels, N2O from intensiveagriculture, CFCs from spray propellants andrefrigerators and CH4 from intensive cattle farming.SF6has infra-red absorbtion characteristics andis considered as a minuscule greenhouse gas,having a very long life duration in the atmosphere ;however, its contribution to global warming isvery small, due to the extremelylowconcentration of SF6in the atmosphere (seefig. 26).

The contribution of SF6is less than one part inten thousand of the total contribution of the otheragents and is thus negligible. However, verylong term environmental considerations make itadvisable to SF6 reclaim,during maintenance orend of life treatment, in order to minimize itsaccumulation in the atmosphere.

fig. 26 : estimated contribution of various gases to the

greenhouse effect.

Gas Concentration Contribution(ppbv)

CO2 353 x 103 60

CH4 1.7 x 103 15

N2O 310 5

O3 10-50 8

CFC-11 0.28 4

CFC-12 0.48 8

SF6 0.002 10-2


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5 Conclusions

The adoption of SF6in switchgear for all operatingconditions has brought advantages in performan-ce, size, weight, global cost and reliability. Thecost of ownership, which includes maintenancecosts, can be much lower than for older types ofswitchgear: for instance, in the case of MV CBs,EDF has reduced the total cumulated mainte-nance time per CB from 350 hours to 30 hours.

Many years of service experience havesubstantiated the assertions made in thisdocument that the use of SF6does notconstitute any threat to personnel or to theenvironment, as long as elementary workingpractices are employed.


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Appendix 1: bibliography

Standards

cIEC TR 1634: High-Voltage Switchgear andControlgear-Use and Handling of SulphurHexafluoride (SF6) in High-Voltage Switchgearand Controlgear First Edition.

cIEC 376: Specification and Acceptance of NewSulphur Hexafluoride First Edition ; (SupplementA-1973) (Supplement B-1974).

Cahiers Techniques Merlin Gerin

cEtude thermique des tableaux lectriques BT,Cahier Technique N145, january 1990 -

C. KILINDJIAN

cBreaking by auto-expansion,Cahier Technique N171, december 1993 -G. BERNARD

Other publications

cCIGRE WG 23-01 Task force 01 (08.96):SF6recycling guide.

cCIRED (1995):Developpement et suivi du palier technique EDFutilisant de lappareillage moyenne tension SF6 -B. JOYEUX, G. PERRISSIN, M. REBOUL

cELECTRA N164 (02.96):SF6 and the global atmosphere (Task force 01 ofWG 23.10).

cIEE (1994):MV switchgear breaking in SF6: the situationafter 20 years in service.


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Edition: Schneider Electric

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