Environmental Effects of Electrically-stressed Sulfur Hexafluoride

High Voltage (HV) equipment such as power switches, current or voltage transformers, and flexible HV transmission lines insulated by pressurized SF6 (or SF6/N2 mixtures) offer component compactness, high reliability and low maintenance demands compared to all conventionally insulating components (i.e. air, organic solid insulants, and mineral oils). Though SF6 insulation for HV applications was initially proposed during late ‘60s, it was spread worldwide rapidly due to offered significant economic advantages, and now SF6 GIS substations dominate the share in electrical networks in densely populated districts. However, it was in mid ‘90s when the first ecological concerns were brought about the SF6 gas use. These mainly stream out by either of the following facts: (i) SF6 is a strong green-house gas with a global warming potential of almost 25,000 greater than that of CO2 and its molecules exhibit an exceptionally high lifetime in earth atmosphere estimated to vary between 750 and 2500 years, and (ii) when electrically stressed (independent of temperature i.e. either high-power arcs developing at 20,000K during the switching actions, or corona discharges developing at 300K due to high electric field effects) toxic byproducts may be formed, some having high cyto-toxicities i.e. S2F10, oxyfluorides, H2S and HF.


Introduction
This work provides information concerning possible global environmental implications and personnel safety aspects that should be considered during the commercial uses of sulphur hexafluoride.Gas insulated systems are now a major component of power transmission and distribution networks all over the world.Gas insulated switchgear (GIS) is exclusively used above 400kV, having all components interconnected and insulated via compressed SF 6 (i.e.circuit breakers, disconnectors, grounding switches, bushbars, potential transformers, power transformers, cable insulation etc.) [1][2][3].Due to their compactness and steel shielding, they offer significant savings in land use, are aesthetically acceptable, have relatively low radio and audible noise emissions, and enable substations to be installed in cities very close to the loads.
Sulphur hexafluoride is an anthropogenically produced slow reacting gas with a relatively high molecular weight.It exhibits excellent arc extinction properties and this gives rise to its high applicability in power technology.In its normal state it is chemically inert, non-toxic and nonflammable.Because of its inertness and non-toxic characteristics it has been treated as an environmentally safe and acceptably non-toxic material, in the sense that it does not react unfavorably with the biomass.However, when dissociated under high-pressure conditions in an electrical discharge the pure SF 6 may readily reform toxic byproducts [4][5].
There are no known natural sources of SF 6 on earth.SF 6 exhibits exceptionally long life times in the earth atmosphere [6][7].The atmospheric lifetime of SF 6 when estimated on the basis of the negative ion model, is found to range between 800 and 2500 years [8][9].SF 6 is an efficient absorber of infrared radiation, particularly in the wave number range from 915 to 960 (cm -1 ) [10].The effective trapping of infrared radiation results in an increase of the average temperature of the atmosphere due to balance shifting between the incoming and outgoing radiation.The effect is known as the 'greenhouse effect' and the responsible gases for it are frequently referred as 'greenhouse gases".Such gases are both man-made and naturally occurring, e.g.H 2 O, CO 2 , CH 4 , N 2 O (which could also be increased during pollution episodes), or purely man-made gases, e.g.fully fluorinated compounds (FFC).Therefore, the global warming effect has two contributors: The natural, originating by naturally occurring gas concentrations and the enhanced, introduced by concentration enhancements or anthopogenically produced compounds.It is now known that SF 6 is a radiatively active [11] and a potent greenhouse gas [12].Its global warming potential is 25000 times greater than that of CO 2 [13][14].The relative contribution of SF 6 to global warming was estimated in late 90s to be only 0.01% and unlike other environmental pollutants, there is no evidence that SF 6 contributes to stratospheric ozone depletion [15].Because SF 6 is now extensively used, concerns have been raised about its long term environmental impact [16].If production and leak rates of SF 6 are maintained at current rates, it is expected that in 80 years from now, its relative contribution to global warming could exceed 0.1%.Due to its enormous global warming potential [17], SF 6 is being systematically monitored by a number of air sampling programs [18][19].These may include high resolution latitudinal profiles over the oceans, weekly flask samples from remote, globally distributed sites, hourly in situ measurements in rural [20] or industrial areas, and a series of archived air samples.The work presented in this paper provides analytical results of toxic byproducts formations obtained by Quadrupole Mass Spectrometry in the SF 6 gas matrix of HV GIS power substations.The gas compartments were examined after developing electrical faults (i.e.corona discharge in 150 kV bus-bars, overheating effects on loosely attached stationary connectors, and a high power arc developed as a result of contaminated gas compartment)

Toxic byproducts of SF 6 in the working environment
The reliability of the insulating gas of six 150 kV GIS power substations operated continuously over a period of thirty years has been investigated.As shown in Figure 1, the design of the substations incorporated single phase double pressure dead tank switches commonly produced in late 60's.During all these years periodic inspection and maintenance (i.e.13X filter substitution, gas dehydration and gas room cleaning) had been primarily based upon water dew point measurements.Recently, quality control and assurance of the insulating gas has been exploited by in situ mass spectrometry and byproduct production yield determination.Such measurements can be performed while the substation is in normal operation, thus offering operational safety and exact maintenance prognosis.View of the failed GIS plant and the employed gas analysis system used for the byproduct detection.Z1 Z2 X-1 X-2 X-3 Gas samples could be obtained from all rooms of the GIS plant, Figure 1, Partial pressure measurements were performed by Quadrupole Mass Spectrometry (QMS).The employed analyzer head consisted of (a) an axial ion source where molecules could be ionized by 100 eV electron beam, (b) a quadrupole mass filter unit, operating in the atomic mass range between 1≤ amu/q ≤ 200, and (c) a Faraday collector offering minimum ion current detection 10 -14 A. Exact ion concentrations were determined by the corresponding ion current values in the obtained spectra, according to the following equation: ppm(ion) = I(ion).10 6/ P(mbar).E(amu/q) where ppm(ion) is the ion concentration in ppm, I(ion) the detected ion current (A), P(mbar) the total pressure -measured in mbar-in the analyzing system, and E(amu/q) the quadrupole sensitivity (A/mbar) at the examined atomic mass unit over charge (amu/q) position.
In the presence of an electrical discharge such as an arc, spark, or corona, a portion of the SF 6 decomposes into lower fluorides of sulphur which can react to form a number of chemically active byproducts.For the electrically stressed SF 6 , the most frequently mentioned molecular ion 04004-p.3byproducts ( 32 S isotope) are detected at the following amu/q positions: H 2 O + = 18, N 2 + = 28, HF + = 20, H 2 S + = 34, CO 2 + = 44, SOF + = 67, SOF 2 + = 86, SOF4 + = 124, SO 2 F + = 83, SO 2 F 2 + = 102, SO 2 F 4 + = 140, and S 2 F 10 + = 254.Much effort has been spent in identifying the species, the generation rates, and some of their properties, under laboratory or practical operating conditions in GIS power systems.Finally, the oxyfluoride levels may be directly detected at higher current sensitivities, as shown in Fig 3 .The displayed results were selected from the low pressure power switch room, following a switch operation.Greater concentrations of detected oxyfluorides were recorded for the following amu/q positions: SOF + = 67, SOF 2 + =86, SOF 3 + =105, SOF 4 + =124, SO 2 F + =83 and SO 2 + =64.

04004-p.4
Figure 4 provides characteristic quadrupole mass analysis spectrum of the gas in a compartment subjected to a thermal-runaway induced power arc.The increased gaseous constituents of byproducts in the SF 6 gas matrix were: C 12 + , O + , HF + , S + , C 3 H 4 + , SO + , SO 2 + , SOF + , SO 2 F + .The gas had a "rotten-egg" smell.The specific sample was selected from a 400 kV Gas Insulated Transmission Line that developed an internal fault in a triple point synergy position, i.e. the location where a solid insulator, a metallic conductor and the insulation gas co-exist in working synergy.The compartment is shown in Fig. 3a where the suspending insulator developed electrical erosion (Fig 3b) and the internal chamber walls were locally melted in the positions of the roots of the internal power arcs (Fig 3c).The most important toxic byproducts, as well as their common concentrations observed during the working cycle of SF 6 as a gaseous dielectric, are summarized in the Tables 1, 2 [21].

Recommendations and Conclusions
It is true that equipment incorporating SF 6 has a demonstrated value for society.However, the environmental risks (either in the global atmosphere or in the working environment) strongly dictate that improved handling procedures and innovative designs of GIS constructions will have to be adopted to minimize leaks to the atmosphere.CIGRE recommends that at the end of life of SF 6 equipment, the electrically stressed gas will either have to be recycled, or reduced to environmentally compatible end-products.SF 6 can be destroyed by thermal decomposition in industrial waste treatment furnaces at elevated temperatures (>1100°C).In this process the constituents of SF 6 , sulphur and fluorine, are converted into naturally occurring materials CaSO 4 (gypsum) and CaF 2 (fluorspar) by reacting with CaCO 3 (calcite).The excess of CaCO 3 is required to assure the reaction with any HF or H 2 S that may be produced in the furnace by the thermal decomposition of SF 6 .Gypsum and fluorspar can be land filled or possibly used as raw materials in other processes.The problems related to SF 6 and the power industry are not without solutions and may eventually lead to new opportunities (e.g.in-situ recycling or disposal; development of alternative gaseous insulators; or even, alternative high voltage technologies).Besides current efforts and recommendations to control the release of electrically stressed SF 6 , the introduction of additional environmental standards concerning recycling procedures or on-site gas purity control have yet to become available.A comprehensive systematic on-line monitoring and in situ analysis of the gas concentrations would be beneficial towards: (i) personnel safety; toxicity control of the SF 6 gas matrix could be based upon disulphur decafluoride or oxyfluoride detection levels.(ii) locating leaks; and help to curtail undesirable release to the atmosphere, and (iii) component reliability and assurance; systematic examination of certain byproduct concentrations can be used as a maintenance prognosis tool (e.g. the presence of high CO 2 levels in the SF 6 matrix of a GIS plant singularly identifies filter sorption saturation, while the presence of metal fluorides [22] possibly originating from the erosion of field smoothing devices should be immediately removed as it may induce surface fatigue (or damage) on the solid insulants of the installation.Thus, the on-line monitoring and analysis techniques, although substantially increase the regular running cost of the equipment, could be proved to be beneficial for the power industry.

Fig 1 .
Fig 1.Typical view of: (a) the examined GIS plant and (b) the gas analysis measuring system.

Fig 2 Fig 2 .
Fig 2. The effect of filter substitution on CO 2 concentration levels as a function of time in GIS Room 1 incorporating the current transformer.(a) before filter substitution; water dew point -23°C, (b) two days following filter substitution; water dew point -37.8°C and (c) 60 days following filter substitution; water dew point -38°C.Concentrations of SF 6 fragments are reduced under identical measuring conditions.

Fig 3 .
Fig 3. Oxyfluoride detection levels in the volume of the switch during a switch operation.

Fig 4 .
Fig 4. Mass spectra in the amu/q range between 1 and 200 of the gas sample collected from the bus-bar chamber after the high power arc development.A logarithmic ion current scale is used for result presentation and the ion current values vary between 10 -14 and 10 -11 A.

Fig 3 .
Fig 3. Arc discharge generated due to triple point synergy problem in the chamber, led to surface damage of the suspending insulator and electrochemical erosion of the internal chamber walls.

Table 1 .
SF 6 decomposition byproducts and their typical concentrations during repeated sparking.A 16kJ total energy deposition in 70 cm 3 SF 6 gas is examined

Table 2 .
Cytotoxicity of SF 6 decomposition products derived by culture cell systems.Data are not provided for cases where insufficient cell disruption was produced at any concentration tested to derive LC50 values