2016 Electrical Insulation Conference (EIC), Montréal, Qc, Canada, 19 - 22 June 2016 Greenhouse Gas Emission Reductions through use of a Sustainable Alternative to SF 6

John G. Owens 3M Electronics Materials Solutions Division Laboratory 3M Company St. Paul, MN USA [email protected]

Abstract — For decades, SF 6 has been a medium 1990s when it became recognized that SF 6 is the most potent commonly used in electrical power applications, particularly for greenhouse gas ever identified. The challenge of identifying an equipment. However, SF 6 has long been recognized as alternative to SF 6 that met the required performance a potent greenhouse gas and governments have sought to reduce characteristics became even more important. emissions from gas-insulated equipment. The electrical power industry has demonstrated an ability and willingness to respond II. REQUIREMENTS FOR AN ALTERNATIVE to these environmental issues. While emission rates have been Identification of a viable alternative to SF 6 is complicated by reduced, the identification of suitable chemical alternatives to SF 6 in electrical power applications has been elusive. the unique combination of properties required in dielectric applications. The material needs to be nonflammable and low The development of alternative dielectric gases is challenging enough in toxicity to allow for handling using practices similar due to the complex combination of performance and safety to those currently used within the industry. An alternative properties required in electrical power applications. A gaseous compound needs to have very high , as similar dielectric medium needs not only high dielectric strength but must to the performance of SF 6 as possible. Since the electrical also have good heat transfer properties and be nonflammable, equipment will be used in a variety of ambient conditions, the thermally stable and low in toxicity. Today, a dielectric gas must material must remain gaseous over the expected operating range also have acceptable environmental properties, specifically, zero of the equipment, typically down to at least -30°C. The dielectric ozone depletion potential and a much lower global warming medium must also be stable over the working life of this potential compared to SF 6. Until recently, achieving all of these equipment without contributing to corrosion or other adverse objectives had appeared to be mutually exclusive. However, an effects on the device. In addition, to be a sustainable alternative environmentally sustainable solution that is both effective and low any new compound would need to have an acceptable in climate impact is now available for some of the applications combination of environmental properties, including no ozone which have traditionally used SF 6. This presentation will review depletion potential and a significantly lower global warming this recently-developed, gaseous dielectric compound. This potential (GWP) compared to SF . material has significantly lower with the 6 capability of substantially reducing greenhouse gas emissions After decades of research, it appeared that this combination from gas-insulated equipment. of properties would be nearly impossible to achieve. Each new compound investigated showed a significant deficiency in one Keywords—dielectric medium; alternative; SF 6; emission of the essential performance, safety or environmental properties. reduction Yet, in the last few years, after the evaluation of dozens of compounds, one material has shown the right balance of I. INTRODUCTION properties to function as an SF 6 alternative. hexafluoride, SF 6, has been a critical component in medium and high voltage electric equipment for several III. THE FUNDEMENTAL BALANCE OF PROPERTIES decades. Its combination of excellent dielectric properties with The demanding combination of performance, safety and high stability, low toxicity and nonflammability made it the environmental properties required for today’s electrical preferred dielectric medium in many electrical applications. A applications is not easily found in either organic or inorganic significant percentage of the , circuit breakers and compounds. In order to be nonflammable and achieve the high other equipment used in the current electrical grid depends upon dielectric properties found in SF 6, it was clear that any new the use of SF 6. Nevertheless, the industry has known for quite some time that an alternative would be desirable. For many alternative would need to be highly halogenated. However, years, researchers had continued to evaluate different many and bromine containing materials can be compounds with the hope of finding a material that was even transported to the stratosphere where those participate better than SF 6. This search took on greater urgency in the early in the well-established catalytic ozone-depletion cycle.

978-1-4673-8706-4/16/$31.00 ©2016 IEEE 535 Consequently, much of the effort to develop alternatives shown in Fig. 1, is being commercialized under the tradename focused on fluorinated compounds since these have been found 3M™ Novec™ 4710 Dielectric Fluid. to be non-ozone depleting [1]. The larger challenge was combining this property with all of the other necessary features. IV. PROPERTIES OF 2,3,3,3-TETRAFLUORO -2- (TRIFLUOROMETHYL )PROPANENITRILE By far the most difficult combination of properties to achieve A. Physical Properties is a compound that is high in dielectric strength and stability The physical properties of Novec 4710 fluid are shown in and at the same time much lower in Global Warming Potential Table 1 in comparison to SF 6. A number of the properties of this (GWP). All organic compounds that are emitted to the material are similar to SF 6. Novec 4710 fluid is a high density, atmosphere have the potential to contribute to climate change nonflammable gas with very high dielectric strength. However, by functioning as greenhouse gases. The potency of a the normal boiling point of Novec 4710 fluid is much higher greenhouse gas is reflected in its GWP and is a function of its than that of SF 6, meaning this compound has a much lower infrared (IR) absorbance and atmospheric lifetime. vapor pressure at any temperature. Nevertheless, Novec 4710 fluid is sufficiently volatile to form gaseous mixtures that can be The GWP is an index that provides a relative measure of the used at temperatures well below its boiling point. Fig. 2 is a possible climate impact of a compound which acts as a graphical representation of the condensation temperatures for greenhouse gas in the atmosphere. It effectively calculates the gas mixtures containing Novec 4710 fluid as a function of amount of energy absorbed by a compound over a period of concentration and total gas pressure when mixed with a non- time relative to that of a reference compound, CO 2. The GWP condensable gas such as dry air, or CO 2. As illustrated as defined by the Intergovernmental Panel on Climate Change by these curves, gas mixtures with lower Novec 4710 fluid (IPCC) [2] is calculated as the integrated radiative forcing due concentration and/or lower total pressure can achieve relatively to the release of 1 kilogram of that compound relative to the low condensation temperatures (the temperature at which condensation would begin to occur or its dew point). warming due to 1 kilogram of CO 2 over the same time interval (the integration time horizon (ITH)), as shown in equation 1: B. Thermal Stability Novec 4710 fluid displays a high degree of thermal stability. Gas samples aged for 6 months at 120°C show no measureable (1) change in composition. Exposure to higher temperatures was evaluated via flow through a tube furnace, the results of which where R is the radiative forcing per unit mass of a compound are shown in Fig. 3 and Fig. 4. The concentration of each product (the change in the flux of radiation through the atmosphere due was analyzed continuously by Fourier Transform Infrared to the IR absorbance of that compound), C is the atmospheric Spectroscopy (FTIR). When gas is flowed through a heated concentration of a compound, τ is the atmospheric lifetime of a ceramic tube, Novec 4710 fluid begins significant degradation compound, t is time and x is the compound of interest. The at approximately 700°C compared to about 900°C for SF 6. commonly accepted ITH is 100 years representing a However, when the heated surface is metal (Inconel 600 alloy) compromise between short-term effects (20 years) and longer- the order of degradation is reversed. Novec 4710 fluid begins to term effects (500 years or longer). degrade at approximately 625°C on a metal tube, at least 100°C higher than the onset of degradation for SF 6. Only two variables in the GWP calculation are affected by the physical characteristics of the compound - the radiative forcing due to IR absorbance and the atmospheric lifetime. All Table I. Properties of Novec 4710 fluid compared to SF 6 fluorinated compounds absorb IR energy in the “window” at 8 Property (at 25° C) Novec 4710 SF 6 to 12 µm which is largely transparent in the natural atmosphere. Molecular Weight (g/mol) 195 146 This IR absorbance coupled with long atmospheric lifetimes Flash Point (°C) none none results in the high GWPs of many perfluorinated compounds Boiling Point (°C) -4.7 -68.3 * such as SF 6. As a result, the most effective approach to Freezing Point (°C) -118 -50.7 producing a lower GWP alternative is to develop a compound Vapor Pressure (kPa) 297 2149 with a significantly shorter atmospheric lifetime. For highly Gas Density at 1 bar (kg/m3) 7.9 5.9 fluorinated compounds, this means developing a molecule Dielectric Strength at 1 bar (kV) 27.5 14.0 containing functionality or structural features that allow it to Atmospheric Lifetime (years) 30 3200 decompose more readily in the atmosphere. Global Warming Potential (100 -yr ITH) 2100 23500 Ozone Depletion Potential 0 0 The decades-long search for an alternative identified a new *sublimation point class of compounds that incorporates this type of functionality. Fluoronitriles were found to have both reduced atmospheric lifetimes and increased dielectric strength. One compound in particular, 2,3,3,3-tetrafluoro-2-(trifluoromethyl)propanenitrile, [(CF 3)2CFCN] exhibits a combination of properties that can make it an effective alternative to SF 6 in many dielectric applications. This new compound, the structure of which is Fig. 1 Molecular Structure of Novec 4710 fluid

536 C. Dielectic Properties As shown in Fig. 5, the pure Novec 4710 fluid displays dielectric breakdown strength that is nearly twice that of SF 6, particularly at higher pressures. The data in this figure were measured on the pure gases using a method similar to that described in ASTM D877 using a 2.5 mm gap with disk electrodes (i.e., a relatively uniform field). In many cases, however, it is likely that Novec 4710 fluid will be used in a gaseous mixture rather than as a pure gas since gas-filled equipment installations are often designed to operate at temperatures well below the -5°C boiling point of the fluoronitrile. A series of curves such as those shown in Fig. 6 – Fig. 8 result when Novec 4710 fluid is mixed with nitrogen, air or CO 2 to form gaseous mixtures. The dielectric breakdown Fig. 2. Condensation Temperatures for Gas Mixtures Containing strength of the gas mixture increases at higher pressures and Novec 4710 fluid increased Novec 4710 fluid concentration (using disk electrodes, 2.5 mm gap). The most pronounced effect occurs in mixtures with CO 2. Mixtures with 20 mole% Novec 4710 fluid in air or N 2 display dielectric strengths comparable to SF 6. Similar results are found with CO 2 mixtures except that at higher pressures the SF6 curve is more closely replicated with a 15% mixture. These dielectric results are achieved at relatively low concentrations with gas mixtures that are much lower in density than pure SF 6 at the same pressure.

Fig. 3. Thermal Degradation of Novec 4710 fluid compared to SF 6 (1 liter/min feed of nominal 520-550 ppmv in dry N 2 through a ceramic tube)

Fig. 5. Dielectric of Pure Gas Compared to SF 6

Fig. 4. Thermal Degradation of Novec 4710 fluid compared to SF 6 (1 liter/min feed of nominal 500 ppmv in dry N2 through an Inconel tube)

Fig. 6. Dielectric Strength of Novec 4710 Fluid/N 2 Mixtures vs pure SF 6

537 E. Environmental Properties

A key aspect for any alternative to SF 6 is its environmental profile. Novec 4710 fluid contains no chlorine or bromine and therefore does not affect stratospheric ozone. A study conducted by the 3M Environmental Laboratory examining the atmospheric loss mechanisms for Novec 4710 fluid determined that this compound does not directly photolyze but does show measureable reactivity with hydroxyl radicals. This rate of decomposition corresponds to an atmospheric lifetime of approximately 30 years which is more than 100 times shorter- lived than SF 6. The potential for Novec 4710 fluid to impact the radiative balance in the atmosphere is limited by this significantly reduced atmospheric lifetime relative to SF 6. Using the IPCC 2013 calculation method [2], the GWP for Novec 4710 fluid is 2100.

Fig. 7. Dielectric Strength of Novec 4710 fluid/air Mixtures vs pure SF 6 This is more than an order of magnitude lower than the GWP of SF 6 (23,500) which has the highest GWP of any compound measured to date. When used as an alternative for SF 6, Novec 4710 fluid can enable significant reductions in greenhouse gas emissions in dielectric applications. The combination of its lower GWP with a reduced concentration in a gas mixture and lower gas density results in greenhouse gas emission reductions of as much as 99% when compared to the emissions from SF 6- filled equipment (assuming similar leak rates).

V. CONCLUSIONS A new compound has been developed that is nonflammable, stable, and acceptable in toxicity, providing a sufficient margin of safety for use as a dielectric medium. The material is higher in dielectric strength and atmospherically much shorter lived than SF 6. The shorter atmospheric lifetime results in a much lower GWP which leads to substantially reduced greenhouse gas

Fig. 8. Dielectric Strength of Novec 4710 fluid/CO 2 Mixtures vs SF 6 emissions from gas-insulated, electric power equipment. This combination of safety, physical and environmental properties makes a compound such as Novec 4710 fluid a sustainable D. Toxicological Properties alternative to SF 6 in many dielectric applications. The safety of Novec 4710 fluid has been evaluated through a series of toxicological tests [3]. The compound has demonstrated low toxicity in acute inhalation studies. The 4 hour REFERENCES rodent (rat) LC 50 (lethal concentration at 50% mortality) is between 10,000 ppmv and 15,000 ppmv. Based on the results of [1] T. Wallington, W. Schneider, D. Worsnop, O. Nielsen, J. Sehested, W. a number of tests including a 28-day repeat dose inhalation Debruyn, J. Shorter, “The environmental impact of CFC replacements – study, Novec 4710 fluid was found to also be low in repeat dose HFCs and HCFCs,” Environ. Sci. Technol., vol. 28, pp. 320-328, 1994. inhalation. Novec 4710 fluid was not mutagenic at atmospheric [2] IPCC, 2013: Climate Change 2013: The Physical Science Basis. concentrations up to 40% v/v in a bacterial reverse mutation Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.- assay. Considering the results of all of these tests, the 3M K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Medical Department has established an occupational exposure Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, limit of 65 ppm (8-hour time weighted average) for this material. United Kingdom and New York, NY, USA, 1535 pp. Workplace airborne SF 6 concentrations typically observed in [3] Safety Data Sheet, 3M™ Novec™ 4710 Dielectric Fluid, 3M Company, indoor gas-insulated switchgear applications are well below 100 St. Paul, MN, 2015. ppmv [4]. Since Novec 4710 fluid will most likely be used in [4] J. Castonguay, “In-situ measurements of SF 6 leak rates in indoor gas- relatively dilute gas mixtures, this occupational exposure limit insulated (GIS),” Gaseous IX, L. Christophorou and J. Olthoff editors, Kluwer Academic/Plenum Publishers, New York, provides a sufficient margin of safety in these applications. NY, pp. 549-554, 2001.

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