Future Challenges for the Grid - Integration of environment friendly gasinsulated. B. Lutz, M. Kuschel, P. Glaubitz Siemens AG Germany

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1 PS3 Advances in substation technology and reliability Future Challenges for the Grid - Integration of environment friendly gasinsulated substations B. Lutz, M. Kuschel, P. Glaubitz Siemens AG Germany SUMMARY SF 6 gas is nowadays the most applied and reliable medium as arc-quenching and insulating gas in high-voltage products and switchgear for grid stations in transmission networks. It has proven worldwide its technical capabilities for more than 50 years for gas-insulated switchgear, circuit breakers and instrument transformers as well as for gas-insulated transmission lines and even transformers and reactors. SF 6 gas technology represents the state-of-the art, with very good arcquenching properties and dielectric strength for insulation purposes. SF 6 allows an easy and accurate gas density and leakage measurement, respectively. Moreover it can be handled without health risks for human beings or the environment as it is non-toxic. SF 6 in the electric power industry is kept in closed cycles and re-use concepts are in place. In the last decades, state-of-the-art gas-insulated substations design has been optimized with respect to both, low amount of installed SF 6 gas and very low leakage rates (e.g. 0.1% per year and compartment). Nevertheless, SF 6 has a significant potential impact on world ecological system with its contribution to the Global Warming Potential (GWP), whenever emitted. For that reason, GIS manufacturers, SF 6 manufacturers and utilities in Europe have signed on national base voluntary commitments to reduce SF 6 emissions over the whole product life of electrical equipment having SF 6 as insulating and arc-quenching gas. Moreover, restrictions and guidelines for the responsible application of SF 6 gas are given by the revised European F-gas directive (binding since 2014). Latest political developments like the Paris treaty or the coming revision of the European F-gas directive in 2020 indicate that even stricter regulations for the application of F-gases are to be expected in the future. That is why the demand for even more environmental friendly substation design is increasing and several alternative pilot solutions for high-voltage products have been presented during the last 20 years. For example, products from Asian and European manufacturers are already available with Dry Air/CO 2 insulation. Applications with CO 2 as arc-quenching gas have also been presented and installed. At CIGRÉ 2014 and following CIGRÉ 2016, alternative gases based on fluoroketones or fluoronitriles with mixtures of N 2, O 2 or CO 2 were presented for arc-quenching and insulating purposes. Another alternative technology to SF 6 for switching is the utilization of vacuum interrupter units for arc-quenching, which is a proven technology in medium voltage for 4 decades. Vacuum interrupter units have proven reliable making and breaking capabilities in many switching operations without any significant ageing of materials and depreciation of functionality. Siemens has started to implement GIS technology based on fluorine-free gas insulation with Clean Air (i.e. 20.5% oxygen with 79.5% nitrogen) and vacuum circuit breaker. Hence this paper provides an overview of the technical and ecological benefits of this new solution. Operational experiences are presented as well as results from testing of specific high-voltage products. Moreover, the environmental impact is discussed based on life cycle assessment. Keyword GIS, Alternative Gases, Clean Air, Global Warming Potential, Life Cycle Assessment Bernhard Lutz (bernhard.lutz@siemens.com)

2 1. Introduction Investigations on alternative gases for replacement of sulfur hexafluoride (SF 6) as insulating and arc quenching medium in gas insulated switchgear have been performed since the early 1980s. Wootton [1] identified some gases with higher dielectric strength compared to SF 6 which were either toxic, with high ozone depletion potential (ODP) or soot generating when exposed to electrical arcs. A study of Niemeyer et al [2] focused on additional parameters like the Global Warming Potential (GWP), the long-term stability in electric fields and the critical liquefaction temperature of alternative gases. Another comprehensive study of Christophorou et al [3] focused on gas mixtures including SF 6 to reduce its GWP. A synergistic effect was found allowing for significant improvement of dielectric strength with low amount of SF 6 diluted in nitrogen (N 2). However, the GWP of SF 6/N 2 mixtures still remains high. Further investigations focused on the applicability of trifluoroiodemethane (CF 3I) diluted in nitrogen and carbon dioxide (CO 2). Kamarudin et al [4] observed that a mixture CF 3I / CO 2 (ratio 30:70, 0.1 MPa) can reach 70% of the dielectric strength of SF 6 under same pressure. However, the application of CF 3I in GIS has to be questioned due to the possible risk of irreversible effects (R68) and its classification as harmful within the European Union. In the last years, also natural gases with fluorinated additives e.g. C5 / C6 fluoroketone (having 5 or 6 carbon atoms) and fluoronitrile have been introduced for medium and high voltage application ( [5], [6], [7]). Investigations were performed to show the improvement of dielectric and switching performance compared to already existing SF 6 replacement candidates. Siemens has implemented Clean Air (i.e. 79.5% N % O 2) as insulating gas for next generation GIS products. Main reasons for the choice of Clean Air are the excellent long-term stability, liquefaction temperature below -30 C, Global Warming Potential of zero, no toxicity and positive life cycle aspects. 2. Insulation performance of alternative gases Natural-origin gases and its mixtures like nitrogen, carbon dioxide, N 2/CO 2 mixtures and Clean Air (i.e. 79.5% N % O 2) are non-toxic and chemically stable and thus potential candidates for insulation in high voltage GIS. A major drawback is the reduced dielectric strength of these gases which is by 60% to 70% lower compared to SF 6 under equivalent pressure [8]. This has to be compensated by appropriate design and pressure coordination for development of new GIS products. Other possible candidates for SF 6 replacement have been presented recently, e.g. gas mixtures including fluoroketone or fluoronitrile. Mantilla et al [9] describe the effect of pressure on the dielectric strength of gas mixtures with different perfluoroketones. The dielectric strength of SF 6 can be reached by a pressure increase of 3 bar when using a gas mixture of 95% air and 5% C5-Ketone (fluoroketone having 5 carbon atoms). Simka et al [10] postulate that air comprising 6% C5-Ketone at a pressure of 0.7 MPa shows 95% of dielectric strength of SF 6 gas at 0.45 MPa. However, a major drawback of the gas mixtures tested in [9], [10] is its dew point temperature greater than 0 C so that application for outdoor GIS or dead tank switchgear (ambient temperature -30 C) is excluded. The same drawback is given for gas mixtures comprising hydrofluoroolefin and fluoroketones which are mentioned in [11]. Gas mixtures consisting of CO 2 and fluoronitrile were presented in [5]. Depending on the mixing ratio, those mixtures can reach 80% to 100% of the dielectric strength of SF 6. Gas mixtures with liquefaction temperature of -25 C were observed to have 87% (0.67 MPa with 5.6 Vol% fluoronitrile diluted in CO 2) and 92% (0.77 MPa with 4.6% fluoronitrile diluted in CO 2) of the dielectric strength of SF 6 gas with pressure 0.55 MPa. The insulation performance of recently discussed alternative gases for high voltage GIS application is summarized in Table 1. Finally, it has to be stated that no fully equivalent alternative for the replacement of SF 6 as insulating gas is available up to now. In any case, compromises and modifications in product design (e.g. pressure increase) or product specifications have to be realized for SF 6-free GIS.

3 Table 1: Insulation performance of alternative insulation gases for HV switchgear application SF6 Clean Air Fluoronitrile C5-Fluoroketone Chem. Formula SF6 N2 + O2 (79.5%/20.5%) (CF3)2CFCN CO2-Equivalent [12] 1 [13] Boiling point ( Celsius) Gas mixture Carrier gas (CF3)2CFC(O)CF3-64 < [12] [13] Pure or mixed with N2, CF4 Not applicable 96% CO2 for GIL [14] 94% CO2 for GIS [15] CO2-Equivalent < [14] < 1 [13] Boiling point ( Celsius) Dielectric strength at same pressure <- 64 (variable) < [14,15] 0 [16] 1 (normalized) ~ for GIL [15] >0.75 for GIS [15] 83% CO2 / 11% O2 [16] ~ 0.7 [9] in mixture with air 3.1 Dielectric design criteria for Clean Air Siemens performed comprehensive investigations to evaluate the insulation capabilities of Clean Air in gas insulated switchgear. Many tests were carried out with small-sized and real-sized GIS specimens and prototypes to investigate the factors of influence (Fig. 1) on dielectric breakdown in Clean Air. Furthermore the influence of defect type (e.g. metallic particle) and pressure on breakdown performance and partial discharge behavior was investigated. Based on the comprehensive test results and statistical analysis, criteria for reliable and safe dimensioning of SF 6-free GIS products were gained. An exemplary relationship between electric breakdown field strength and gas pressure for different surface quality is shown in Fig 2. Figure 1: Investigated factors of influence on dielectric breakdown strength of Clean Air [17]

4 Figure 2: Normalized electric breakdown field strength versus gas pressure for Clean Air and different voltage type and surface quality without coating 3. Switching performance of alternative gases The arc quenching capability of alternative gases is in principal lower compared to SF 6. Therefore arc switching investigations are necessary for all switching components of GIS, i.e. circuit breaker, disconnector and earthing switch. In this section, the literature information about switching performance of alternative gases is briefly summarized. In addition, to qualify Clean Air as an arc quenching gas, the results of several tests performed with real GIS modules are shown and compared with results obtained with SF 6 and other alternative gases. 3.1 Circuit breaker In general, there are two possible alternatives for SF 6 self-compression circuit breakers: - Self-compression circuit breaker with alternative gas as arc quenching medium - Vacuum circuit breaker with SF 6-free insulation In the late 1990s investigations on arc extinguishing capabilities of alternative gases and their mixtures with SF 6 were performed [18]. In the following years, focus was laid on CO 2 as arc interruption medium. In 2012 a live tank circuit breaker application was presented [19]. Investigation were also performed on CF 3I [20] for current interruption. Recently GIS with self-compression circuit breakers containing gas mixtures with fluoroketone [16] and fluoronitrile [15] were presented. Until today, there are open points to be clarified before having a clear understanding of the benefits and drawbacks of alternative gases for application in self-compression circuit breakers, e.g. [21]: - Long-term interrupting capability - Mechanical long-term performance considering higher required energy and contact speed - Consumption of gas components by switching operations - Environment, health and safety aspects (e.g. toxicity of by-products) Vacuum technology is well known in medium voltage application and used for decades. Advantages of vacuum technology are e.g.: - High mechanical endurance and no maintenance of switching unit - High number of breaking operations at normal and short circuit currents - No gas liquefaction at low temperatures and long operational life

5 It was shown recently, that the positive vacuum switching properties can be transferred from medium voltage to high voltage range. Circuit breakers for 72.5 kv for live tank applications [22] and GIS application [23] with vacuum interrupters were developed. Circuit breakers for 126 kv [24] and 145 kv live tank [26] are also available. Latest developments show first applications for Clean Air and vacuum interrupter units for high-voltage GIS products (Fig. 3). 2.3 m 2.9 m 2.5 m 3.7 m Figure 3: Examples of Clean Air GIS: (left) 66 kv GIS for specific Windtower application [25] (right) 145 kv GIS with non-conventional instrument transformers [26] 3.2 Disconnector Switch Two test duties have to be performed by a disconnector switch: - Bus transfer current switching (BTC) - Bus charging current switching (BCC) Bus transfer current switching (BTC) was performed according to IEC (2013) with test current of 1600 A and test voltage of 20 V. The arcing times of 100 breaking operations with real GIS equipment containing Clean Air and SF 6 are shown in Fig. 4. It can be seen that the arcing times with Clean Air show no significant change compared to the arcing times with SF 6. Moreover, tests with other alternative gases showed no significant change of arcing times [27]. However, conductive soot generation was observed after BTC switching tests with gas mixtures of fluoronitrile and CO 2. Investigations show that additional share of oxygen in C4-Nitrile gas mixtures improve the soot generation during low energy arcing what should be taken into account for switchgear developments. Figure 4: Measured mean arcing times during bus transfer current tests for SF 6 gas and Clean Air; the black bars indicate the range of arcing times of one test series

6 Switching of bus charging currents by the disconnector switch is not a mandatory type test for rated voltages below 300 kv. However for research purposes the switching capabilities of Clean Air were investigated according to IEC Annex F (2013), including condition checks. All tests were passed without any break-outs of the arc to the housing. 3.3 Earthing switch The earthing switch in high voltage application has to perform the following switching operations: - Making of rated short-circuit current - Switching of electrostatically and electromagnetically induced currents Tests according to IEC have shown that the arc switching performance of Clean Air is sufficient for making of short-circuit currents and switching of electrostatically induced currents with no measures to be taken to pass the tests. In order to pass the electromagnetically induced current switching tests for Class B (see IEC Annex C), additional measures on the design of the arcing contacts have to be realized for all alternative gases. 4. Safety, operation and maintenance aspects Alternative gases should preferably have no restrictions on technical performance of GIS and on safety requirements. Besides the physical properties of pure gases shown in section 2, additional environmental requirements with respect to toxicity, water solubility or decomposition products after arcing have to be considered. Table 2 shows some environment-relevant properties of different SF 6 alternatives. Table 2: Environment-relevant properties of different alternative insulation gases for HV switchgear application; *LC50 = 50% lethal concentration after 4h inhalation by rats, **TWA = time weighted average concentration SF6 Clean Air Fluoronitrile C5-Fluoroketone CO2-Equivalent [12] 1 [13] LC50* (ppm) - - > < [12] > [13] TWA** (ppm) [12] 225 [13] Arcing impact Dissociation / decomposition Decomposition products found ~ 2000 K (reversib.) Reversible (N2 ~ 7000 K O2 ~ 4000 K) HF, SO2, sulfur compounds Only if failure: Nitrogen oxides, ozone > 920 K (irrev.) [14] ~ 970 K (irrev.) [28] HF, CO, COF2,CF3CN, C2F5CN, C2F6 [14] HF, CF4, C2F6, C5F10O, C3F8 [16] For both: C4F10,C3HF7; C4F8, C4F6, C3F6, C2F3N, C2N2 in MV GIS mixed with air [28] The Clean Air approach represents the fluorine-gas free insulation. While SF 6 has a Global Warming Potential (GWP) of , Clean Air shows a GWP of 0. Moreover due to the lowest boiling point it can be used in the entire temperature range even lower than -65 C without any restrictions. Leakages of Clean Air are not considered to be reported to any public authority. Recycling cost of the gas does not apply. Moreover Clean Air is like SF 6 a non-toxic, non-harmful and safe medium. No specially trained employees are required for the gas handling works compared to F-gas solutions. Clean Air can be released via filters in the atmosphere without any environmental impact, where F-gas solution needs to be carefully evacuated via filters and stored in dedicated vessels. To be noted are also the different decomposition products in the event of arcing. With Clean Air no acids occur in comparison to F-gases. In planned or unplanned maintenance therefore more efforts are needed to protect employees using F-gases. Overall Clean Air has clear advantages from health and environmental point of view.

7 5. Field experience In order to secure the safe application of vacuum switching technology in the High Voltage grid a huge variety of performance tests in the high power laboratories i.e. capacitance switching, inductance switching etc. were performed [29]. Significant number of 72.5 kv live tank vacuum breakers was installed in 2010 in the rough environment of the trans-european grid and operators agreed to start a co-operation with Siemens to gain field experience with this completely type tested outdoor vacuum circuit breaker. After installation of the breakers in the substations (Fig. 5 left), they have performed more than 6500 switching operations of different load and short-circuit currents. Neither in the laboratory nor in the grid, the vacuum breakers showed any difference to the existing SF 6 technology. The applications are mainly switching of outgoing feeder bays or charging bus bars [30]. The feedbacks given by the end-users are fully positive and no abnormalities have been observed during the test application phase. This gives the evidence that the extension of the vacuum switching technology to high voltage applications will not be limited by technical reasons. This technology is equal and for specific applications even superior to SF 6-breakers and is a suitable candidate for SF 6- free high-voltage switches in the near future. Moreover, there is already positive long-term experience available for dried compressed air as insulating gas as well as arc quenching medium (Fig. 5 right). Figure 5: 72.5 kv live tank circuit breaker with vacuum interrupter and nitrogen insulation (left) and 170 kv GIS with dried compressed air as insulation and arc quenching medium (right) [31] 6. Life cycle aspects The development of environment friendly GIS products is driven by several aspects to be considered over the whole life cycle. For that purpose, life cycle assessment according to ISO 14040/44 - with respect to the impact category global warming potential (GWP 100) - was performed for two different GIS product scenarios for three-phase encapsulated cable bays for the European market. 1. SF 6 insulation and SF 6 arc-quenching interrupter unit for standard GIS 2. Clean Air and vacuum interrupter unit technology The global warming potential (GWP) of Clean Air GIS can be significantly reduced compared to SF 6 GIS. Fig. 6 depicts the GWP over the whole life cycle that is reduced by more than 30% mainly due to the following reasons: - Although more hardware material like aluminum or steel is used for Clean Air GIS, the contribution of material production to GWP is almost equal to SF 6 GIS. This is due to a lower energy consumption to produce Clean Air in comparison to SF 6 as insulating gas. - GWP caused by gas losses during manufacturing, transport, operation & maintenance are significantly reduced as GWP of Clean Air is equal to zero. - Reduction of total GWP by recycling is slightly higher for Clean Air filled GIS due to greater amount of recyclable materials like aluminum or steel. The absolute results of this life cycle assessment are design-specific and are not representative for other GIS configuration and voltage ratings. A more detailed description of the data used for the life cycle assessment can be found in [32].

8 Figure 6: Normalized CO 2 equivalent over the whole life cycle for a 145 kv GIS filled with SF 6 compared to a 145 kv GIS filled with Clean Air. If gas mixtures with F-gases are applied to GIS, then further effects on global warming potential and life cycle costs might be considered, e.g.: 1. Heating of indoor GIS filled with gas mixtures including fluoroketones is needed to avoid liquefaction (<5 C ambient temperature: this leads to overall additional energy losses) 2. Gas mixtures with fluoronitriles or fluoroketones gases are currently non-recyclable, therefore the disposal after end-of-life needs additional energy 3. Negative effect of fluorinated compounds due to GWP greater than zero 4. Energy consumption of additional safety measures and measurement equipment for the handling of toxic gases and their decomposition products, e.g. monitoring of gas concentration 5. Limited long-term stability of fluorinated gases with low GWP, which are e.g. sensitive to decomposition by electrical discharge, leads eventually to shorter maintenance intervals Finally, positive life cycle aspect with respect to the global warming potential can be attributed to GIS products with Clean Air insulation and vacuum switching technology. Compared to SF 6 filled GIS, the global warming potential can be significantly reduced. 7. Conclusion Siemens implements high voltage GIS technology based on fluorine-free gas insulation with Clean Air and vacuum circuit breaker. Comprehensive investigations were carried out to define criteria for reliable and secure design of GIS products with Clean Air insulation. Several tests were performed to prove the functionality of disconnecting and earthing switches with Clean Air as arc quenching medium. Positive test results and long-term experience with 72.5 kv vacuum interrupter switches installed in the field is available. Further advantages of 145kV GIS with Clean Air insulation and vacuum circuit breaker are summarized as follows: - Clean Air GIS has a positive impact over the total life cycle, with a lower GWP than a SF 6 GIS - Clean Air has excellent long-term stability, is not toxic and has an intrinsic GWP=0 - Clean Air can be applied for temperatures below -30 C - Clean Air is more simple with respect to gas handling, resulting in lower life cycle costs - Vacuum switching technology is available and proven with long-term field experience - Vacuum switching behaviour is comparable to SF 6 self-compression breakers Considering these positive aspects, GIS products with vacuum circuit breaker and Clean Air insulation are recommended for application in next generation high voltage substations as environmental friendly alternative for GIS products based on SF 6 technology.

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