Axpo Power Ltd. 2) GE Grid Solutions

PS 3 SF6-free 123 kv Substation Solution to meet Axpo s sustainability strategy C. LINDNDER 1), D. GAUTSCHI 2) 1) Axpo Power Ltd. 2) GE Grid Solutions SUMMARY As it is the strategic goal of Axpo Power AG to lower the CO2 footprint of its assets significantly in the future it is one of the primary goals to reduce the amount of SF6 in its network. Together with the transmission system operator EKZ, the Axpo Power AG is installing the world s first 123 kv GIS switchgear which runs with a mixture of C4F7N and CO2 close to the lake of Zurich in Switzerland. The substation is containing 4 bays of GIS of type F35g-145 kv which are connected via double secionalisers on the busbar. The two line bays are connected via high-voltage cables, the two transformers which are installed outdoor are directly connected using gas-insulated busducts and air bushings. The whole switchgear is designed to operate at low temperatures down to -25 C. While the substation is already under construction the release for production of the new type of GIS was given in the manufacturers plant. Gas handling carts, gas analysers and densimeters have been developed in parallel to the development of the switchgear. The gas is delivered on site premixed in bottles and the filling and evacuation process is similar to the one when using SF6. In general, most of the processes which apply to SF6 are also valid when using the new gas. This includes intervention procedures and maintenance cycles. An environmental analysis comparing different type alternative gases and gas mixtures was made and the results show that the new type of switchgear has the lowest CO2 footprint of all analysed types. The reduction in the CO2 footprint is around 70 % to the same switchgear filled with SF6. This leads to a reduction of about 700 t in the lifetime of the switchgear. The installation of this world s first of its type will contribute to achieve Axpo s ambitious goals in terms of CO2 reduction. Keywords Alternative Gases to SF 6 Fluoronited Nitriles GIS and GIB Global warming potential reduction Gas handling of alternative gases Christian Lindner, Christian.lindner@axpo.com David Gautschi, david.gautschi@ge.com 1

1. AXPO S SUSTAINABILITY STRATEGY Axpo Power AG has a strategic goal to be the most sustainable Swiss energy company. Besides the continuous increase of energy efficiency in production and distribution of electric energy, the continuous reduction of greenhouse gas emissions is part of Axpo s ecological sustainability goal. Because SF6 has an enormous greenhouse potential which is 23 500 times higher than CO2 it is also one of the keys to reduce the amount of SF6 installed in the switchgear. In the past major reductions could be achieved by the installation of switchgear with low SF6 volumes and low leakage rates. In Figure 1 the evolution of the gas mass of the circuit-breaker (CB) and the common point (CP) of 145 kv GIS of different generations from the manufacturer GE are presented. The dotted blue line shows an exponential trend line for three-phase encapsulated GIS, the dashed red line the exponential trend line for single-phase encapsulated GIS. SF6 mass of CB & CP in kg Figure 1: Evolution of the SF 6 mass of 145 kv GIS (circuit-breaker and common point) [1] The dramatic decrease of the SF6 mass inside the switchgear and the reduction of the leakage rate lead to lower SF6 emissions in the past. But Axpo did not only take the SF6 emissions into account. For new substations the overall CO2 footprint of the substation over the full lifecycle from production and operation to dismantling was calculated and the results were used for the final product decision [2]. This approach is complex and many details must be provided by the manufacturers to be able to get an accurate picture. This includes for example the determination of the CO2 footprint of frames, heaters, instrument transformer burden etc. 2. SUBSTATION ETZEL For the 123 kv GIS substation Etzel in Switzerland a similar approach was used. But in the first time in history the new gas mixture containing the fluoronitrile C4F7N and CO2 as background gas will be used as insulation, switching and interruption gas. This gas mixture developed by GE and 3M combines the ability to achieve low ambient operating temperatures as today (-25 C) while using the same most compact design and for most components the same enclosures as the switchgear operating with SF6. This leads to the same physical footprint as the SF6 pendant. The substation will consist of 4 bays of 3 phase encapsulated 123 kv GIS (see Figure 2a) which are connected via a single busbar including a double sectionalizer. In addition, 10 bays of 16 kv Duplex Medium voltage cubicles in double bus bar arrangement and two 50-110 kv / 16 kv power transfomers will be installed. The power transformers, which are installed in 2

outdoor condition, are connected with the switchgear using gas-insulated bushings and threephase encapsulated busducts. These components as well as the switchgear can operate at minimum temperatures of -25 C. The substation building is now under construction (see Figure 2b). The GIS will be delivered in autumn 2017 and it will be put in operation by spring 2018. a) b) Figures 2: a) 3D view of the GIS substation Etzel with transformer connections via GIL and bushings, b) Picture of the site beginning of June 2017 The substation Etzel is placed and put in operation in the Region Freienbach/Höfe near the lake of Zurich where the demand on electric energy is growing strongly above Switzerland s average growth rate. The project will be realized jointly between Axpo and the DSO EKZ of the region, which provide together electric energy to about 3.5 Million people. Figure 3: Geographic location of the substation Etzel and integration in the existing network During the quotation process, which had been launched in autumn 2015, GE proposed to Axpo the SF6-free GIS using 6 % C4F7N and CO2 as background gas. At that time, several type tests were not yet performed, but within a time frame of about 1 year, GE successfully proofed the design of this new GIS with all relevant type tests for all components (circuitbreaker, disconnector, earthing switches, instrument transformers, bushings and busducts). 3

3. BASIC GAS PROPERTIES During more than 20 years of research of alternative gases as a replacement of SF6 many different candidates have been tested. Possible candidate gases were N2, CO2 and O2 and gas mixtures like dry air [3]. The results showed all the same picture: All the candidates are not competitive because they need either much higher operating pressures or bigger dimensions to compensate the lack of dielectric withstand compared to SF6. Significantly higher operating pressures lead to bigger wall thickness which impact mass of enclosure material with negative influence on environmental figures, switchgear cost, etc. The research was therefore extended to gas mixtures based on synthetic gases. The gas which was introduced in the F35g GIS family is based on the fluoronitrile C4F7N and the background gas CO2. Basic data about the gas properties can be found in [4]. a) b) c) Figures 4: a) Breakdown voltage / pressure curve of SF 6 and C 4F 7N, b) Liquefaction curve of C 4F 7N c) Dielectric withstand of C 4F 7N when mixed with the background gas CO2 compared to SF6 The figure 4a shows that the nitrile C4F7N has in pure state around twice the dielectric withstand of SF6. The measurements were taken according to the ASTM D877 method using disk electrodes and a gap with 2.5 mm distance. In Figure 4b the liquefaction curve of C4F7N is presented. Due to the high boiling point of the pure molecule only around 0.5 bar pressure can be filled in when application temperatures down to -25 C are requested. This is the case in the substation Etzel, where the transformers are installed outside in ambient air and where the connection is done with gas insulated lines and air bushings. In Figure 4c the dielectric performance of the nitrile when mixed with the background gas CO2 is presented. Fortunately, the curve is not linearly depending on the mixing ratio. When the mixing ratio is between 4 % and 10 % the dielectric performance is in the range of 70 % to 90 % of SF6 at the same pressure. This lead to the concentration of 6 % C4F7N in 94 % CO2 for the substation Etzel. The missing 20% of the dielectric performance was compensated by a slight operating pressure increase of around 2 bars. Even though the dielectric behaviour of N2 when mixed with C4F7N looks like the curve in Figure 4c CO2 was used as background gas due to its remarkably higher breaking performance in the circuit-breaker. 4

4. INFORMATION ABOUT GAS HANDLING AND MAINTENANCE For equipment operators like Axpo it is of key importance to know the differences in gas handling, gas quality check and maintenance when talking about alternative gases. The following chapters answer these questions. In general, the application of the nitrile / CO2 mixture is not more complicated than the use of SF6 today. From the beginning GE had also an eye to develop gas handling equipment always together with at least two independent manufacturers to have a certain competition starting right from the beginning of the introduction of this new type of switchgear. To avoid any mistakes in gas handling the filling valve was adapted using another thread diameter than todays SF6 valves (see Figure 7c). 4.1 General information regarding the gas handling The mixture of the fluorinated nitrile C4F7N and CO2 is used in a closed cycle like SF6. The gas is delivered on site already premixed in bottles (Figure 5a). The mixing ratio of the delivered gas is checked in the manufacturing site. The filling or evacuation procedure is done in a similar way like today with SF6 directly from the bottle. A dedicated gas handling cart is used to fill or recover the gas from / to the GIS compartment. Filling carts are available from two different manufacturers (see Figures 5 b & c). Existing SF6 handling carts cannot be used because of the risk of liquefaction of C4F7N during the pumping operation which could result in a wrong mixing ratio in the equipment. a) b) c) Figures 5: a) Premixed gas in bottles, b) & c) Gas filling and recovery equipment from different manufacturers modified for the use of C 4F 7N The gas registration for example for the transport of the gas mixture is finished for most of the countries. The procedure in case of a major failure when a pressure relief device opens and the gas flows into the substation room remains the same as today with SF6. Also, the neutralisation of byproducts after a major failure or in case of maintenance remains the same. 4.2 Leek seakers and integral tightness test Before the delivery on site the GIS components are subjected to an integral tightness test. Test systems were developed and are existing (see Figure 6a). Multiple leek seekers to detect the exact location of a leek were developed and qualified. In Figure 6b a simple leek detector with acoustic alarm is displayed, in Figure 6c a more advanced handheld device which measures 5

also the ppm value of C4F7N. Detection systems for permanent monitoring of the C4F7N and CO2 quantitiy in the substation room are available also. a) b) c) Figures 6: a) Integral tightness test of the first with C 4F 7N and CO 2 filled 245 kv air-insulated current transformer, b) Simple acoustic leek seeker to detect small leaks, c) leak detector with integrated C 4F 7N concentration measurement 4.3 Gas quality check and gas monitoring Specially after the erection the percentage of the gases and the moisture content needs to be checked. In Figure 7a a portable gas quality analyzer is shown. These equipments are based on SF6 analyzers but of course some of the hardware components and the software were adapted to cope with the physical properties of the new gas mixture. a) b) c) a) Figures 7: a) C 4F 7N gas analyser, b) conventional densimeter adapted for the C 4F 7N gas, c) adapted DN20 filling valve For gas monitoring an adapted temperature compensated densimeter is sufficient. The densimeter presented in Figure 7b is used in the substation Etzel to check the gas pressure. These densimeters are available with different alarm contacts and analoge output. Also, pure electronic densimeters are available which can be integrated in an online-monitoring system for permanent temperature, pressure and density observation. For the substation Etzel conventional densimeters per Figure 7b will be installed. To avoid filling the components by mistake with SF6 a filling valve using a different thread diameter is used (Figure 7c). 6

4.4 Maintenance Even though that the actual IEC standards would allow higher leakage rates for non-sf6 gases than 0.5 % / year the actual design was made to be below this value. It was decided to follow the same rules as for SF6 today without the need to change the actual maintenance cycles. Tests have also shown that the leakage rate of C4F7N/CO2-switchgear with an adapted sealing system is in the same range as for similar SF6 equipment. The maintenance cycles therefore remain the same as for SF6 switchgear. 5. CO2 FOOTPRINT COMPARISON OF DIFFERENT ALTERNATIVE GASES 5.1. Studied cases As Axpo had already a broad know-how about life-cycle assessments of substations [2] it was clear that not only the CO2 footprint of the gas alone needs to be considered during the assessement of new technologies. As written in chapter 1 the whole life-cycle was illuminated and integrated in the study. The switchgear of type F35g-145kV which is delivered in the substation Etzel was compared with the same switchgear model using SF6. In addition, a switchgear using the alternative gas mixture based on C5-Ketone/CO2/O2 and a switchgear based on dry air and vacuum breaker technology were calculated and compared. 5.2. Calculation basis The CO2-footprint of the following parts were considered in the calculation: Material during the production of the GIS Material used for the substation building Joule losses during the whole life cycle Gas production Gas losses during the life cycle The following parameters were used for the calculation: Operation period: 40 years Nominal current: 2500 A Current profile: 80 % of the time at 625 A (25 % nominal current) 20 % of the time at 1500 A (60 % nominal current) Electricity mix: European electricity mix Leakage rate: 0.2 % / year 5.3. Results of the calculation In Figure 8 the results of the calculation are summarised. It is visible that the CO2-footprint of the new gas mixture F35g-145kv (g3) is around 70 % lower than the SF6 switchgear F35-145kV (SF6). If only the gas losses are compared, then the reduction of the global warming potential impact is more than 99 %. If the leakage rate is increased from 0.2 % / year to the value of 0.5 % / year as stated in IEC 62271-1 then the reduction is even higher. The switchgear of type F35g-145kV has the lowest CO2-footprint of all the compared alternative gases. This is the case because the switchgear using C4F7N has the same size as the equipment filled with SF6. Most of the components are the same. The switchgear of type 7

F35g-145kV is the most compact switchgear on the world market. For the other alternatives, additional limitations for example in the operating temperature range may occur in addition. Figure 8: CO 2 footprint comparison of a 145 kv GIS SF 6 switchgear with alternative gas technologies. 6. CONCLUSION AND OUTLOOK The substation Etzel is under construction at the moment. The release for production for the new type of switchgear F35g-145kV was also given in the manufacturing site of GE in Oberentfelden, Switzerland. It will be the first of its kind by using the alternative gas C4F7N and CO2 as background gas. By introducing this type of switchgear, it is possible for the utility Axpo to save around 700 t of CO2 during its lifetime of 40 years when assuming a leakage rate of 0.2 % / year. This corresponds to around 470 flights from Zurich to Miami in eonomy class. The introduction of this pilot project in the network will support to reach the ambitious sustainability strategy of Axpo. BIBLIOGRAPHY [1] D. Gautschi, K. Pohlink, R. Lüscher, Y. Kieffel: Limitations, trends and potentials in the design of modern gas-insulated high voltage switchgear. CIGRE Session 2014, Paper B3-112, Paris, August 2014 [2] C. Lindner et al.: Environmental analysis of different technologies for as Swiss high-voltage substation, CIGRE Session 2010, Paper B3-205, Paris, August 2010 [3] M. Hairbour et al.: Dielectric Withstand of N 2, CO 2 and SF 6 in GIS, CIGRE SC B3, Publication No. 203, Berlin, 2007 [4] Y. Kieffel et al.: SF 6 alternative development for high voltage switchgears. CIGRE Session 2014, Paper D1-305, Paris, August 2014 8