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The Current Use of SF6 Gas by Electric Power Industry in Saudi Arabia and its Possible Replacement by Diluted SF6 Gas Mixtures By N.H. Malik, M.I. Qureshi and A.A. Al-Arainy College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia. ABSTRACT SF6 gas insulated equipment plays an important role in electric power networks all over the world including GCC countries, due to its merits as compared to traditional air and oil insulated equipment. As a result, its production as well as its amount that is released in atmosphere by utilities worldwide due to equipment leakages and maintenance problems has increased. Recently, it has been declared as a potent greenhouse gas. Therefore, worldwide efforts are being made to develop its universal substitute. However, in the absence of its alternate and to reduce its used content, its mixtures with other insulated gases are being investigated. In this investigation, initially a survey was carried out about the types and number of SF6 filled equipment that are in use in the Kingdom of Saudi Arabia by the local power utilities, and the amount of SF6 gas released due to leakages and maintenance, etc. In addition, systematic experiments were also carried out to evaluate the performance of 8% SF6+N2 and 8% SF6+CO2 gas mixtures subjected under positive 150/1500 s switching impulse applications in non-uniform field electrode system. The gas mixture's pressure was varied in a range of 1 – 5 bar which is the pressure in which most of SF6 insulated equipment operates in the local electric power utilities. The results are discussed in the light of physical phenomena that control the breakdown process in such gas mixtures. Keywords: SF6 gas, gas insulated equipment, SF6-CO2 mixtures, SF6-N2 mixtures, switching impulse breakdown. 1. INTRODUCTION Sulfur hexafluoride (SF6) is a man-made gas that became commercially available in 1947 [1]. It exhibits many properties that make it suitable for equipment utilized in electrical transmission and distribution systems. SF6 is a strong electronegative or electron attaching gas both at room temperature and at temperatures well above the ambient, which principally accounts for its relatively high dielectric strength and good arc-interruption properties. The breakdown voltage of SF6 is nearly three times higher than air at atmospheric pressure [2]. Furthermore, it has good heat transfer properties and it readily reforms itself when dissociated under high gas pressure conditions in an electrical discharge or an arc so that it has a fast recovery and it is self-healing. Most of its stable decomposition byproducts do not significantly degrade its dielectric strength and are removable by filtering. It produces no polymerization, carbon, or other conductive deposits during arcing, and it is chemically compatible with most solid insulating and conducting materials used in electrical power equipment at temperatures up to 200 C. It offers significant savings in land use, is aesthetically acceptable, has relatively low radio and audible noise emissions, and enables substations to be installed in cities very close to the load centers [3,4]. However, SF6 has some undesirable properties as well. For example, it forms highly toxic and corrosive compounds when subjected to electrical discharges. Moreover, nonpolar contaminants, e.g., air. are not easily removed from it. Its breakdown voltage is sensitive to water vapor, conducting particles, and conductor surface roughness. Decomposition of SF6 due to electrical discharges may proceed as per the following reactions: SF6 + e SF4 + 2F + e SF6 + e SF2 + 4F + e SF6 + e SF5 + F + e SF5 + SF5 S2F10 SF4 + H2O SOF2 + 2HF SF4 + O SOF4 SOF2 + H2O SO2 + 2HF Some of the byproducts as shown above are formed under high power arc discharges while others are produced under lower power corona discharges in presence of oxygen and moisture. SF6 itself is nontoxic and a human can survive indefinitely in a mixture of 20% O2 and 80% SF6. However, SF6 will not support life and can cause suffocation. Being extremely dense, it will accumulate in low-laying areas, requiring care if being exhausted to atmosphere. Moreover, some of the discharge byproducts of SF6, such as S2F10, SOF4, and HF, can be very toxic having working day exposure limits in the range of a few ppm. Therefore, it is prudent to assume that any SF6 equipment which has been in service for some time may contain some toxic species, thereby requiring extreme care in its handling. It is generally believed that if moisture is absorbed by molecular sieves and oxygen can be prevented from being present in SF6 equipment, the metal fluorides which constitute the main discharge byproducts are relatively harmless. Thus chemical adsorbents such as soda-lime and activated alumina are used in some SF6 equipment to keep the harmful byproduct concentration within tolerable limits [4]. SF6 gas insulated equipment forms a major component of electrical power networks all over the world including GCC countries, due to its magnificent merits as compared to traditional air and oil insulated equipment. On global basis, its production has exponentially increased in the last decade and so does its amount that is being released into atmosphere by utilities due to equipment leakages and maintenance problems. Recently, it has been declared as a potent greenhouse gas, since it is an efficient absorber of infrared radiation particularly at wavelengths near to 10.5 m. It possesses a high global warming potential, which for 100 years horizon is estimated to be about 24000 times greater than that of CO2 [4]. Worldwide efforts made to develop its universal substitute have not met a success, yet. However, to reduce its used content, its mixtures with other gases are being investigated in several research centers around the world. Recently, efforts have been made to study diluted mixtures of SF6N2 and SF6-CO2 gases and reports show promising results. In Saudi Arabia and other GCC countries, electric power utilities use large number of SF6 gas insulated equipment. Present research work was executed in two phases. In the first phase, a Kingdom-wise survey was carried out to determine the types and number of SF6 filled equipments that are in use in the local power utilities and industry. In addition, the total amount of SF6 gas used and its percentage released to the atmosphere during maintenance, refilling etc. was estimated. In the second phase, a systematic experimental study was undertaken to study the switching impulse breakdown characteristics of diluted mixtures of SF6 gas with N2 and CO2 having 8% of SF6. The paper presents the results and analysis and implications of this survey and the experimentally observed characteristics. 2. USE OF SF6 IN INDUSTRY AND ITS RELEASE TO ATMOSPHERE Generally, there are four major types of electrical equipment that use SF6 for insulation and/or interruption purposes: gas-insulated circuit breakers, gas-insulated transmission lines, gas-insulated transformers, and gasinsulated substations. It is reported in reference [5] that for these applications, the electric power industry uses about 80% of the SF6 produced worldwide, with circuit breaker applications accounting for most of this amount. Gas-insulated equipment is now a major component of almost all power transmission and distribution systems allover the world and it employs SF6 almost exclusively. Depending on the particular function of the gasinsulated equipment, the gas properties that are the most significant vary. For circuit breakers, the excellent thermal conductivity and high dielectric strength of SF6 along with its fast thermal and dielectric recovery are the main reasons for its high interruption capability. SF6-based circuit breakers are superior in their performance to alternative systems such as highpressure air blast or vacuum circuit breakers. For gasinsulated transmission lines, the dielectric strength of the gaseous medium under industrial conditions is of paramount importance, especially the behavior of the gaseous dielectric under metallic particle contamination, switching and lightning impulses, and fast transient electrical surges. The gas must also have a high efficiency for transfer of heat from the conductor to the enclosure and be stable for long periods of time of 40 years. SF6-insulated transmission lines offer distinct advantages due to their cost effectiveness, high-carrying capacity, low losses, availability at all voltage ratings, no fire risk, higher reliability, a compact alternative to the overhead high-voltage transmission lines in congested areas, and avoidance of public concerns about high-voltage overhead transmission lines. Since, the use of SF6 has increased in recent decades, more quantities of this gas are being released to the atmosphere. In turn, this has resulted in increased concentration of SF6 in the atmosphere [6]. Measurements have shown that the amount of SF6 in the atmosphere has been increasing at a rate of ~8.7%/yr, from barely measurable quantities in 1980 to levels of ~3.2 pptv (~3.2 parts in 1012 parts) by volume in 1992. The atmospheric concentration of SF6 could reach 10 pptv by the year 2010 depending upon the assumptions of release rates and economic growth forecasts [4]. In many industrial applications SF6 is not recoverable, and its release into the environment by the electric power industry comes from normal equipment leakage, maintenance, reclaiming, handling, testing, etc. At present efforts are being made so that SF6 leakage rate from the electric power-system equipment is limited to 0.5% to 1% per year. Nevertheless, decreasing the rate of SF6 leakage and increasing the level of SF6 recycling are high priorities since they will both curtail production needs of SF6 and thus will reduce the quantities of SF6 that are eventually released into the environment. Indeed, efforts are being undertaken by the electric power industry to better monitor the gas pressure in SF6-insulated equipment and the amount of SF6 released into the environment [4]. These efforts include improved methods to quantify and stop leakages, better pumping and storage procedures, setting of standards for recycling, manufacturing tighter and more compact equipment, development of sealed-for-life electrical apparatus, gradual replacement of older equipment which normally leaks at higher rates, and implementation of a sound overall policy of using, handling, and tracing SF6. 3. POTENTIAL OF SF6 AS A GREENHOUSE GAS Greenhouse gases are atmospheric gases that absorb a portion of the infrared radiation emitted by the earth and return it to earth by emitting it back. Potent greenhouse gases have strong infrared absorption in the wavelength range from ~7 m to 13 m and occur naturally in the environment. Examples of such gases are H2O, CO2, CH4 and N2O, etc. Other examples of such gases are man-made gases that are released into the environment, such as fully fluorinated compounds (FFC); combustion products such as CO2, nitrogen and sulfur oxides and SF6. The effective trapping of infrared radiation by the greenhouse gases and its re-radiation back to earth results in an increase in the average temperature of the earth's atmosphere. The effect is known as the "greenhouse effect". The man-produced contribution to the greenhouse effect shifts the balance between incoming and outgoing radiation at the top of the earth's troposphere toward the former, causing "global warming". Sulphur hexafluoride is an efficient absorber of infrared radiation, particularly at wavelengths near 10.5 m. Additionally, unlike most other naturally occurring greenhouse gases (e.g., CO2, CH4), SF6 is largely immune to chemical and photolytic degradation and therefore its contribution to global warming is expected to be cumulative and virtually permanent. Although the determination of the lifetime of SF6 in the environment (the time taken for a given quantity of SF6 released into the atmosphere to be reduced via natural processes to ~37% of the original quantity) is highly uncertain because of the lack of knowledge concerning the predominant mechanisms of its destruction, it is very long; estimates range from 800 years to 3,200 years [7]. The strong infrared absorption of SF6 and its long life time in the environment are the reasons for its extremely high global warming potential, which for a 100-year horizon is estimated to be ~24,000 times greater than that of CO2, the predominant contributor to the greenhouse effect. Therefore the concern about the presence of SF6 in the environment derives exclusively from this very high value of its potency as a greenhouse gas. While the potency of SF6 as a greenhouse gas is extremely high, the amount of SF6 in the atmosphere is too small to have significant environmental consequences. Estimates of the relative contribution of SF6 to nonnatural global warming, using estimated SF6 concentration levels range from 0.01% to 0.07% and in 100 years this value could become as high as 0.2% [5]. However, government and environmental protection agencies, electrical, chemical, and other industries using or interested in the use of SF6 have expressed concerns over the possible long-term environmental impact of SF6 and the electric power industry is responding in a multiplicity of ways to better control SF6 than in the past and to reduce its releases into the environment. The best response to the concerns over the possible impact of SF6 on global warming is to prevent the release of SF6 into the environment. Clearly the most effective way to do this is not to use SF6 at all. Nonelectronegative gases that are benign and environmentally ideal, such as N2, normally have low dielectric strengths. For example, N2 has a dielectric strength about three times lower than SF6 and lacks the fundamental requirements for use by itself in circuit breakers. Nonetheless, such environmentally friendly gases might be used by themselves at higher or lower pressures, as the main component in mixtures with electronegative gases, including SF6 at partial concentrations of a some percent. Suggestions have been made repeatedly over the last two decades to use high-pressure N2 and mixtures of N2 with SF6 for insulation, arc quenching, and current interruption. Mixtures of N2/ SF6 have been and are being used in circuit breakers under severe weather conditions. Besides SF6/N2, other mixtures in use include SF6/CF4 and SF6/He. Dielectric properties of SF6 gas mixtures with other gases are also being investigated since 1970s and considerable literature exists on this subject. In GCC countries, as have been done by other utilities worldwide, industrial users, manufacturers and installation contractors of SF6 filled equipment, should be encouraged to minimize the intentional release of SF6 gas to the atmosphere. Moreover, the issue can be discussed at various administrative levels in order to highlight the importance of the problem. Such efforts will result in a reduction of the total amount of this gas released to the atmosphere, thereby, decreasing its adverse environmental effects. 4. SF6 EQUIPMENT IN SAUDI ARABIA Uniform field AC breakdown data was retrieved from the literature [8] and was used to derive an equation that can be used to estimate breakdown values in a large range of 'pd' values. A survey carried out by the authors show that SF6 filled equipment is extensively being used in the Kingdom of Saudi Arabia. The equipment used by electric utilities includes: (a) SF6 circuit breakers with voltage rating from 13.8 kV to 380 kV. At present, the total number of such breakers is more than 3000. The gas used in these breakers has a pressure which varies in the range of 0.5 bar to 7.5 bars (gauge). Total amount of SF6 gas used in these breakers is estimated to be over 200 tons. (b) SF6 insulated GIS have voltage ratings from 13.8 kV to 380 kV. At present, the total number of GIS sections in use are over 15000. The gas used in these GIS sections has pressure in the range of 0.2 bar to 7 bars (gauge). Total gas used in this equipment is over 500 tons. (c) Gas insulated lines (GIL) operating at 380 kV have a total length of over 30 km. SF6 gas pressure used is in range of 3.5 bars to 6 bars, while the total amount of SF6 gas used in GIL is about 20 tons. It is clear from these results that large quantities of SF6, is being used in a variety of SF6 filled equipment while some amount is certainly being released annually to atmosphere due to gas leakages, malfunction or other problems. The survey further showed that a variety of methods are being used to monitor SF6 equipment which are aimed at detection of the gas leakage which is accompanied by changes in the gas pressure and density. Usually either pressure or density is monitored with two levels for alarm and tripping. Various types of gas detection sensors are available. Different utilities use different methods for detection of gas leakage, as well. Moreover, for storage of SF6 and its quality check, proper methods exist and are being used by local utilities. Usually gas is stored in cylinders of different size ranges. Before using the gas, its purity and moisture content is checked by suitable tests such as dew point test or gas purity tester, etc. 5. EXPERIMENTAL SYSTEM AND PROCEDURES To simulate non-uniform field stress distribution, a point-plane electrode set up was employed. In this case, point electrode used was in the shape of a 110 mm long rod with a tip radius of 1.0 mm. The plane electrode was made of aluminium with polished surface. The pair of electrodes assembly was inserted inside a pressure test vessel. It was 66 cm long and 15 cm in diameter and constructed from transparent epoxy and designed to operate in the gas pressure range of 1 bar to 6 bars. It could be evacuated with the help of a vacuum pump down to 100 mbar to evacuate air prior to uploading it with gas mixture or gas. Three types of gases i.e. SF6, N2 and CO2 were investigated to evaluate their insulating behavior under the application impulse voltages. All the gases were of technical grade and had a purity of 99%. The mixture of gases was formed based on gas partial pressures and was obtained by initially filling the pressure vessel to a predetermined pressure of SF6 after its prior evacuation, and then it was further filled with the other gas up to a total pressure level of 6.0 bar absolute. Based on the partial pressure values, a mixture of required ratio of 8% SF6N2 or 8% SF6-CO2 gas mixture was used in these studies. High voltage lightning and switching impulses were generated using a multi-stage impulse generator that is capable of delivering output in the voltage range of (0 – 1000) kVp for both polarities. It is a ten stage generator, in which each stage can be charged up to 100 kV to deliver an impulse energy of 4 kJ per stage. Fig. (1) shows the complete experimental system that was used to generate and measure the impulse voltages as applied to the non-uniform field gaps using point-plane electrodes. GC-223 is the control unit of the impulse generator. Fig. (1): Experimental set-up for non-uniform field breakdown study under impulse voltages. To acquire output voltage wave shape and measure its parameters, beside a peak impulse voltmeter (not shown here), a digital impulse measuring system (DIMS) was also used. It is equipped with a 10 bit, 100 MS/s PC-based digitizer, which can reproduce applied output impulses of both polarities with great accuracy. These impulses can be stored and retrieved at will when these are required for comparison and evaluation. RESULTS AND ANALYSIS 6.1 Uniform Field AC Breakdown Voltage of SF6 The uniform field AC breakdown voltage data of SF6, the following regression equation was derived for Vb as a function of pd values obtained from the literature [8]. In this equation Vb is in kV and pd is in bar cm. Vb = 90.2(pd) – 2.164(pd)2 + 0.021(pd)3 (1) In reference [9], the following equation has been proposed to estimate Vb for uniform field gap as a function of pd where Vb is in kV and pd is in kPa cm. Vb = 132 (pd)0.915 (2) A comparison of equations (1) and (2) as shown in Fig. (2) illustrates that equation (1) provides a more accurate fit to the experimental data over wider range of pd values in uniform field gaps for pure SF6. Vb (kVpeak) For lightning and switching impulse studies, the 5% (V5), 50% (V50) and 90% (V90) breakdown voltage values were found using statistical method, where the breakdown probabilities (Pr) were measured at several fixed voltage levels by using ten impulse applications at each set voltage level. Then, from the plotted breakdown probability breakdown voltage curves, the values for V5, V50, and V90 were obtained. On these normal probability plots, the 0.01% of probability has been plotted as = 0%, whereas 100% breakdown voltage is placed at 99.99% value of probability. 6. Fig. (2): Comparison plots of Vb and 'pd' values calculated using equation (2) (dotted line from reference [9]), and using equation (1) (solid line). Data points derived from reference [8]. 6.2 SF6 and its Mixtures under Surge Voltages Power networks are commonly subjected to lightning and switching surge stresses during their life time and so is the fate of gas filled insulated components that form its important integral elements. Study of the impact of such voltage waveforms on the insulating characteristics of gas insulated gaps is very important. In case of lightning impulse voltage, it was shown by Yoshitake et al. [10] that merely an addition of 2% of SF6 to N2 gas increases the breakdown strength of the mixture by 240% than that of pure N2 gas and exhibits strong synergistic effect. Similarly, Hayakawa et al. [11] have shown promising performance of SF6 mixtures with CO2 gas. Moreover, it was shown that though the V50 breakdown voltage in pure SF6 gas exhibits higher values under positive lightning impulse (LI) voltages in coaxial electrode system than under negative polarity LI, yet the behavior is opposite under SF6-N2 gas mixtures. Although breakdown in highly non-uniform field gaps for SF6 gas and its mixtures with other gases have been studied extensively, the switching impulse voltage behavior in dilute mixtures (with SF6 content 10%) has not been studied properly. The experimental results presented here address this aspect of the investigation. Fig. (3) shows the breakdown voltage (Vb) versus breakdown probability distribution (Pr) of SF6 gas maintained at 4.0 bar under lightning impulse 0.9/45 s waveform of both polarities. It is clear from the 'P r-Vb' curves that breakdown probability of gas varies very sharply between 10% and 100% with the applied voltage with the exception of scatter observed between 20~40% Pr values under both polarities. The scatter is slightly more under negative polarity than under positive polarity. However, the breakdown voltages under positive polarity are much smaller than under negative polarity and hence show the important role of positive polarity, both in the design of gas insulated equipment as well as in research of phenomena that lead toward breakdown of gas insulation. Fig. (4) illustrates the breakdown probability distributions as a function of voltage in SF6 gas under the applied switching impulse (SI) voltage of 150/1500 s waveform and having a positive polarity for pressure of 4 bar, respectively. It is clear that the probability distributions are non-Gaussian and are accompanied with large scatter that typically lies in a bigger range (20-80%) of Pr values. Moreover, as the pressure is increased, the range of voltage (V20-80 = V80 – V20) in which this scatter is observed, increases systematically Fig. (3): Effect of polarity on the 'Pr-Vb' curves for 100% SF6 gas maintained at 4.0 bar, under standard lightning impulse voltages of positive and negative polarities. to maximum at around 4 bar and then declines sharply at 6 bar as shown in Fig. (5). This characteristic is similar to the typical Vb-p variation of a rod-plane gap under AC stress, and illustrates that this scatter is related to the corona manifestations in the electrode gap. Anis and Srivastava [12] have reported four different types of pre-breakdown discharges in rod-plane gaps when subjected to positive switching impulse voltages. These were termed as single pulse discharge, multiple-pulse discharge, incomplete breakdown pulses and corona free breakdowns. The relative proportions amongst these forms depend on the degree of field nonuniformity and the applied pulse magnitude. The single discharge pulses prevail at pressures below the critical pressure, while multiple-pulse discharges are associated with leader-like form of discharge which occur at lower pressures and more likely under over-voltages. The incomplete breakdown pulses occur at intermediate pressures just below the critical pressure. The final one is the direct breakdown that occurs in stresses above critical pressure level. In addition, the maximum scatter in discharge onset voltage is seen to occur at gas Fig. (4): pressure corresponding to corona stabilization region. Corona stabilization has the indirect effect of increasing the scatter in the onset voltages. However, under positive polarity LI, it was shown more recently by Yoshitake et al. [10] that at intermediate gas pressures where the Vb is higher, the partial discharges are composed of three successive pulses. The first one is associated with streamer discharge while the other two occur with some time delay and are shown using streak camera to be associated with the leader discharge. The impulse breakdown was verified to propagate stepwise. It is most likely that the multiple corona pulses observed by Anis and Srivastava [10] are a similar manifestation like the triple pulse phenomena observed by Yoshitaka, under lightning impulses. The only difference is that in case of switching impulse (SI) the field gets comparatively more time to play a role in establishing an avalanche or number of avalanches resulting in the formation of several pulses. However, in this case too, it is anticipated that the last two or more pulses in the bunch occur with time delay and are associated with the formation of a leader. Pr-Vb curve for SF6 gas maintained at 4.0 bar, under positive switching impulse of 150/1500 s waveform. d = 29 mm. Fig. (5): Comparison of the variation of V(20-80) as a function of gas pressure due to SF6 gas, SF6-N2 and SF6-CO2 mixtures having 8% SF6. Figs. (6) displays an example of the breakdown probability distribution profiles as function of SI applied voltage for SF6-N2 gas mixture. It is clear that in this gas mixture too, like in pure SF6 gas, a large scatter occurs in Vb values in the probability range of 20-80%. It was found that (20-80) increases with the increase in pressure up to a level of 5 bar, where after it exhibits a sharp decline, as shown in Fig. (5). Fig. (6): Pr-Vb curve for 8% SF6-N2 gas mixture maintained at 5.0 bar, under positive switching impulse of 150/1500 s waveform. d = 29 mm. Figs. (7) illustrates a selected example of breakdown probability distribution as a function of applied voltage for SF6-CO2 mixture. A similar trend in scatter is observed in this gas mixture, too. However, in this case the scatter (20-80) increase almost monotonically up to a gas mixture presence of 5.0 bar before observing a sharp decline as in other two cases. From the (Pr–Vb) plots for SF6 gas, SF6-N2 and SF6-CO2 mixtures and for each gas pressure, the breakdown values corresponding to 5%, 50% and 95% probabilities i.e. V5, V50 and V95 were determined. Figs. (8) through (10) illustrate the variation of these parameters with the applied gas pressure. The V5 values represent the near maximum dielectric withstand capability of SF6 and its gas mixtures and are used in the design of gas insulated systems and show how such equipment will perform under non-uniform field and positive switching impulse voltages. From the results, it becomes clear that in point-plane gap spaced at 29 mm, the SF6 gas pressure of ~2 bar gives the optimum value of withstand voltage level. However, if the applied stress level is enhanced then 2.0 bar pressure is still the best choice for SF6 gas insulated equipment, since both Fig. (7): the V50 and V95 values exhibit the maximum at this pressure. In case of 8% SF6-N2 gas mixture, the optimum pressure level where all the three voltage parameters investigated exhibit maximum value also stands at 2.0 bar. Interestingly, both the SF6 and SF6-N2 mixtures exhibit a voltage withstand level of ~130 kVp at this pressure level. In case of 8% SF6-CO2 mixtures almost all the three voltage parameters increase monotonically up to a pressure of 4.0 bar, while beyond that pressure level, a decline can be noticed in V5, V50, and V95 values. At 2.0 bars the V5 value of SF6-CO2 mixtures exhibits relatively 76% of withstand voltage level as compared to that of pure SF6 and SF6-N2 gas mixture. However, in 8% SF6+CO2 mixture this voltage withstand level is achieved at an elevated gas pressure of 4.0 bar. This shows that SF6-CO2 gas mixtures could also be promising substitutes for SF6 but it will need the GIS to be designed to operate at much higher gas pressure. Pr-Vb curve for 8% SF6- CO2 gas mixture maintained at 6.0 bar, under positive switching impulse of 150/1500 s waveform. d = 29 mm. Fig. (8): Breakdown voltage versus gas pressure curves for SF6. Fig. (9): Breakdown voltage versus gas pressure curves for SF6-N2 mixture with 8% SF6. Fig. (10): Breakdown voltage versus gas pressure curves for SF6-CO2 mixture with 8% SF6. 7. CONCLUSIONS [5] G. Mauthe, L. Niemeyer, B.M. Pryor, R. Probst, H. Brautigam, P.A. O'Connell, K. Pettersson, H.D. Morrison, J. Poblotzki, and D. Koenig, "SF6 and the Global Atmosphere", Electra, No. 164, pp. 121-131, February, 1996. [6] L.G. Christophorou and R.J. Van Brunt, "SF6 Insulation: Possible Greenhouse Problems and Solutions", NIST-IR 5685, July 1995. [7] U.S. Environmental Protection Agency, "Electrical Transmission and Distribution Systems, Sulfur Hexafluoride, and Atmospheric Effects of Greenhouse Gas Emissions Conference", EPA Conference Proceedings Report, August, 1995. [8] N.H. Malik, and A.H. Qureshi, "Calculation of Discharge Inception Voltages in SF6-N2 Mixtures", IEEE Trans. on Electrical Insulation, Vol. EI-14, No. 2, pp. 70-76, 1979. [9] R.S. Nema, S.V. Kulkarni and E. Hasain, "Calculation of Sparking Potentials of SF6 and SF6-gas mixtures in Uniform and Non-Uniform Electric Fields", IEEE Trans. on Elect. Insul., Vol. 17, No. 2, pp. 70-75, 1982. The following pertinent conclusions can be derived from this study: 1. 2. 3. SF6 equipment used in Kingdom of Saudi Arabia includes circuit breakers (CBs), GIS, and GIL. The CBs and GIS operate in the voltage range of 13.8 kV to 380 kV in which gas pressure varies from 0.2 bar to 7.0 bar gauge. The number of CBs that are currently in use, well exceeds 3000, whereas GIS sections installed stand more than 15,000. Accumulative length of GIL which operates mainly in 380 kV networks exceeds 30 km. The total amount of SF6 gas contained in these units at present exceeds 700 tons. Beside that, large quantity of this gas is kept stored as spare for future use or maintenance purpose at different sites. Certain amount of SF6 gas is also released annually to the atmosphere due to malfunction or faults etc. Dielectric behavior of SF6 gas and its mixtures with other insulated gases is reasonably well understood for uniform field configurations. An empirical relation is proposed which fits the experimental data over large range of 'pd' values. Under surge voltages, addition of small quantities ( 8% of SF6) to N2 or CO2 can significantly improve their impulse withstand characteristics. In such electrode configurations the pre-breakdown discharges play a significant role in shaping the overall profile of the breakdown phenomena. Under switching surges, such discharges lead to large scatter in the breakdown probability distribution, which was found to depend on gas pressure, mixture type as well as impulse waveform parameters. In this regard both the wavefront as well as wavetail values play important role. REFERENCES [1] G. Gamilli, "Gas-insulated Power Transformers", Proc. IEE, Vol. 107 A, pp. 375-382, 1960. [2] L.G. Christophorou and R.J. Van Brunt, "SF6/N2 Mixtures, Basic and HV Insulation Properties", IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 2, pp. 952-1003, 1995. [3] L.G. Christophorou, J.K. Olthoff and R.J. Van Brunt, "Sulphur Hexafluoride and the Electric Power Industry", IEEE Electrical Insulation Magazine, Vol. 13, No. 5, pp. 20-24, 1997. [4] N.H. Malik, A.A. Al-Arainy, and M.I. Qureshi, "Electrical Insulation in Power Systems", Marcel Dekker Inc., New York, USA, 1997. [10] Y. Yoshitake et al., "Impulse Partial Discharge Propagation Mechanism Under Non-Uniform Electric Field in N2/SF6 Gas Mixtures", Proceedings of 13th Int'l Symposium on High Voltage Engineering, 2003. [11] N. Hayakawa et al., "Impulse Partial Discharge and Breakdown Characteristics Under NonUniform Field in CO2 and N2/CO2 Gas Mixtures", i.b.i.d., 2003. [12] H. Anis and K.D. Srivastava, "Pre-breakdown Discharges in Rod-Plane Gaps in SF6 Under Positive Switching Impulses", IEEE Trans. on Electrical Insulation, Vol. EI-16, No. 6, pp. 552563, 1981.