Breakdown Mechanism of Different Sulphur Hexafluoride Gas Mixtures

Sulphur hexafluoride (SF6) gas and SF6 gas mixtures are widely used in gas-insulated systems (GIS) because they have good dielectric and thermal properties. Among the various gas mixtures investigated so far, SF6-air, SF6-CO2, and SF6-N2 are most used gas mixtures. Also, these mixtures have much technical superiority in GIS. )ese are nontoxic and nonflammable gases, and they have less sensitivity to nonuniformities and higher liquefaction temperature at high pressure. GIS, switchgears, circuit breakers, and substations are systems that work with alternating current and have nonuniform electric fields. For this reason, in this study the breakdown mechanisms of 0.125, 0.5, 1, and 20% SF6 gas mixtures with rod-plane configuration under AC voltage are investigated and explained breakdown mechanism.


Introduction
Sulphur hexafluoride (SF 6 ) is the most popular insulating gas in the electric power systems such as switchgears, gas circuit breakers, and gas-insulated stations because of having good electrical, thermal, and chemical properties.However, the dielctric strength of SF 6 at nonuniform electric field falls drastically to very low values because SF 6 is very much sensitive to nonuniformities like impurities and electrode configuration.Since the molecule of SF 6 is large and it has small free pathways besides the electrons that are easily attached to SF 6 molecules, it has a high dielectric strength.SF 6 is a very strong greenhouse gas; for this reason, its global use is restricted on the world.Some greenhouse gases, such as CO 2 , are naturally found in nature.Some of them are industrial gases like CO, CH, CFCs, SF 6 , and HCFC-22.Greenhouse gases hold longwave radiation while they allow shortwave radiation to pass to earth. is case causes Earth to warm up. is is an undesirable situation.Today, voluntary programs work to reduce the emission of SF 6 into the atmosphere.ese programs encourage us to work efficiently at power systems by using less SF 6 .Our study supports this scientific mission.Besides, SF 6 is more expensive than other insulating gases.us, it is essential to improve the different dielectric gas or gas mixtures having more appropriate insulating properties, inexpensive, and without greenhouse effect.So, SF 6 gas and its mixtures with such as air, nitrogen (N 2 ), and carbon dioxide (CO 2 ) have been extensively studied [1][2][3][4][5][6].e aim of this study is to investigate SF 6 mixtures in terms of their electrical strength and to clarify the mechanisms of breakdown.Breakdown properties of SF 6 gas mixtures in uniform and quasi-uniform electric field are nearly similar, but their breakdown characteristics in nonuniform electric field depend on various factors such as gas pressure, gas temperature, electrode gap spacing, and gas ratios in the mixture and electrode configuration.
e breakdown properties of SF 6 gas mixtures in uniform field under impulse and direct voltage have been studied in earlier researches [7][8][9], but the examination of the characteristics in nonuniform fields is open to research.
Earlier studies have shown that the breakdown voltage of mixtures of SF 6 -N 2 with below containing 30% SF 6 are less than that of pure SF 6 at the pressure of 5 bar.At the same pressure and negative polarity, 0.1% SF 6 -N 2 mixture has half dielectric strength compared with that of 100% SF 6 .e breakdown voltage of SF 6 -CO 2 mixtures is higher than that of SF 6 -N 2 mixtures at the pressure of approximately 4-5 bar, while the dielectric strengths of two mixtures are the same at low pressures at negative and positive polarities for direct voltage.However, the strength of negative polarity is always higher than that of positive polarity as in all nonuniform elds.e breakdown voltage of SF 6 -CO 2 mixtures is higher than that of pure SF 6 at some pressures for impulse voltages.
As an example at above the pressure of 3 bar, the strengths of 1% SF 6 -CO 2 mixtures are slightly higher than those of pure CO 2 at negative impulse voltage [10].

Test Setup
In the experimental study, rod-plane electrode con guration is used to simulate the nonuniform elds in practical systems (Figure 1) because rod-plane electrode system represents the best nonuniformities.e rod electrode with tip radius of 1 mm and the plane electrode with diameter of 75 mm are used.
All experiments are performed in the pressure range from 1 bar to 5 bar and at 25 mm xed gap spacing.e electrodes are located in a pressure tube as test vessel that has 120 mm diameter and 600 mm length.In rod-plane electrode con guration, high voltage is applied to the rod electrode while the plane electrode is earthed (Figure 1).e test vessel is rstly evacuated and then lled with the gas mixtures to the pressure of 5 bar.For the 50 Hz AC tests, 100 kVrms, and 5 kVA, a high-voltage test transformer is used.A capacitive divider and voltmeter are used to measure the applied high voltage.Experiments were carried out in Istanbul Technical University (ITU) High Voltage Laboratory (Figure 2).
In this study, the breakdown voltages of CO 2 , N 2 , air, SF 6 , and the mixtures of SF 6 -CO 2 , SF 6 -N 2 , and SF 6 -air with 20, 1, 0.5, and 0.125% of SF 6 are measured from 1 bar to 5 bar with 1 bar steps at rod-plane electrode con guration.

Experimental Results
SF 6 gas mixtures are lot in nature and cheap.
ese gas mixtures are selected as air, N 2 , and CO 2 .e breakdown voltages of these gas mixtures depending on pressure are shown in Figures 3-5, respectively.Since the decompositions of these gas mixtures under pressure are important, air, N 2 , and CO 2 gases are selected in this study.
As seen in Figures 3-5, a little bit addition of SF 6 to these gases has signi cantly increased the breakdown voltage values.
e high breakdown of voltage means that high electrical strength is very important in insulation technique in power systems.As can be seen from the above discussion, dielectric strength of only N 2 , CO 2 , and air increases linearly with increasing pressure, whereas the breakdown voltages of SF 6 mixtures are approximately the same at a pressure of higher than 4 bar.e mixture having the lowest dielectric strength among the three mixtures is the mixture of SF 6 -N 2 .
e e ect of N 2 on breakdown strength is less than that of air and CO 2 .e reason for this is electron mobility and ionization energy.e gas additive to SF 6 a ects the charge density and the width of streamer between the electrodes.Besides, electron mobility a ects the delay of leader streamer in gas mixtures.For this reason, the breakdown mechanisms of SF 6 gas mixtures are di erent from each other.It is known that the breakdown mechanism of SF 6 is streamer and stepped leader breakdowns.As the SF 6 ratio increases, the stepped leader mechanism is valid.e leader step time is the interval time between two consecutive leader steps and is where p is the gas pressure, U is the voltage applied between the electrodes, and K is the constant depending on the SF 6 gas mixtures ratio.For SF 6 -N 2 mixtures, K is smaller than that for SF 6 -CO 2 and SF 6 -air mixtures [11].ese mixtures have many superiorities compared to pure SF 6 .e addition of relatively small amount of SF 6 to nitrogen, carbon dioxide, and air causes a signi cant increase in dielectric strength.As an example, breakdown voltage of 0.125% of SF 6 -air is 1.5 times higher than that of pure air at the pressure of 4 bar.
Figure 6 gives the variation of breakdown voltage of pure SF 6 and 0.125% SF 6 mixtures depending on gas pressure.As seen in Figure 6, for 25 mm rod-plane gap, breakdown voltage of pure SF 6 is about 3 times higher than that of air, N 2 , and CO 2 at pressure of 1 bar.However, as the gas pressure is increased to about 3.5 bar, breakdown voltage of SF 6 is 20% less than that of air.At the pressure of 5 bar, pure SF 6 is 20% higher than that of air.e breakdown voltages of CO 2 and N 2 are lower than that of SF 6 .As seen in Figures 3-5, the breakdown voltages of pure N 2 , CO 2 , and air are increased approximately linear with pressure; however, pure SF 6 has a maximum-minimum breakdown characteristic.
e experiments have shown that addition of relatively small amount of SF 6 to air, N 2 , and CO 2 causes important increase in dielectric strength.As an example, the breakdown voltage of 0.125 SF 6 -air is approximately twice than that of pure SF 6 at pressure of 3.5 bar.Breakdown strength of SF 6 -CO 2 is higher than that of other mixtures at the pressure range of 1-3 bar, but SF 6 -air mixtures are better at higher pressures.From experimental results, SF 6 -air mixtures show less degree of saturation at higher pressure when compared to SF 6 -N 2 and SF 6 -CO 2 mixtures.Since breakdown phenomena occur in the positive polarity, the breakdown mechanism explanations about AC voltage are related to the mechanism of positive impulse and direct voltage.It is needed to mention the breakdown mechanism to explain the abnormal case.In this study, the electrode gap is of 25 mm.e breaker contact spacings used in practice are in these ranges, and most of them have nonuniform elds.e best model to simulate these nonuniformities is the rod-plane electrode con guration.e contact ranges of the switchgears vary between 1 and 10 cm according to the nominal voltage and current.However, for GIS, these gaps may be 25-30 cm, but the investigations show that they have the same breakdown mechanism.As is known, there are two types of breakdown mechanisms in gas mixtures.
ese are Townsend theory and streamer theory.Townsend breakdown mechanism is valid for smaller product of the gas pressure (mmHg) and the electrode gap (cm) of 500 mmHg•cm.

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Especially at large electrode gap spacing, Townsend theory is insufficient to explain the discharge mechanism.Streamers in short gaps develop with the increase of the applied voltage until spark takes place.Physical and chemical interpretations can be made about some of the characteristic cases.Maximum-minimum behaviour due to gas pressures is indicated as abnormal discharges in the literature.e reason why breakdown voltage has maximum-minimum characteristics according to the pressure is related to the following situations: nonuniform field, SF 6 gas, positive polarity voltage, or AC voltage.
e breakdown voltage increases with the increasing pressure.is is a very normal situation.If the ambient has nonuniform field, SF 6 gas and AC voltage as in our system, variation of breakdown voltage has the maximum-minimum characteristic.Earlier research has shown that the rise time duration of the test voltage increases when the possibility of effective corona stabilization raises.is behaviour is significant for gas pressures of 1 and 2 bar.Also for longer rise time duration and higher gas pressure of 3 bar, the effect of corona stabilization is weak.
is means with high gas pressure and the low leader inception voltage, the breakdown strength reduces. is period is the diffusion time of a thermic wave. is wave starts from the high-field electrode system and spreads the whole gap.e temperature rises to as high as 900 °K under AC voltages in the ionization region.So it is important to reduce the temperature at this region.e electrical strength rises at point and the spark becomes easily.
is theory is investigated as "a narrow jet model" at dc voltage by R. S. Sigmond.As a result, the diffusion and heat convection in the pressure range of 2-3 bar decrease, and the thermal ionization brought by the electron avalanche plays an important role.In this case, breakdown follows the shortest ionized path.e reason of this abnormal breakdown is the opposite effect of ionization and diffusion phenomena related to breakdown voltage.Diffusion is reduced by the pressure increase; this case leads to increase of space charge and electrical strength.
is phenomenon causes the breakdown voltage to decrease.Ionization effect is active at positive polarity since heavy positive ions cannot reach the opposite electrode easily, and the positive ions are collected at the tip of the rod electrode, so they shorten the space gap.Diffusion also causes the thermal convection, and the breakdown follows the shortest way between the electrodes.As seen in Figures 3-5, the maximum point shifts to the right as the SF 6 ratio decreases [12].
is is also an abnormal condition in SF 6 gas mixtures for nonuniform fields.Small SF 6 mixtures may fall to a minimum at higher pressures.Ionization and diffusion events are also effective here.

Conclusion
e test results show that SF 6 mixtures have an important electrical strength at the pressure range of 1 -4 bar according to electrical strengths of pure air, N 2 , and CO 2 .SF 6 has disadvantage properties particularly between the pressure of 2-3 bar, which is the range of use of electrical devices, but SF 6 gas mixtures have many superiorities compared to pure SF 6 .
ese mixtures are technically and economically attractive alternatives.ese mixtures are cheaper and more environmentally friendly than pure SF 6 .It is important to check the chemical decomposition of SF 6 mixture under electrical stress at various mixture ratios.Further studies will focus on decomposition of SF 6 mixtures under pressure at ambient with water vapour and oxygen because at this case decomposition of SF 6 forms as solid state.