energies

Article AC Breakdown Characteristics of c-C4F8/N2 Gas Mixtures in an Extremely Non-Uniform Electric Field

Lin Lin 1,* , Qingguo Chen 1,*, Xinyu Wang 1,*, Hui Zhang 1 , Haoran Feng 2 and Cong Zhang 1

1 Key Laboratory of Engineering and Its Application of Ministry of Education & College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, China; [email protected] (H.Z.); [email protected] (C.Z.) 2 State Grid Zhejiang Xinchang Power Supply Co. LTD, Shaoxing 312000, China; [email protected] * Correspondence: [email protected] (L.L.); [email protected] (Q.C.); [email protected] (X.W.); Fax: +86-0451-86391625 (L.L.)  Received: 5 November 2018; Accepted: 13 December 2018; Published: 19 December 2018 

Abstract: Octafluorocyclobutane (c-C4F8) is one of the environmentally friendly gases with the potential to replace SF6. In this paper, the AC breakdown characteristics of c-C4F8/N2 mixture in an extremely non-uniform electric field were studied; the effects of the gas pressure, electrode distance, and c-C4F8 volume fraction on the were analyzed; and the feasibility of replacing SF6 with c-C4F8/N2 was verified. The results show that the breakdown voltage of c-C4F8 /N2 increases with an increase in pressure, electrode spacing, and volume fraction of c-C4F8 in an extremely non-uniform electric field. Under the same conditions, the breakdown voltage of 20%c-C4F8/80%N2 is 46–90% that of 20%SF6/80%N2. When the pressure is 0.3 MPa, the 20%c-C4F8/80%N2 breakdown voltage can reach over 57% that of SF6. Taking into consideration the environment, liquefaction temperature, and insulation strength, 20%c-C4F8/80%N2 may replace SF6 used for medium- and low-voltage equipment.

Keywords: gas insulation; alternative gas of SF6; breakdown voltage; extremely non-uniform electric field; c-C4F8/N2

1. Introduction

Sulfur hexafluoride (SF6) is widely used in power systems and electrical equipment owing to its high insulation level and arc-extinguishing capacity [1,2]. However, the non-homogeneity of an electric field, the conducting particles, and the surface roughness of the electrode all have a significant influence on the strength of SF6 during use [3–5]. Above all, SF6 is a potent greenhouse gas whose (GWP) is 23,900 times that of CO2, and whose life expectancy is 3200 years in the atmosphere, which causes a severe greenhouse effect [6,7]. Therefore, in the Kyoto protocol adopted by Japan on December 1997, SF6 was listed as one of six gases requiring global regulation [8]. Considering the environment, it has become an inevitable tendency to find a new environment-friendly gas to replace SF6. Thus, many gases with high insulating strength have been concentrated on by researchers to replace SF6, such as octurobutane (c-C4F8), trifluoroiodomethane(CF3I), heptafluorobutyronitrile (C4F7N) and trifluorovinyl ether (C5F10O). However, the boiling points of C4F7N and C5F10O are high, which restrict the use for low temperature environments. Although CF3I has low toxicity, the decomposition product of CF3I contains iodine, and the effect of iodine on breakdown is not clear [9], while c-C4F8 is an electronegative gas with stable chemical properties, and displays non-toxicity, non-combustibility, and non-ozone effects [10]. The breakdown voltage of c-C4F8 is 1.3 times of SF6 under a uniform electric field, while the GWP of c-C4F8 is 8700, which is about 1/3

Energies 2018, 11, 3533; doi:10.3390/en11123533 www.mdpi.com/journal/energies Energies 2018, 11, 3533 2 of 9

of SF6, so the impact on the environment is much smaller than that of SF6 [11,12]. c-C4F8 mixed gas was listed as an insulating gas for future long-term research at the 1997 technical conference of the National Association of Standards and Technology [13]. Owing to the high liquefaction temperature of c-C4F8 gas, it cannot be directly used in electrical equipment, and buffer gases such as N2 or CO2 are often added in use. So far, researchers have conducted many studies on the electrical properties of c-C4F8 mixtures [6,14]. Xiao et al. investigated the AC breakdown characteristics of c-C4F8/N2 and c-C4F8/CO2 under a slightly non-uniform electric field, analyzed the breakdown products, and obtained the synergetic coefficients of mixed gases under different volume fractions. Results indicate that c-C4F8/N2 and c-C4F8/CO2 can be used as an alternative gas to SF6 under certain conditions [15,16]. Xing et al. studied the partial discharge characteristics of a c-C4F8/N2 mixed gas, and demonstrated the feasibility of substituting c-C4F8/N2 for SF6 in terms of the liquid temperature, environmental influence, and partial discharge characteristics [17]. Li et al. analyzed the decomposition products of a c-C4F8/N2 mixed gas under typical faults such as local overheating, a partial discharge, spark, and arc discharges, and confirmed that the breakdown products of a c-C4F8/N2 mixture under several common faults are less toxic, thus c-C4F8/N2 can be used as an alternative to SF6 [18]. Thus far, researchers have studied the insulation properties of a c-C4F8/N2 mixture under a uniform and slightly non-uniform electric field. However, in practical application, owing to the limitation of processing technology, burrs or suspended impurity particles are present in gas-insulated high-voltage equipment, which will lead to a distortion of their local electric field. Therefore, it is also particularly important to investigate the breakdown characteristics of the gas insulated under an extremely non-uniform electric field. In this paper, a discharge device was established and an AC test platform was set up. Based on the use conditions, the volume fraction of c-C4F8 in the test was determined, and the breakdown voltages of c-C4F8/N2 under different pressures, electrode distances, and volume fractions of c-C4F8 were tested. The trend of breakdown voltage changes with pressure, electrode distance and the volume fraction of c-C4F8 was then analyzed. The feasibility of replacing SF6 with c-C4F8/N2 is discussed.

2. Experimental Setup

2.1. Experimental Circuit The experimental circuit adopted in this paper is shown in Figure1. The adjustable range of the voltage regulator was 0–220V and the was a YWDT-15kVA/150kV model (Yingkou Jingcheng Special Transformer Co. Ltd., Yingkou, China). Resistance in the circuit was 10 kΩ, used to reduce the output current and protect the transformer. The current of the relay in the voltage regulator was set at 2.1 A. The circuit was arranged so that when the gas broke down, the current reached a set value, the relay opened, and the breakdown voltage measured. The values of capacitor C1 and C2 were, respectively, 309 pF and 0.307 µF[19,20]. Before the test, the discharge device was vacuumed, and then filled with mixed gas. During the test, the voltage was manually increased until breakdown of the mixed gas. When the voltage is much less than the breakdown voltage, the boost speed of the voltage is high; conversely, the boost speed slows when the voltage is close to the breakdown voltage. After five duplicate tests, the average value was taken as the breakdown voltage. The interval between the tests was 5 min. The discharge device is shown in Figure2. As the pressure of the insulating gas used is generally 0.3–0.5 MPa [7,21], the discharge device must safely withstand a pressure of at least 0.5 MPa. The cavity of the discharge device was enclosed by a stainless steel 304 seamless steel tube with a diameter of 300 mm, height of 470 mm, and thickness of 10 mm. The upper and lower closure plates were made of stainless steel 304 with a thickness of 20 mm. In addition, two observation windows with a diameter of 50 mm were placed on both sides of the cavity to observe the experimental phenomena. The observation windows were made of a stainless steel flange and tempered glass. A high-voltage electrode was introduced through high-voltage bushing. Energies 2018, 11, x FOR PEER REVIEW 2 of 10

45 insulating gas for future long-term research at the 1997 technical conference of the National 46 Association of Standards and Technology [13]. Owing to the high liquefaction temperature of c-C4F8 47 gas, it cannot be directly used in electrical equipment, and buffer gases such as N2 or CO2 are often 48 added in use. So far, researchers have conducted many studies on the electrical properties of c-C4F8 49 mixtures [6,14]. Xiao et al. investigated the AC breakdown characteristics of c-C4F8/N2 and 50 c-C4F8/CO2 under a slightly non-uniform electric field, analyzed the breakdown products, and 51 obtained the synergetic coefficients of mixed gases under different volume fractions. Results indicate 52 that c-C4F8/N2 and c-C4F8/CO2 can be used as an alternative gas to SF6 under certain conditions 53 [15,16]. Xing et al. studied the partial discharge characteristics of a c-C4F8/N2 mixed gas, and 54 demonstrated the feasibility of substituting c-C4F8/N2 for SF6 in terms of the liquid temperature, 55 environmental influence, and partial discharge characteristics [17]. Li et al. analyzed the 56 decomposition products of a c-C4F8/N2 mixed gas under typical faults such as local overheating, a 57 partial discharge, spark, and arc discharges, and confirmed that the breakdown products of a 58 c-C4F8/N2 mixture under several common faults are less toxic, thus c-C4F8/N2 can be used as an 59 alternative to SF6 [18]. 60 Thus far, researchers have studied the insulation properties of a c-C4F8/N2 mixture under a 61 uniform and slightly non-uniform electric field. However, in practical application, owing to the 62 limitation of processing technology, burrs or suspended impurity particles are present in 63 gas-insulated high-voltage equipment, which will lead to a distortion of their local electric field. 64 Therefore, it is also particularly important to investigate the breakdown characteristics of the gas 65 insulated under an extremely non-uniform electric field. In this paper, a discharge device was 66 established and an AC test platform was set up. Based on the use conditions, the volume fraction of 67 c-C4F8 in the test was determined, and the breakdown voltages of c-C4F8/N2 under different 68 pressures, electrode distances, and volume fractions of c-C4F8 were tested. The trend of breakdown 69 voltage changes with pressure, electrode distance and the volume fraction of c-C4F8 was then 70 analyzed. The feasibility of replacing SF6 with c-C4F8/N2 is discussed.

71 2. Experimental Setup

72 2.1. Experimental Circuit 73 The experimental circuit adopted in this paper is shown in Figure 1. The adjustable range of the 74 voltage regulator was 0–220V and the transformer was a YWDT-15kVA/150kV model (Yingkou 75 Jingcheng Special Transformer Co. Ltd., Yingkou,China). Resistance in the circuit was 10 kΩ, used to 76 reduce the output current and protect the transformer. The current of the relay in the voltage 77 regulator was set at 2.1 A. The circuit was arranged so that when the gas broke down, the current 78 reached a set value, the relay opened, and the breakdown voltage measured. The values of capacitor 79 C1 and C2 were, respectively, 309 pF and 0.307 F [19,20]. Before the test, the discharge device was 80 vacuumed，and then filled with mixed gas. During the test, the voltage was manually increased 81 until breakdown of the mixed gas. When the voltage is much less than the breakdown voltage, the 82 boost speed of the voltage is high; conversely, the boost speed slows when the voltage is close to the

83 breakdownEnergies 2018, 11 voltage., 3533 After five duplicate tests, the average value was taken as the breakdown3 of 9 84 voltage. The interval between the tests was 5 min. Energies 2018, 11, x FOR PEER REVIEW 3 of 10

86 Figure 1. Experiment circuit.

87 The discharge device is shown in Figure 2. As the pressure of the insulating gas used is 88 generally 0.3–0.5 MPa[7,21], the discharge device must safely withstand a pressure of at least 0.5 89 MPa. The cavity of the discharge device was enclosed by a stainless steel 304 seamless steel tube with 90 a diameter of 300 mm, height of 470 mm, and thickness of 10 mm. The upper and lower closure 91 plates were made of stainless steel 304 with a thickness of 20 mm. In addition, two observation 92 windows with a diameter of 50 mm were placed on both sides of the cavity to observe the 8593 experimental phenomena. The observation windows were made of a stainless steel flange and 94 tempered glass. A high-voltage electrodeFigure wa 1.s Experimentintroduced circuit. through high-voltage bushing.

95 96 Figure 2. Photograph of discharge device.

97 2.2. Electrode and Electric Field Simulation 98 In this study, aa needle-plateneedle-plate electrodeelectrode waswas usedused toto generategenerate anan extremelyextremely non-uniformnon-uniform electric 99 field.field. The electrodeelectrode distancedistance ranged ranged from from 0 to0 to 20 20 mm. mm. The The diameter diameter of theof the plate plate electrode electrode was was 140 mm,140 100 themm, thickness the thickness was 25 was mm, 25 and mm, the chamferand the radiuschamfer was radius 8 mm. was The 8 diameter mm. The of thediameter needle of electrode the needle was 101 5electrode mm, the was length 5 mm, was the 50 mm,length the was length 50 mm, of the the tip length was 5 mm,of the and tip the was tip 5 curvaturemm, and radiusthe tip wascurvature about 102 0.15radius mm; was in about addition, 0.15 mm; both in needle-plate addition, both electrodes needle-p werelate electrodes made of brass. were Becausemade of thebrass. discharge Because will the 103 leaddischarge to material will lead loss to at material the tip and loss increase at the tip the and curvature increase radius the curvature of the electrode, radius of the the needle electrode, electrode the 104 shouldneedle electrode be regularly should replaced. be regularly replaced. 105 During the test, the electrode distancesdistances werewere 5,5, 10,10, 15,15, andand 2020 mm.mm. Here we calculate the 106 non-uniformity coefficientscoefficientsf underf under different different electrode electrode distances distances with thewith following the following expression expression [20,22]: 107 [20,22]: E f = max (1) EEmaxav f= (1) EavU E = (2) av d U where Emax is the maximum electric field strength (V/m), Eav is the average electric field strength Eav= (2) (V/m), U is voltage applied to the electrode (V), anddd is the electrode distance (m). We used COMSOL software to simulate the electric field distribution, and obtained the Emax 108 where Emax is the maximum electric field strength (V/m), Eav is the average electric field strength between the needle-plates with different electrode distances. The simulation result when the electrode 109 (V/m), U is voltage applied to the electrode (V), and d is the electrode distance (m). distance was 20 mm is shown in Figure3. 110 We used COMSOL software to simulate the electric field distribution, and obtained the Emax 111 between the needle-plates with different electrode distances. The simulation result when the 112 electrode distance was 20 mm is shown in Figure 3.

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113 Figure 3. Electric field simulation of needle-plate electrode at 20 mm. 114 Figure 3. Electric field simulation of needle-plate electrode at 20 mm. Based on the simulation results, the non-uniformity coefficients calculated under different 115 electrodeBased distances on the simulation are shown inresults, Table1 .the non-uniformity coefficients calculated under different 116 electrode distances are shown in Table 1. Table 1. Non-uniformity coefficients under different electrode distances. 117 Table 1. Non-uniformity coefficients under different electrode distances. Electrode gap /mm 5 10 15 20 Non-uniformityElectrode gap coefficients /mm 4.25 5.7510 15 6.9 20 7.9 Non-uniformity coefficients 4.2 5.75 6.9 7.9

It can be seen from the calculation results that the non-uniformity coefficients of the electric field 118 It can be seen from the calculation results that the non-uniformity coefficients of the electric calculated at different electrode distances are all greater than 4 [22], and thus the distribution of the 119 field calculated at different electrode distances are all greater than 4 [22], and thus the distribution of electric field between the inter-electrode conforms to the requirement of an extremely non-uniform 120 the electric field between the inter-electrode conforms to the requirement of an extremely electric field. 121 non-uniform electric field. 2.3. Volume Fraction of Mixture 122 2.3. Volume Fraction of Mixture c-C4F8 and N2 used were manufactured by Huate Gas Co., Ltd (Foshan, China), and the prices 123 c-C4F8 and N2 used were manufactured by Huate Gas Co., Ltd (Foshan, China), and the prices were USD 75.00 and USD 0.70 per kilogram, respectively. The purity of c-C4F8 and N2 in the 124 were USD 75.00 and USD 0.70 per kilogram, respectively. The purity of ◦c-C4F8 and N2 in the experiment was above 99.9%. The liquefaction temperature of N2 is −195.8 C, which is far lower 125 experiment was above◦ 99.9%. The liquefaction temperature of N2 is −195.8◦ °C, which is far lower than c-C4F8 (−8 C). In the test, the mixture was set at 0.5 MPa and −10 C without liquefaction. 126 thanActually, c-C4F the8 (− liquefaction8 °C). In the temperature test, the mixture of the mixturewas set should at 0.5 beMPa lower and than −10 the°C operatingwithout liquefaction. temperature. 127 Actually, the liquefaction temperature of the mixture should be lower than the operating The maximum volume fraction of c-C4F8 in the mixture can be calculated through the following 128 temperature.formula [23]: The maximum volume fraction of c-C4F8 in the mixture can be calculated through the 129 following formula [23]: T T = b (3) mb 1 − ln(10kP)/10.5 Tb Tmb = (3) where Tmb is the liquefaction temperature of1- a mixed ln() 10kP gas (K), / 10.5Tb is the liquefaction temperature value of c-C4F8 (K), P is the gas pressure (MPa), and k is the volume fraction of c-C4F8 when the gas pressure 130 is P.where Tmb is the liquefaction temperature of a mixed gas (K), Tb is the liquefaction temperature 131 valueIt of can c-C be4F8 determined (K), P is the through gas pressure Equation (MPa), (3) and that k the is the volume volume fraction fraction of c-C of 4c-CF8 4inF8 awhen mixed the gas gas is 132 pressurenot more is than P . 20%. In the test, the pressure ranged from 0.1 to 0.5 MPa; the testing step length was 133 0.05It MPa; can thebe determined volume fractions through of c-CEquation4F8 were (3) 5%,that 10%, the volume 15%, and fraction 20%; the of accuracyc-C4F8 in a of mixed mixed gas gases is 134 notwas more±1%; than and 20%. the experiment In the test, wasthe pressure conducted ranged at 20 ◦fromC. 0.1 to 0.5 MPa; the testing step length was 135 0.05 MPa; the volume fractions of c-C4F8 were 5%, 10%, 15%, and 20%; the accuracy of mixed gases 136 was ±1%; and the experiment was conducted at 20 °C.

137 3. Results and Discussion

138 3.1. Breakdown Voltages of c-C4F8/N2 Mixture Changes with Pressure

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3. Results and Discussion

3.1.Energies Breakdown 2018, 11, Voltagesx FOR PEER of REVIEW c-C4F8/N 2 Mixture Changes with Pressure 5 of 10

The AC breakdown characteristics of c-C4F8/N2 in an extremely non-uniform electric field were 139 The AC breakdown characteristics of c-C4F8/N2 in an extremely non-uniform electric field were 140 obtainedobtained from from the the test, test, and and the the breakdown breakdown voltages voltages of the of gasthe mixture gas mixture changed changed with the with gas the pressure gas 141 arepressure shown are in Figureshown4 in. Figure 4.

142 FigureFigure 4. 4.The Thebreakdown breakdown voltagesvoltages of the mixture va varyingrying with with pressure: pressure: electrode electrode distance distance d ofd of (a) ( a5,) 5, 143 (b(b)) 10, 10, ( c()c) 15, 15, and and ( d(d)) 20 20 mm.mm.

144 ItIt can can be be seen seen from from Figure Figure4 that 4 that the pressurethe pressure has a has significant a significant impact impact on the on breakdown the breakdown voltage. 145 Onvoltage. the whole, On the the whole, AC breakdown the AC breakdown voltages ofvoltages c-C4F8 /Nof c-C2 mixture4F8/N2 mixture increase increase with the with increase the increase in pressure. in 146 Whenpressure. the gapsWhen between the gaps the between electrodes the are electrodes 5, 10, or 15are mm, 5, 10, the or AC 15 breakdownmm, the AC voltage breakdown shows voltage the same 147 increaseshows the trend same as increase the change trend in as pressure. the change When in pres the electrodesure. When distance the electrode is 5 or distance 10 mm, is the 5 or breakdown 10 mm, 148 voltagethe breakdown of the gas voltage at a pressure of the ofgas 0.5 at MPa a pressure increases of approximately0.5 MPa increases two-fold approximately to 0.1 MPa, two-fold whereas to when 0.1 149 theMPa, electrode whereas distance when isthe 15 electrode mm, the distance breakdown is 15 voltages mm, the increases breakdown by about voltages 1.5-fold. increases The reasonby about why 150 the1.5-fold. breakdown The reason voltages why increase the breakdown with pressure voltages is incr because,ease with as the pressure pressure is be increases,cause, as thethe numberpressure of 151 moleculesincreases, perthevolume number increases, of molecules which per shortens volume the increases, free path which of the electrons.shortens the The free amount path of of energy the 152 accumulatedelectrons. The by amount the electrons of energy decreases accumulated in the free by path, the thuselectrons affecting decreases the development in the free of path, an electron thus 153 avalanche.affecting the At thedevelopment same time, of c-C an4F electr8 is anon electronegative avalanche. At gas,the withsame atime, relatively c-C4F large8 is an cross-section electronegative in the 154 lowgas, energy with a region, relatively and canlarge attract cross-section low-energy in electrons,the low energy thereby region, generating and negativecan attract low-energy and further 155 increasingelectrons, thethereby breakdown generating voltage. negative When ions theand electrode further increasing distance isthe 20 breakdown mm, the breakdown voltage. When voltages the 156 showedelectrode different distance trends is 20 withmm, the pressure.breakdown Under voltag aes pressure showed ofdifferent 0.15 MPa, trends the with breakdown the pressure. voltage 157 increasesUnder a rapidly,pressure andof 0.15 breakdown MPa, the voltagesbreakdown between voltage 0.15 increases and 0.40 rapidly, MPa increaseand breakdown slowly. Whenvoltages the 158 between 0.15 and 0.40 MPa increase slowly. When the pressure exceeds 0.4 MPa, the breakdown 159 voltages increases sharply, that is, a “hump” appears. This phenomenon exists in the breakdown of 160 mixed gases such as SF6, C3F8, and CF3I [24]. The main reason for this is that a stable corona can be 161 formed at the tip of the needle in an extremely non-uniform electric field, and the electric field

Energies 2018, 11, 3533 6 of 9

pressure exceeds 0.4 MPa, the breakdown voltages increases sharply, that is, a “hump” appears. This phenomenon exists in the breakdown of mixed gases such as SF6,C3F8, and CF3I[24]. The main Energies 2018, 11, x FOR PEER REVIEW 6 of 10 reason for this is that a stable corona can be formed at the tip of the needle in an extremely non-uniform electric field, and the electric field distribution around the needle electrode can be improved to 162 distribution around the needle electrode can be improved to further increase the breakdown 163 furthervoltages; increase thus, the the breakdown breakdown voltage voltages; is thus,higher the wh breakdownen the pressure voltage is 0.15 is higher MPa. whenHowever, the pressureas the 164 ispressure 0.15 MPa. increases However, further, as thethe pressurediffusion increasesmotion of further,the space the charge diffusion decreases, motion the of size the of space the corona charge 165 decreases,decreases, the the size effect of theon the corona improvement decreases, of the the effect electric on the field improvement diminishes, ofand the the electric breakdown field diminishes, voltage 166 andincreases the breakdown slowly. voltage increases slowly.

3.2. Breakdown Voltages of c-C4F8/N2 Mixture Varying with Electrode Distance 167 3.2. Breakdown Voltages of c-C4F8/N2 Mixture Varying with Electrode Distance The breakdown voltage of c-C F /N gas varied with the electrode distance, as shown in Figure5. 168 The breakdown voltage of c-C44F8/N8 2 gas2 varied with the electrode distance, as shown in Figure 5.

Figure 5. The breakdown voltage of c-C4F8/N2 gas varying with the electrode distance for k of (a) 5%,

169 (bFigure) 10%, 5. (c The) 15%, breakdown and (d) 20%. voltage of c-C4F8/N2 gas varying with the electrode distance for k of (a) 5%, 170 (b) 10%, (c) 15%, and (d) 20%. As shown in Figure5, c-C 4F8/N2 shows the same growth trend with the change in electrode

171 distanceAs undershown ain different Figure 5, volume c-C4F8/N fraction.2 shows Whenthe same the growth electrode trend distance with the is 10 change mm, noin electrode significant 172 improvementdistance under is showna different when volume compared fraction. with When the breakdown the electrode voltage distance of 5is mm. 10 mm, When no the significant electrode 173 distanceimprovement is larger is shown than 10 when mm, thecompared breakdown with voltagethe breakdown significantly voltage increases of 5 mm. in aWhen linear the fashion. electrode 174 distance is larger than 10 mm, the breakdown voltage significantly increases in a linear fashion. 3.3. Breakdown Voltages of c-C4F8/N2 Mixture Varying with Volume Fraction

175 3.3. Breakdown Voltages of c-C4F8/N2 Mixture Varying with Volume Fraction When the electrode distance is 5 or 15 mm, the breakdown voltages of the c-C4F8/N2 mixture

176 changeWhen with the the electrode volume fraction distancek ofis 5 c-C or4 F158, mm, as shown the breakdown in Figure6 .voltages of the c-C4F8/N2 mixture 177 change with the volume fraction k of c-C4F8, as shown in Figure 6.

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4 8 2 178178 FigureFigureFigure 6. 6. The The breakdown breakdown voltage voltage of of c-C c-C 4F48F/NF8/N/N2 gas gas2 gas varying varying varying with with with the the thevolume volume volume fraction fraction fraction for for d for dof of d( aof()a )5 ( 5aand )and 5 and 179179 (b((bb) ))15 1515 mm. mm.mm.

According to Figure6, as the volume fraction of c-C4 48F8 increases, the of 180180 AccordingAccording to to Figure Figure 6, 6, as as the the volume volume fraction fraction of of c-C c-C4FF8 increases, increases, the the dielectric dielectric strength strength of of c-C44F8/N2 2 increases. When the electrode distance is 5 mm,4 c-C8 4F28/N2 increases nearly linearly with 181181 c-Cc-C4FF8/N/N2 increases. increases. When When the the electrode electrode distance distance is is 5 5mm, mm, c-C c-C4FF8/N/N2 increases increases nearly nearly linearly linearly with with the the the increase in the volume fraction of4 c-C8 F . When the electrode distance is 15 mm, the pressure is 182182 increaseincrease in in the the volume volume fraction fraction of of c-C c-C4FF8. .When When4 8 the the electrode electrode distance distance is is 15 15 mm, mm, the the pressure pressure is is 0.45 0.45 183183 and0.45and 0.50 and0.50 MPa; 0.50MPa; MPa;in in addition, addition, in addition, the the volume thevolume volume fraction fraction fraction k kincreases increasesk increases from from from 5% 5% to 5%to 10%, 10%, to 10%, and and andthe the thebreakdown breakdown breakdown 184184 voltagevoltagevoltage significantly significantlysignificantly improves.improves. TheThe The reasonreason reason that that that breakdown breakdown breakdown voltage voltage voltage increases increases increases is that,is is that, withthat, with thewith increasethe the 4 8 185185 increaseinincrease volume in in volume fraction, volume fraction, thefraction, probability the the probability probability of c-C4F of8 of attractingc-C c-C4FF8 attracting attracting electrons electrons electrons increases, increases, increases, and the and electronand the the electron electron avalanche 186186 avalanchedevelopmentavalanche development development slows, resulting slows, slows, resulting inresulting an increase in in an an increase in increase the breakdown in in the the breakdown breakdown voltage. voltage. Itvoltage. can be It It seencan can be frombe seen seen this from from trend 4 8 4 8 2 187187 thisthatthis trend iftrend the that that volume if if the the fractionvolume volume offraction fraction c-C4F of8 ofis c-C c-C less4FF8 thanis is less less 20%, than than the 20%, 20%, breakdown the the breakdown breakdown voltage voltage voltage of c-C of 4ofF c-C8 c-C/N4F2F8/Ncan/N2 be 4 8 188188 canincreasedcan be be increased increased through through through an increase an an increase increase in the in volumein the the volume volume fraction fraction fraction of c-C of 4of Fc-C 8c-C. 4FF8. .

3.4. Feasibility of Replacing SF6 /N2 with c-C4 8F /N2 189189 3.4.3.4. Feasibility Feasibility of of Replacing Replacing SF SF6/N6/N2 with 2with c-C c-C4FF84/N/N82 2

4 8 2 190190 ToToTo verify verifyverify the the insulation insulation insulation level level level of of ofc-C c-C c-C4FF8/N4/NF28 /Nunder under2 under an an extremely extremely an extremely non-uniform non-uniform non-uniform electric electric electricfield, field, the the field, 4 8 2 6 6 2 191191 breakdownthebreakdown breakdown voltages voltages voltages of of c-C c-C4 ofFF8/N c-C/N2 4were Fwere8/N compared 2comparedwere compared with with those those with of of SF thoseSF6 and andof 20%SF 20%SF SF6 and6/80%N/80%N 20%SF2 under under6/80%N the the same 2sameunder 6 6 2 192192 experimentaltheexperimental same experimental conditions. conditions. The conditions. The breakdown breakdown The voltages breakdownvoltages of of SF SF voltages6 and and 20%SF 20%SF of SF6/80%N/80%N6 and2 were 20%SF were taken taken6/80%N from from2 [23], were[23], and and taken 193193 thefromthe comparison comparison [23], and results the results comparison are are shown shown results in in Figure Figure are 7. shown7. in Figure7.

194 Figure 7. Comparisons of the breakdown voltages of SF6, c-C4F8/N2, and 20%SF6/80%N2 in an 194 FigureFigure 7. 7. ComparisonsComparisons of of the the breakdown breakdown voltages voltages of of SF SF6, ,c-C c-C4F8F/N/N2, and, and 20%SF 20%SF6/80%N/80%N2 in anin an 195 extremely non-uniform electric field for d of (a) 10 and (b) 206 mm. 4 8 2 6 2 195 extremelyextremely non-uniform non-uniform electric electric field field for for d ofd of(a) ( a10) 10and and (b) ( b20) 20mm. mm.

196196 ItItIt can can be be be seen seen seen from from from Figure Figure Figure 7 7that7 that that when when when the the theel electrodeectrode electrode distance distance distance is is 10 10 isor or 10 20 20 ormm, mm, 20 on mm, on the the on whole whole the wholethe the 197 breakdown voltages of 20%c-C4F8/80%N2 are lower than those of SF6 and 20%SF6/80%N2. When the 197 breakdownthe breakdown voltages voltages of 20%c-C of 20%c-C4F8/80%N4F82 /80%Nare lower2 are than lower those than of SF those6 and 20%SF of SF66/80%Nand 20%SF2. When6/80%N the 2. 198 electrode distance is 10 mm, the breakdown voltage of 20%c-C4F8/80%N2 is 46–64% that of 198 electrodeWhen the distance electrode is distance 10 mm, is the 10 mm,breakdown the breakdown voltage voltageof 20%c-C of 20%c-C4F8/80%N4F82 /80%Nis 46–64%2 is 46–64% that of that 6 2 4 8 2 6 199199 20%SF20%SF6/80%N/80%N2. At. At 0.3 0.3 MPa, MPa, the the breakdown breakdown voltage voltage of of 20%c-C 20%c-C4FF8/80%N/80%N2 is is 57% 57% that that of of SF SF6. When. When the the

Energies 2018, 11, 3533 8 of 9

of 20%SF6/80%N2. At 0.3 MPa, the breakdown voltage of 20%c-C4F8/80%N2 is 57% that of SF6. When the electrode distance is 20 mm, the breakdown voltage of 20%c-C4F8/80%N2 is 60–90% that of 20%SF6/80%N2. At 0.3 MPa, the 20%c-C4F8/80%N2 breakdown voltage is 66.1 kV, or 91% of the breakdown voltage of SF6 (72.4 kV). In some cases (Figure7a, 0.20MPa and Figure7b, 0.15MPa) the performance of SF6 is approximately two–three times better than the c-C4F8 mixture. By comparing the breakdown voltages, it can be seen that 20%c-C4F8/80%N2 has the potential to replace SF6 in an extremely non-uniform electric field. As an alternative to SF6, many other aspects of c-C4F8 electrical properties are relevant, including the breakdown characteristics under a slightly non-uniform electric field, partial discharge characteristics, and toxicity of decomposition products. In [15], results showed that the AC breakdown voltage of c-C4F8 is 1.4 times that of SF6 under a slightly non-uniform electric field. Results from the literature [17] also indicated that the partial discharge performance of 20%C4F8/80%N2 is similar to that of 20%SF6/80%N2. The authors of [18] found that the product components of c-C4F8 are mainly fluorocarbon gas, such as CF4,C2F6 C2F4 C3F8 and C3F6. From the above electrical properties, c-C4F8 has potential to replace SF6 used in low- and medium-voltage equipment.

4. Conclusions

1. In an extremely non-uniform electric field, the breakdown voltages of 20%c-C4F8/80%N2 increase with the increase in pressure, electrode distance, and the volume fraction of c-C4F8. As the pressure increases, a “hump” occurs owing to the homogenization of an electric field through the corona. With the increase in electrode distance, the AC breakdown voltages of c-C4F8/N2 increase slightly in the case of a short gap, and increase significantly in the case of a long gap.

2. Based on a comparison between SF6 and 20%SF6/80%N2, it was found that under the same conditions, the breakdown voltage of 20%c-C4F8/80%N2 can reach up to 46–90% that of 20%SF6/80%N2. At 0.3 MPa, the breakdown voltage of 20%c-C4F8/80%N2 can reach over 57% that of SF6. 3. In an extremely non-uniform electric field, considering the insulation level and liquefaction temperature, a 20%c-C4F8/80%N2 mixture has the potential to replace SF6 in low- and medium-voltage equipment.

Author Contributions: Conceptualization, X.W. and L.L.; methodology, Q.C. and L.L.; experiment, H.F. and C.Z.; software, L.L.; formal analysis, L.L.; investigation, L.L.; resources, Q.C.; data curation, L.L.; writing—original draft preparation, L.L.; writing—review and editing, L.L.; supervision, H.Z.; project administration, Q.C. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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