J. Cent. South Univ. (2017) 24: 861-865

DOI: 10.1007/s11771-017-3488-7

Breakdown characteristics of CF₄ and CF₄/N₂ hybrid gas in refrigeration temperature range

HOU Meng-xi(侯孟希)¹, LI Wei-guo(李卫国)¹, YUAN Chuang-ye(袁创业)¹,

YANG Yi-xin(阳以歆)¹, OUYANG Jie(欧阳洁)²

1. School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, China;

2. Measuring Center of State Grid Hunan Electric Power Corp, Changsha 410000, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: Common insulation gas cannot normally work in refrigeration temperature range (153-243 K), especially in extremely cold regions. To solve this problem, this essay uses cubic equation combined with two-parameter model in theorem of corresponding states to estimate dew-point of hybrid gas. The influence of temperature on mixing ratio is studied by using van der Waals equation. The result shows that the mixing ratio is stable during temperature-fall period. Insulation property of CF₄ and CF₄/N₂ in refrigeration temperature range is studied through self-designed low-temperature test system. The result shows when the density of hybrid gas is invariable, temperature changing has less influence on breakdown voltage, and when the mixing ratio is 20%, CF₄/N₂ is the greatest potential hybrid gas.

Key words: dew-point; refrigeration temperature range; CF₄; gas insulation

1 Introduction

SF₆ has excellent dielectric strength and interruption performance. For this reason, it is widely used in GIS, GIL, GIB. However SF₆ is very sensitive to field utilization factor, and SF₆ has high GWP index, being 23900 times that of CO₂ [1]. More, its liquefaction temperature is relatively high which can’t be used in extremely cold regions [2].

From the result of recent research, c-C₄F₈ and CF₃I are possible substitute gas for SF₆ at room temperature [3]. The dielectric strength in homogeneous field of these two gases is higher than that of SF₆ [4], in which c-C₄F₈ is 1.4 times that of SF₆ [5] and CF₃I is 1.21 times that of SF₆ [6]. In extremely non-uniform fields, the dielectric strength of c-C₄F₈ is also higher than that of SF₆, but CF₃I is more sensitive than SF₆ [7, 8]. The dielectric strength of c-C₄F₈ and its CO₂ and N₂ mixture was studied in Refs. [9, 10] and the result shows that GWP of c-C₄F₈ and its binary mixtures is only 1/10 of SF₆ when they have the same dielectric strength. SST experiment of c-C₄F₈/CF₄ was studied in Ref. [11] and the result shows that the (E/N)_lim of 50% c-C₄F₈/CF₄ is 21% higher than that of SF₆. AC insulation characteristics of c-C₄F₈/CF4 are studied in Ref. [12] and the result shows that the hybrid gas is in its best insulation state with the hybrid ratio of 5% under 20 mm electrode distance and 0.3 MPa. The possibility of CF₃I as an substitute gas for SF₆ was studied in Refs. [13, 14]. The partial discharge characteristics of CF₃I/CO₂ and CF₃I/N₂ were studied in Ref. [15], and the result shows that 25%-30% CF₃I/CO₂ mixture is more suitable for being used in low medium voltage gas insulated equipment.

But the liquefaction temperature of CF₃I or c-C₄F₈ is very high. c-C₄F₈ liquefaction temperature is -8 °C. CF₃I will liquefy above 0 °C under 0.5 MPa [16]. Although the liquefaction temperature of the CO₂ and N₂ mixture will be lower than that of pure gas, it still can’t be used in refrigeration temperature range. Meanwhile, CF₃I will decompose when exposed to light [17], and its insulation property will steeply drop. Moreover, the price of c-C₄F₈ or CF₃I is far higher than that of SF6.

CF₄ as a kind of PFC gas has very low liquefaction temperature with moderate price and its GWP index is only 1/3 that of SF₆. But recent research fastened on its arc extinction property [18] or regarded it as a buffer gas mixed with c-C₄F₈ and CF₃I [19]. Few research articles reported the insulation property of CF₄ [20]. Also few research articles reported the insulation property of gas dielectric in refrigeration temperature range. The AC breakdown characteristics of CF₄ and CF₄/N₂ mixture are studied from room temperature to -153 °C in this work. Test result of this work is an extension of the study on substitute gas for SF₆, which has guiding significance for gas insulation design in extremely cold regions.

2 Experimental setup and procedures

The structure of low temperature test chamber [21] which is made of stainless steel is shown in Fig. 1. In order to form homogeneous field, plane-plane is adopted.Plane-plane electrode is made of stainless steel and its edge is disposed according to Zhukovskiy curve. The parameters of plane-plane electrode are Ψ=50 mm, thickness d=15 mm. The transformer model is YDQ200KV/20KVA accompany with divider whose precision is 1% and the voltage can be adjusted continuously.

Fig. 1 Organizational structure of chamber:

The test chamber is cooled by liquid nitrogen. In order to decrease heat leak, the condensation copper pipe is covered by vacuum layer. Temperature meterage system consists of PC, four Pt resistance sensors (three of them located on chamber jamb wall and one in high-voltage electrode), NI-PC6221 DAQ. Temperature data will be shown as figures after being tackled by Labview.

Chamber will be cleaned with absolute alcohol before being used, and vacuum pump will be used to make the chamber pressure lower than 1 Pa. Simple substance should stand 30 min before testing. Minority proportion gas will be injected before majority proportion when testing mixture gas and after injection mixture gas, it should stand 24 h before testing. The interval between breakdowns is 5 min to let gas recover its insulation property. Each group of gases is tested 10 times, and the average value of these 10 times will be the final BD voltage of dielectric. In order to save cost and time, lower pressure test data are gained through deflation.

3 Results and analyses

3.1 Physical property analyses

Gas dielectric will lose most insulation property when turning into liquid. In order to insure the test material not to have phase changing in refrigeration temperature range, dew-point temperature (dew-point temperature refers to the gas cooling to saturation temperature when pressure and moisture content are stable) [22] of mixture gas should be checked.

Cubic equation state is used to analyze dew-point and is expressed as

                  (1)

Parameters Ψ, γ, σ, η are defined by calculation model, which can be constant or function of T. According to CSP (theorem corresponding states), two-parameter model is suitable for binary gas mixture. So, γ=b, σ=η=0 and Ψ(T)=a(T). Equation (1) can be simplified as

                             (2)

where a is energy parameter; b is covolume. a and b are separately defined as

                    (3)

                            (4)

 (5)

where p_c and T_c are critical pressure and critical temperature, respectively; ω is acentric factor; T_r=T/T_c.

Combine with Panagiotopolous—Reid rule and Lewis-Randal rule

        (6)

Dew-point temperature of binary gas can be calculated. Dew-point temperatures of C₃F₈/N₂ and CF₄/N₂ mixture under 0.1-0.4 MPa were theoretically calculated above. The result is shown in Tab1e 1, and experiment result is shown in Table 2.

From the result of related research, C₃F₈ has better insulation property than CF₄ [23], so is its N₂ mixture. But considering the test material will be tested in refrigeration temperature range, C₃F₈ and its N₂ mixture will be liquid in this temperature range. But CF₄/N₂mixture will not have phase changing when mixing ratio k<50% under 0.1-0.4 MPa. CF₄/N₂ mixture is more suitable and is of much value for research.

Table 1 Calculation results of dew-point temperatures of C₃F₈/N₂ and CF₄/N₂ mixture under 0.1-0.4 MPa

Table 2 Experiment results of dew-point temperature of C₃F₈/N₂ and CF₄/N₂ mixture under 0.1-0.4 MPa

3.2 Influence of temperature on mixing ratio

Mixing ratio, k, at room temperature condition is determined by Daltons law which is suitable for ideal gas.

                                 (7)

During temperature-fall period, the pressure variation of different gases is different. It is necessary to analyze the influence of temperature on mixing ratio k.

For 1 mol non-ideal gas, p is determined by van der Waals equation:

                              (8)

The mixing ratio k is

          (9)

van der Waals constant of CF₄ is 1.78×10⁶ Pa·L²/mol², 0.55 L/mol and van der Waals constant of N₂ is 1.41×10⁵ Pa·L²/mol², 0.039 L/mol [24]. Figure 2 shows the variation tendency of k. V=0, meaning the gas turning into liquid, is a distortional point where k rises sharply with the changing of T.

Under actual condition V >>0, V is a constant in test chamber. Take partials of T:

 (10)

Figure 3 shows mixing ratio k raising with T. When T changes from 153 K to 273 K, the variation of k is 7.4×10^-4, which can be ignored. So, we take k as a constant during temperature-fall period.

Fig. 2 Function image of k and T, V

Fig. 3 Relationship between k and T

3.3 Breakdown characteristics of CF₄/N₂

The BD voltage of CF₄ and CF₄/N₂ hybrid gas at room temperature is shown in Fig. 4. When p<0.35 MPa, the BD voltages of CF₄ and CF₄/N₂ rise linearly with pressure. When p>0.35 MPa, CF₄ shows obvious saturation phenomenon and CF₄/N₂ also shows varying degree of saturation with different mixing ratios. The dielectric strength of CF4 is 65% of SF₆ and when mixing ratio k=20%; the dielectric strength of CF₄/N₂ is 55% of SF6. Fitting formula of BD voltage and p is:

The BD voltage of CF₄ under 0.1-0.3 MPa in refrigeration temperature range is shown in Fig. 5. Both BD voltage and its rate of increasing rise with temperature decreasing when p is stable. CF₄ loses its insulation property very fast when reaching boiling point. The fitting formula of BD voltage and T is:

The BD voltage of CF₄/N₂ in homogeneous field under 0.1-0.3 MPa in refrigeration temperature range, mixing ratio k=1:4,1:1,4:1 is shown in Fig. 6.

Fig. 4 Breakdown voltage of CF₄ and its N₂ mixtures at room temperature

Under certain conditions of pressure and k, BD voltage of CF₄/N₂ rises with temperature decreasing but not linearly. Under certain conditions of temperature and k, BD voltage of CF₄/N₂ rises linearly with pressure.

Fig. 5 BD voltage of CF₄ in refrigeration temperature range under 0.1-0.3 MPa

Fig. 6 BD voltage of CF₄/N₂ under conditions p=0.1-0.3 MPa, T=153-243 K, k(CF₄:N₂)=1:4, 1:1, 4:1

Also CF₄/N₂ hybrid gas BD voltage shows clearly saturation phenomenon with mixing ratio k. When k is rising from 20% to 80%, the BD voltage rises less than 15%. 20% CF₄/N₂ has the lowest dew-point and liquefaction temperature in this experiment and the BD voltage of 20% CF₄/N₂ hybrid gas is 80% of that of pure CF₄, which shows a good dielectric strength. So, 20% CF₄/N₂ is the greatest potential hybrid gas.

Both CF₄ and CF₄/N₂ hybrid gas BD voltage increase with temperature decreasing when p is stable. During isovolumic cooling, variation of BD voltage is not observed. According to Maxwell velocity distribution, from 273 K to 153 K, the most probable velocity v_B of electron decreases by 26.3%, but compared with drift velocity of electron, the variation of v_B can be ignored. So, temperature changing in refrigeration temperature range makes nonsense to ionization coefficient, which means that when mixing ratio k is a constant, the BD voltage of CF₄/N₂ is the function of E/ρ. Before pressure reaching saturation point (0.35 MPa), BD voltage of CF₄/N₂ raises linearly with ρ. Table 3 shows the calculation result. The value of E/ρ decreases with CF₄ increasing, which proves saturation phenomenon of BD voltage increasing with a mixing ratio.

Table 3 E/ρ-k calculation result for CF₄/N₂

During isovolumic and isobaric cooling shown in Figs. 5 and 6, molecular density in chamber changes, leading to mean-free-path length of electron decreasing. So, ionization coefficient decreases, leading BD voltage to increasing. During isovolumic and isobaric process, molecular density increases not linearly with temperature, nor does the BD voltage.

4 Conclusions

1) Dew-point temperature of binary gas can be estimated by cubic equation. When k<50%, p<0.4 MPa, CF₄/N₂ hybrid gas will not have phase changing in the refrigeration temperature range. Influence of temperature on mixing ratio can be ignored in the testing temperature range.

2) During isovolumic cooling, temperature has no effect on BD voltage of CF₄ and CF₄/N₂ hybrid gas in the refrigeration temperature range. During isovolumic and isobaric cooling, BD voltages of CF₄ and CF₄/N₂ hybrid gas do not rise linearly with temperature. This is because of molecular density increasing not linearly with temperature.

3) Taking GWP, dew-point temperature, dielectric strength and economic cost into consideration, 20% CF₄/N₂ is the greatest potential mixture in the refrigeration temperature range.

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(Edited by YANG Hua)

Cite this article as: HOU Meng-xi, LI Wei-guo, YUAN Chuang-ye, YANG Yi-xin, OUYANG Jie. Breakdown characteristics of CF₄ and CF₄/N₂ hybrid gas in refrigeration temperature range [J]. Journal of Central South University, 2017, 24(4): 861-865. DOI: 10.1007/s11771-017-3488-7.

Foundation item: Project(51277063) supported by the National Natural Science Foundation of China

Received date: 2016-05-31; Accepted date: 2016-08-27

Corresponding author: LI Wei-guo, Professor, PhD; Tel: +86-18701289872; E-mail:18810102619@139.com