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

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 breakdown voltage 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.

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 dielectric strength of SF₆ during use [3,4,5]. Above all, SF₆ is a potent greenhouse gas whose global warming potential (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 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 Figure 1. The adjustable range of the voltage regulator was 0–220V and the transformer 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 Figure 2. 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.

2.2. Electrode and Electric Field Simulation

In this study, a needle-plate electrode was used to generate an extremely non-uniform electric field. The electrode distance ranged from 0 to 20 mm. The diameter of the plate electrode was 140 mm, the thickness was 25 mm, and the chamfer radius was 8 mm. The diameter of the needle electrode was 5 mm, the length was 50 mm, the length of the tip was 5 mm, and the tip curvature radius was about 0.15 mm; in addition, both needle-plate electrodes were made of brass. Because the discharge will lead to material loss at the tip and increase the curvature radius of the electrode, the needle electrode should be regularly replaced.

During the test, the electrode distances were 5, 10, 15, and 20 mm. Here we calculate the non-uniformity coefficients f under different electrode distances with the following expression [20,22]:

$$f=\frac{E_{\max }}{E_{\mathrm{av}}}$$

(1)

$$E_{\mathrm{av}}=\frac{U}{d}$$

(2)

where E_max is the maximum electric field strength (V/m), E_av is the average electric field strength (V/m), U is voltage applied to the electrode (V), and d is the electrode distance (m).

We used COMSOL software to simulate the electric field distribution, and obtained the E_max between the needle-plates with different electrode distances. The simulation result when the electrode distance was 20 mm is shown in Figure 3.

Based on the simulation results, the non-uniformity coefficients calculated under different electrode distances are shown in

Table 1. Non-uniformity coefficients under different electrode distances.

Electrode gap /mm	5	10	15	20
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 calculated at different electrode distances are all greater than 4 [22], and thus the distribution of the electric field between the inter-electrode conforms to the requirement of an extremely non-uniform electric field.

2.3. Volume Fraction of Mixture

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 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. Actually, the liquefaction temperature of the mixture should be lower than the operating temperature. The maximum volume fraction of c-C₄F₈ in the mixture can be calculated through the following formula [23]:

$$T_{\mathrm{mb}}=\frac{T_{\mathrm{b}}}{1-\ln \left(10kP\right)/10.5}$$

(3)

where T_mb is the liquefaction temperature of a mixed gas (K), T_b 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 is P.

It can be determined through Equation (3) that the volume fraction of c-C₄F₈ in a mixed gas is not more than 20%. In the test, the pressure ranged from 0.1 to 0.5 MPa; the testing step length was 0.05 MPa; the volume fractions of c-C₄F₈ were 5%, 10%, 15%, and 20%; the accuracy of mixed gases was ±1%; and the experiment was conducted at 20 °C.

3. Results and Discussion

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

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

It can be seen from Figure 4 that the pressure has a significant impact on the breakdown voltage. On the whole, the AC breakdown voltages of c-C₄F₈/N₂ mixture increase with the increase in pressure. When the gaps between the electrodes are 5, 10, or 15 mm, the AC breakdown voltage shows the same increase trend as the change in pressure. When the electrode distance is 5 or 10 mm, the breakdown voltage of the gas at a pressure of 0.5 MPa increases approximately two-fold to 0.1 MPa, whereas when the electrode distance is 15 mm, the breakdown voltages increases by about 1.5-fold. The reason why the breakdown voltages increase with pressure is because, as the pressure increases, the number of molecules per volume increases, which shortens the free path of the electrons. The amount of energy accumulated by the electrons decreases in the free path, thus affecting the development of an electron avalanche. At the same time, c-C₄F₈ is an electronegative gas, with a relatively large cross-section in the low energy region, and can attract low-energy electrons, thereby generating negative ions and further increasing the breakdown voltage. When the electrode distance is 20 mm, the breakdown voltages showed different trends with the pressure. Under a pressure of 0.15 MPa, the breakdown voltage increases rapidly, and breakdown voltages between 0.15 and 0.40 MPa increase slowly. When the 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 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 further increase the breakdown voltages; thus, the breakdown voltage is higher when the pressure is 0.15 MPa. However, as the pressure increases further, the diffusion motion of the space charge decreases, the size of the corona decreases, the effect on the improvement of the electric field diminishes, and the breakdown voltage increases slowly.

3.2. Breakdown Voltages of c-C₄F₈/N₂ Mixture Varying with Electrode Distance

The breakdown voltage of c-C₄F₈/N₂ gas varied with the electrode distance, as shown in Figure 5.

As shown in Figure 5, c-C₄F₈/N₂ shows the same growth trend with the change in electrode distance under a different volume fraction. When the electrode distance is 10 mm, no significant improvement is shown when compared with the breakdown voltage of 5 mm. When the electrode 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

When the electrode distance is 5 or 15 mm, the breakdown voltages of the c-C₄F₈/N₂ mixture change with the volume fraction k of c-C₄F₈, as shown in Figure 6.

According to Figure 6, as the volume fraction of c-C₄F₈ increases, the dielectric strength of c-C₄F₈/N₂ increases. When the electrode distance is 5 mm, c-C₄F₈/N₂ increases nearly linearly with the increase in the volume fraction of c-C₄F₈. When the electrode distance is 15 mm, the pressure is 0.45 and 0.50 MPa; in addition, the volume fraction k increases from 5% to 10%, and the breakdown voltage significantly improves. The reason that breakdown voltage increases is that, with the increase in volume fraction, the probability of c-C₄F₈ attracting electrons increases, and the electron avalanche development slows, resulting in an increase in the breakdown voltage. It can be seen from this trend that if the volume fraction of c-C₄F₈ is less than 20%, the breakdown voltage of c-C₄F₈/N₂ can be increased through an increase in the volume fraction of c-C₄F₈.

3.4. Feasibility of Replacing SF₆/N₂ with c-C₄F₈/N₂

To verify the insulation level of c-C₄F₈/N₂ under an extremely non-uniform electric field, the breakdown voltages of c-C₄F₈/N₂ were compared with those of SF₆ and 20%SF₆/80%N₂ under the same experimental conditions. The breakdown voltages of SF₆ and 20%SF₆/80%N₂ were taken from [23], and the comparison results are shown in Figure 7.

It can be seen from Figure 7 that when the electrode distance is 10 or 20 mm, on the whole the breakdown voltages of 20%c-C₄F₈/80%N₂ are lower than those of SF₆ and 20%SF₆/80%N₂. When the electrode distance is 10 mm, the breakdown voltage of 20%c-C₄F₈/80%N₂ is 46–64% that 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 (Figure 7a, 0.20MPa and Figure 7b, 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

• 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.
• 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.
• 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.