Study on Dispersion of Impulse Discharge in SF₆ and Eco-Friendly Insulating Gas C₄F₇N/CO₂

Abstract

In recent years, C₄F₇N/CO₂ gas has been widely studied as an eco-friendly alternative to SF₆, which is commonly used in electrical equipment. To ensure electrical equipment reliability, the dispersion of impulse discharge voltage of insulated gas is generally required to be less than 3%. However, experimental results indicate that under fault conditions, such as sudden pressure changes or electric field distortion, the discharge dispersion of both C₄F₇N/CO₂ and SF₆ often exceeds 3%. This paper investigates the impact of pressure and electric field nonuniformity on the dispersion of impulse discharge voltage for conventional and eco-friendly insulating gases under varying degrees of electric field nonuniformity. Experiments reveal that under identical conditions, the 9%C₄F₇N/91%CO₂ mixture exhibits lower impulse discharge voltage dispersion compared with SF6. As pressure increases, the dispersion decreases for both gases. Conversely, dispersion increases with higher electric field nonuniformity, and the 9%C₄F₇N/91%CO₂ mixture demonstrates greater sensitivity to electric field nonuniformity than SF₆. In practical applications, electrical equipment typically operates under slightly nonuniform electric fields and high pressure, meeting dispersion requirements. However, if electric field distortion causes the nonuniformity factor (f) to exceed 2.4 or if pressure drops below 0.3 MPa then dispersion increases significantly, reducing the reliability of insulation performance data.

1. Introduction

SF₆ is extensively used in electrical insulation due to its excellent insulating and arc-quenching properties [1]. However, SF₆ has a global warming potential (GWP) 23,900 times higher than CO₂ and an atmospheric lifetime of approximately 3200 years. It was listed as a restricted greenhouse gas in the 1997 Kyoto Protocol [2], accelerating global warming. For example, the Sutong 1000 kV GIL project in China uses 19 tons of SF₆ per kilometer, totaling 665 tons [3]. Assuming an annual leakage rate of 0.1%, 0.665 tons of SF₆ would leak annually, equivalent to 16,000 tons of CO₂ emissions. To mitigate SF₆ usage, eco-friendly alternatives with comparable physicochemical properties are urgently needed.

C₄F₇N has emerged as a promising SF₆ substitute, offering twice the insulation strength of pure SF₆. According to Yannick Kieffel et al., C₄F₇N has a GWP of 2100 and zero ozone depletion potential [4]. Commercialized by 3M (Saint Paul, MN, USA), its high cost (60 times that of SF₆) and high liquefaction temperature (−4.7 °C at 0.1 MPa) limit its standalone use. Recent research focuses on C₄F₇N/CO₂ mixtures to balance insulation performance, lower liquefaction temperatures, and reduce costs. Alstom (2014 CIGRE, Mumbai, Maharashtra) and GE (2015 CIRED, Boston, MA, USA) proposed C₄F₇N/CO₂ mixtures for gas-insulated equipment, deploying 420 kV GIL, 245 kV GIT, and 145 kV GIS [5]. Studies by Li Xingwen (Xi’an Jiaotong University) calculated the insulation strength and liquefaction temperature of C₄F₇N/CO₂ [6], while Tu Youping (North China Electric Power University) investigated partial discharge inception and breakdown voltages under DC [7]. Zhou Wenjun’s team (Wuhan University) analyzed the effects of electric field nonuniformity, pressure, and mixing ratios on AC/impulse discharge characteristics, demonstrating that 9%C₄F₇N/91%CO₂ achieves over 80% of SF₆’s insulation strength [8,9,10,11].

Existing studies on SF₆ and its alternatives often use the step-up and step-down method [12,13,14,15], requiring breakdown voltage dispersion ≤3% [16]. However, under fault conditions such as sudden pressure changes or electric field distortion, dispersion for both SF₆ and C₄F₇N/CO₂ exceeds 3%, complicating data acquisition and indicating poorer insulation stability.

However, the greater the dispersion of discharge voltage under impulse voltage, in order to obtain accurate test data, the more the test workload is required to increase and the longer the test time. This also shows that the impact insulation stability of the gas is poor. According to the gas discharge theory, the dispersion of gas discharge voltage is the randomness of the formation of different discharge channels in the gas gap, and the randomness of the formation of discharge channels is related to many factors. At present, the dispersion of SF₆ and its alternative gas discharge voltage is affected by the nonuniformity of the electric field and gas pressure, and there is still no conclusion as there is a lack of relevant research. At the same time, how to avoid gas discharge dispersion exceeding the specified value has not been reported. Therefore, this paper compares the breakdown voltage dispersion of C₄F₇N/CO₂ gas mixture and SF₆ gas through standard lightning impulse voltage discharge tests under different electrode forms and reveals the impulse discharge dispersion of C₄F₇N/CO₂ gas mixture. The variation in discharge dispersion with pressure and electric field nonuniformity under different polarity impulse voltages is obtained, which provides new data for the feasibility of replacing SF₆ gas with C₄F₇N/CO₂ gas mixture, provides a reference for C₄F₇N/CO₂ and SF₆ to obtain more reliable impulse breakdown data, and also provides a method for gas-insulated equipment to avoid discharge dispersion exceeding the standard requirements.

2. Experimental Setup and Methodology

2.1. Experimental Setup

The impulse test platform (Figure 1) includes a 600 kV impulse voltage generator, a capacitive voltage divider (ratio 1870:1) and a Tektronix DPO2022B oscilloscope (1 GHz sampling rate, 200 MHz bandwidth).The manufacturer of the first two is China Leiyu Company (Yangzhou, China). The oscilloscope is from Tektronix in Beaverton, OR, USA. The test chamber (490 kV withstand voltage, 1 MPa pressure limit) features adjustable electrodes (spacing accuracy: 0.01 mm) and quartz observation windows.

The high melting point advantage of tungsten is as follows: The melting point of tungsten (3410 °C) is much higher than that of copper (1085 °C), and tungsten–copper alloys can maintain structural stability at high temperatures, avoiding electrode melting or deformation caused by local high temperatures. Anti-arc erosion: In electrical discharge machining or high-voltage discharge, the electrode surface is susceptible to instantaneous high-temperature arc impact. The high-temperature resistance of tungsten–copper alloys can significantly reduce electrode wear and extend service life [17]. Due to the above advantages, we chose tungsten–copper alloy electrodes under frequent arc impact conditions, as shown in Figure 2.

The nonuniformity factor f is defined as follows:

$$f={\displaystyle \frac{E_{\max }}{E_{\mathrm{ave}}}}$$

(1)

$$E_{\mathrm{ave}}={\displaystyle \frac{U_d}{d}}$$

(2)

Electrode configurations (plate-plate, sphere-plate, and needle-plate) simulate uniform (f ≈ 1), slightly nonuniform (1 < f < 2), and highly nonuniform (f > 4) electric fields. The highly nonuniform (f > 4) electric field used to simulate faults in electrical equipment.

The calculation is shown in Formula (1), where E_max is the maximum electric field strength between electrodes, which is obtained by electrostatic-field simulation calculation.

The electrode material is set to copper. The relative dielectric constant of the SF₆ or C₄F₇N/CO₂ mixed gas is also set to 1.00 here. In Formula (1), E_ave is the average electric field strength between electrodes, d is the electrode spacing, and U_d is the applied voltage. Use Formula (2) to calculate E_ave. In this paper, different electrode forms are used, and the electrode gap distance is 2.5~10 mm, as shown in

Table 1. The electric field nonuniformity of different electrode types.

Electrode Types	Radius/mm	d/mm	Electric Field Nonuniformity f
Plate-plate electrodes	/	2.5	1.01
Sphere-plate electrodes	3	5	2.4
	3	20	6.9
Needle-plate electrodes	0.5	10	13.8
	0.5	15	18.9
	0.5	20	23.6

.

2.2. Methodology

In order to reduce the interference of moisture on the test results, when the C₄F₇N/CO₂ mixed gas is configured, the purity of the C₄F₇N and CO₂ pure gas used is 99.99%, and the steel cylinder configured with the mixed gas is vacuumized for at least 2 h.

$${\displaystyle \frac{n_1}{n_2}}={\displaystyle \frac{P_1}{P_2}}$$

(3)

In this paper, the percentages of C₄F₇N in the C₄F₇N/CO₂ mixture are all molar percentages. If the gas is regarded as an ideal gas, because the volume of the cavity is constant, according to Dalton’s partial pressure law, as shown in Formula (3), n₁ and n₂ are the moles of the two gases, and P₁ and P₂ are the gas pressures of the two gases. The molar ratio of C₄F₇N to C₄F₇N/CO₂ gas mixture is the ratio of the pressure of the gas to that of C₄F₇N/CO₂ gas mixture, which can be used for gas distribution according to the pressure ratio. After mixing C₄F₇N with CO₂ and standing, the C₄F₇N component analyzer calibrated by the China Institute of Metrology is used for secondary analysis to ensure the mixing proportion accuracy.

Before the test, the interior of the chamber shall be wiped with dust-free paper dipped with anhydrous alcohol and then gas-dried. Fill the test chamber with test gas through the gas distribution pipeline on the gas distribution vehicle. Vacuumize the cavity and then charge the gas to be tested for gas washing. Before the discharge test, the gas shall be washed at least three times to avoid the interference of other gases in the chamber. First, fill the chamber with the test gas with the highest pressure in the form of this group of electrodes. After the gas pressure test, use the valve to discharge the gas and vacuum it. Take out the electrode for grinding and cleaning with alcohol. After reinstalling the electrode in the test chamber, fill in the mixed gas with another pressure value to test the pressure of the next group. Repeat the previous operation method, so as to avoid the error of electrode ablation and gas-discharge decomposition on the next group of tests. The impulse breakdown test shall be conducted by the voltage rise and fall method, and at least 20 effective breakdowns shall occur in each group.

According to references [8,9], the proportion of mixed gas is 9% C₄F₇N/91% CO₂. The DC, power frequency, and lightning impulse insulation properties are close to SF₆, and the liquefaction temperature can adapt to an environment as low as −15 °C. It has great potential to replace SF₆ as the insulating gas in Gil.

Standard lightning impulse waveforms (1.2/50 µs) comply with IEC 60060 [12]. By up and down measuring method obtain the 50% impulse breakdown voltage in Figure 3. Then, 50% impulse breakdown voltage was found through the red line.

After statistical processing of each group of test data, calculate the standard deviation σ under each group of test conditions according to Formula (4), and then compare the standard deviation with the 50% impulse breakdown voltage U_50% under this group of test conditions to obtain the size of dispersion, see Formula (5). For convenience, we use DISP to express it. The formulas are as follows:

$$\sigma =\sqrt{{\displaystyle \frac{1}{N}}{\displaystyle \sum _{i=1}^N(x_i-\mu )}}$$

(4)

$$DISP={\displaystyle \frac{\sigma }{U_{50\%}}}\times 100 (\%)$$

(5)

3. Results and Analysis

3.1. Effect of Pressure on Dispersion

(1) Under the uniform-electric-field plate electrode, the electrode spacing is 2.5 mm and the electric field nonuniformity is f = 1.05. The gas pressure values described in this paper are absolute gas pressure values. And the variation trend of the impulse discharge voltage dispersion of C₄F₇N/CO₂ mixed gas and SF₆ gas with the change in gas pressure under positive lightning impulse voltage (+Li) and negative lightning impulse voltage (−Li) is shown in Figure 4a. The DISP of the C₄F₇N/CO₂ mixture and SF₆ gas decreased with the increase in pressure under positive and negative impulse voltage and showed a linear trend. If linear fitting was used, the linear goodness of fit R² of each curve reached 0.97~0.99. It can also be seen from the figure that the DISP of 9%C₄F₇N/91%CO₂ mixed gas is significantly less than that of SF₆ gas under both positive and negative lightning impulse voltages. When SF₆ gas is under atmospheric pressure, the positive DISP exceeds the general standard of 3%.

(2) Considering the electric field nonuniformity in actual electrical equipment, the electric field nonuniformity f is in the range of a slightly nonuniform electric field [3].

Under the condition of sphere-plate electrodes d = 5 mm, the trend of DISP change with the change in gas pressure is shown in Figure 4b.

Similarly to Figure 4a, the DISP of C₄F₇N/CO₂ mixture and SF₆ gas decreases with the increase in pressure under positive and negative impulse voltages, but the linearity is significantly different from that of uniform electric field. If linear fitting is considered, the linearity R² of each curve is only 0.88~0.93. It can be seen from Figure 4a,b that the dispersion of positive polarity DISP is significantly higher than that of negative polarity DISP under a uniform electric field and a slightly uneven electric field for the same gas under an impulse voltage of different polarity. This is consistent with the polarity effect law of lightning impulse 50% breakdown voltage U_50% under positive and negative impulse voltages.

When the electrode spacing in the sphere-plate electrode increases, the electric field changes from slightly uneven to extremely uneven to simulate faults in electrical equipment.

At this time, when the gas pressure is low, the dispersion of positive and negative polarity impulse voltage exceeds the general requirement of 3%. With the increase in gas pressure, the dispersed DISP of breakdown voltage at positive lightning impulse voltage decreases slightly at first, then decreases abruptly, and finally gradually tends to saturation. When the gas pressure is 0.2~0.3 MPa there is a sudden drop, while when the gas pressure is about 0.4 MPa the DISP tends to saturation. At negative lightning impulse voltage, DISP gradually decreased with the increase in gas pressure, and the goodness of linear fitting R² was 0.96~0.98. The DISP of C₄F₇N/CO₂ gas mixture under positive and negative polarity is crossed, and the DISP of the negative polarity exceeds that of the positive polarity with the increase in gas pressure.

(3) When the electrodes are needle-plate electrodes, as in Figure 4d–f, the DISP of 9% C₄F₇N/91% CO₂ gas mixture is almost unchanged with the increase in gas pressure, while the dispersion of the positive polarity begins to drop sharply at 0.2 MPa.

For SF₆ gas at a pressure of 0.1 MPa under uniform and slightly nonuniform electric fields, the DISP% range is 2.85–3.42%. Under extremely nonuniform electric fields, the DISP% range is 5.49–7.31%. When the pressure is 0.2 MPa, the DISP% ranges are 2.80–3.31% and 5.23–7.22%, respectively. When the pressure is 0.3 MPa, the DISP% ranges are 2.62–3.11% and 4.52–7.01%, respectively. For C₄F₇N/CO₂ gas at a pressure of 0.1 MPa, under uniform and slightly nonuniform electric fields, the DISP% range is 2.71–3.11%. Under extremely nonuniform electric fields, the DISP% range is 4.47–6.01%. When the pressure is 0.2 MPa, the DISP% ranges are 2.41–2.96% and 4.36–5.94%, respectively. When the pressure is 0.3 MPa, the DISP% ranges are 2.34–2.61% and 3.12–5.82%, respectively. Please refer to Figure 4a–f for details.

With the increase in electrode spacing d, the electric field nonuniformity increases, and the change trend of each curve is almost unchanged. The DISP of negative polarity is higher than that of positive polarity. Under the same conditions, the dispersion of 9% C₄F₇N/91% CO₂ gas mixture is lower than that of SF₆ gas, and the dispersion decreases with the increase in pressure. The reason may be that the increase in the pressure of the insulating gas increases the number of moles of electronegative gas in the same volume cavity, and the length of the discharge random formation path can be considered to have little change when the electrode and spacing remain unchanged. However, with the increase in the gas density with strong adhesion ability, the ability of adsorbing electrons on each possible discharge path is enhanced, the probability of binding electrons that could have been randomly diffused increases, and the possibility of different paths is significantly reduced. According to the Townsend discharge mechanism, under low pressure, electrons continue to accelerate in a long mean free path, forming cascade ionization (electron avalanche), and the discharge area is relatively uniform. However, under high pressure, ionization is only concentrated in the strong-electric-field area (such as near the electrode), resulting in a narrow discharge channel and reduced dispersion. This mechanism has important guiding significance in high-voltage insulation designs, corona controls, and spark discharge applications.

3.2. Effect of Electric Field NonUniformity

Under the condition of 0.1~0.7 MPa absolute pressure, the variation trend of 9% C₄F₇N/91% CO₂ mixed gas and SF₆ gas impulse discharge voltage dispersion with the increase in electric field nonuniformity is shown in Figure 5.

No matter which type of electrode is used, the dispersion of breakdown voltage increases with the increase in electric field nonuniformity. For example, when the distance between the sphere-plate and the needle-plate is 20 mm, the dispersion of the needle-plate (f = 23.6) is significantly greater than that of the sphere-plate (f = 6.9).

According to the gas discharge theory, the reason may be that the maximum electric field intensity near the tip electrode increases with the increase in the electric field nonuniformity, which makes it easier to ionize a large number of free electrons. At the same time, with the increase in the distance between the tip and plate electrodes, the number of molecules in the electrode gap increases. That is, more free electrons and ions can be produced, but the path is mainly determined by the free electrons moving faster, and the diffusion and drift of free electrons are more intense, which affects the discharge path and makes the randomness of the discharge path increase. That is, the more dispersed DISP results are obtained statistically.

At 0.1~0.7 MPa absolute pressure and under different pressure conditions, the DISP of 9% C₄F₇N/91% CO₂ mixture and SF₆ gas showed different degrees of saturation trends with the increase in electric field heterogeneity. When the pressure is low and the absolute pressure p = 0.1 and 0.2 MPa, the dispersion of SF₆ gas and 9% C₄F₇N/91% CO₂ mixture changes from increasing to saturation when the electric field nonuniformity reaches around 6.9. When the pressure is high, that is, when the pressure reaches 0.3 MPa, the DISP of SF₆ gas will still show a saturation trend; however, the 9% C₄F₇N/91% CO₂ gas mixture is not saturated, showing a trend of increasing with the increase in electric field nonuniformity. Therefore, when the pressure is high, the sensitivity of the DISP of 9%C₄F₇N/91% CO₂ mixture to electric field nonuniformity is greater than that of SF₆.

4. Discussion

(1) The nonuniformity of internal electric field of gas-insulated equipment without defects and flaws generally ranges from 1.6 to 2.5 [13]. Figure 4b test results are in this range. In order to prevent the dispersion of impulse discharge in gas-insulated equipment from exceeding the standard requirements, it is necessary to monitor the gas pressure in the equipment. For example, once the pressure of SF₆ gas is lower than 0.3 MPa absolute pressure, the stability of impact insulation performance of the gas gap will decline, which may lead to a decline in the insulation margin and cause accidents.

(2) According to Figure 5, it can be concluded that if the gas pressure meets the conditions (higher than 0.3 MPa), it also needs to meet certain conditions for the electric field nonuniformity to make the DISP meet the requirements of less than 3%. That is, the electric field nonuniformity should be less than or equal to 2.4, which is a slightly uneven electric field. It is necessary to keep the inside of the closed cavity free of defects and flaws. According to the classical theory of gas discharge, when the nonuniformity of the electric field increases to a certain extent, it may cause the development of electron avalanche to be almost completely concentrated in the high-field-strength region, and unable to expand to the low-field-strength region. At this point, even if the unevenness of the electric field is further increased (such as making the needle sharper or the pole spacing smaller), the discharge can still only develop in the high-field-strength region, resulting in no significant change in dispersion, that is, approaching saturation. This may be related to the accumulation of space charges, as a large number of charges are concentrated in high-field-strength regions, and the generated space-charge field partially offsets the external electric field, limiting the further expansion of discharge.

5. Conclusions

(1) By comparing the DISP of SF₆ and C₄F₇N/CO₂ under the same conditions, the feasibility of replacing SF₆ gas is considered from the perspective of insulation stability. Under the same pressure of 0.1~0.7 MPa, the dispersion DISP of discharge voltage of 9%C₄F₇N/91%CO₂ is less than SF₆ in uniform electric field, slightly uneven electric field and extremely uneven electric field. That is, the stability of lightning impulse insulation performance of 9% C₄F₇N/91% CO₂ is better than SF₆.

(2) With an increase in pressure, 9%C₄F₇N/91%CO₂ and SF₆ lightning impulse discharge voltage DISP gradually decreased. At the same time, the DISP of both increased with an increase in the electric field nonuniformity. In engineering, the interior of the closed cavity is usually in a slightly uneven electric field. When the pressure is lower than 0.3 MPa, 9%C₄F₇N/91%CO₂ and SF₆ gas cannot meet the requirements. Therefore, it is necessary to monitor the pressure in the cavity and adjust the pressure range reasonably. The inflation range of high-voltage equipment in the project is high pressure, and the internal electric field distribution of the cavity is slightly uneven when there is no defect, so 9%C₄F₇N/91%CO₂ and SF₆ can meet the dispersion requirements.

(3) Before operation or during operation, if there are defects or flaws inside the cavity, making the electric field nonuniformity greater than 2.4, although the internal pressure of the cavity has met the conditions the discharge dispersion of 9%C₄F₇N/91%CO₂ and SF₆ cannot meet the requirements. Therefore, the defects and flaws inside the cavity should be avoided as strictly as possible. The variation in pressure and electric field nonuniformity in equipment can affect dispersion, thereby affecting the stable operation of the equipment. Through the research in this article, the appropriate range of air pressure and electric field nonuniformity has been obtained, which is of great significance for the safe operation of electrical equipment using SF₆ gas and C₄F₇N/CO₂ gas. The contribution of new gases to environmental protection will be significant.