Experimental study on the partial discharge and AC breakdown properties of C 4 F 7 N/CO 2 mixture

: As an environmental friendly insulating medium, C 4 F 7 N has received extensive attention in the past two years. In this study, the partial discharge (PD) and breakdown characteristics of C 4 F 7 N/CO 2 gas mixture were investigated using the gas insulation performance test platform. The influence of gas pressure and mixing ratio on the PD initial voltage (PDIV) and breakdown voltage of C 4 F 7 N/CO 2 gas mixture in different electric fields was tested. It was found that the PDIV and breakdown voltage of gas mixture with 2–8% C 4 F 7 N in the highly non-uniform field showed a linear saturation increasing trend with the change of gas pressure and mixing ratio. The breakdown voltage of gas mixture increases linearly with gas pressure in the quasi-uniform electric field. The relative breakdown voltage of gas mixture in the highly non-uniform field under high-pressure conditions is inferior to those of low-pressure conditions. The C 4 F 7 N/CO 2 gas mixture containing 2–8% C 4 F 7 N is sensitive to the non-uniformity of electric field. According to the PD and breakdown tests results, C 4 F 7 N/CO 2 gas mixture with 2–8% C 4 F 7 N has the potential to replace SF 6 using in gas-insulated equipment.


Introduction
SF 6 is widely used in all kinds of medium-voltage and high-voltage electrical equipment due to its high dielectric strength and excellent arc extinguishing properties, such as gas insulated switchgear, gas insulated lines and gas-insulated transformers. However, SF 6 is a strong greenhouse gas with the global warming potential (GWP) value up to 23,500 (100 years' time horizon) and the atmosphere lifetime about 3200 years [1][2][3]. According to statistic, ∼80% of world produced SF 6 is used in power industry and the current equilibrium warming due to SF 6 is 0.004°C (Celsius degrees), with a clear tendency to increase [4]. The Paris climate agreement signed in 2016 aims at holding global warming to well below 2°C and to 'pursue efforts' to limit it to 1.5°C [5]. It is reported that under the worst-case emissions scenario a SF 6 emission cut today could contribute 1.5% of this goal [4]. Thus, it is urgent to search for an environmental friendly gas to partially or completely replace SF 6 using in power industry.
In the past two years, C 4 F 7 N (2, 3, 3, 3-tetrafluoro-2-(trifluoromethyl)-2-propanenitrile) is considered as one of the latest generation of insulation gases. Table 1 shows the basic properties of C 4 F 7 N and SF 6 [4,6,7]. The insulation strength of pure C 4 F 7 N is twice higher of SF 6 and its GWP value is only 2090. C 4 F 7 N is also colourless, non-flammable and chemically stable up to about 700°C [6]. For engineering applications, it is necessary to mix C 4 F 7 N with CO 2 , N 2 or dry air because of its high boiling point. Moreover, the dielectric strength of 18-20% C 4 F 7 N in CO 2 gas mixture is equivalent to pure SF 6 [7], which has the potential to substitute SF 6 .
At present, studies on the insulation and arc-quenching performance of C 4 F 7 N have achieved specific results. Nechmi et al. tested the AC breakdown and lightning impulse voltage of 3.7% C 4 F 7 N/96.3% CO 2 gas mixture using different electrode configurations (plane-plane, plane-sphere, sphere-sphere and rodplane) and investigated the effective ionisation coefficients and critical breakdown electric field of C 4 F 7 N/CO 2 gas mixture. It was found that 3.7% C 4 F 7 N/CO 2 gas mixture is suitable for using in high-voltage apparatus [8,9]. The DC breakdown characteristics of C 4 F 7 N/CO 2 gas mixture were also explored by Hopf et al. [10]. In addition, Kieffel et al. [11] studied the switching performance of 4% C 4 F 7 N/96% CO 2 gas mixture using a 420 kV disconnector and found that the average arcing time over 100 times operations is about 12 ms (lower than the typical value of 15 ms for SF 6 ). The decomposition process during the arc process and the transport properties of C 4 F 7 N/CO 2 gas mixture were also revealed theoretically [12,13].
Moreover, partial discharge (PD) behaviour is another important concern for engineering application of C 4 F 7 N gas mixture [14]. The various kinds of insulation defects produced in the long-term operation of gas insulated equipment will cause PD. When a strong PD is generated inside the device, physical phenomena such as electromagnetic radiation, noise and chemical effects are accompanied. On one hand, the strong electromagnetic steep pulse produced by PD could damage the insulating material, degrade its insulation performance, form a vicious circle and may cause breakdown. On the other hand, PD will cause the decomposition of gas insulating medium to form several byproducts. For example, the decomposition of SF 6 under PD will generate several toxic products such as SOF 2 , SO 2 F 2 [15]. PD is not only the most important reason for accelerating the insulation degradation, but also the most effective characteristic quantity for characterising the insulation state. Therefore, it is of great significance to study the PD characteristics of C 4 F 7 N gas mixture.
At present, there are few reports about the PD characteristics of environmental friendly insulating medium C 4 F 7 N gas mixture. Study on the breakdown characteristics of C 4 F 7 N gas mixture under different electric fields is not comprehensive. In this paper, we tested the PD and breakdown characteristics of C 4 F 7 N/CO 2 gas mixture using the gas insulation performance test platform. The influence of electric fields, gas pressure and mixing ratio on the insulation performance were investigated. Relevant results provide an important reference for the engineering application of C 4 F 7 N/CO 2 gas mixture.

Experimental method
The gas insulation performance test platform is consists of test transformer, protective resistor, capacitive voltage divider, coupling capacitor and gas chamber. The experiment circuit is shown in Fig. 1. The test transformer rated capacity is 50 kW, which can provide 0-100 kV power frequency high voltage. The protective resistor (5 kΩ) is used to prevent overcurrent damage to the test transformer. The capacitive voltage divider (OWF-100 kV/500 pF) and coupling capacitor are adopted to measure the actual high voltage applied to the electrodes and provide low resistance channel for the pulse current produced by PD, respectively. The volume of the gas chamber is 20 L, which is made of 304 L stainless steel and polytetrafluoroethylene.
The sphere electrodes and needle-plate electrodes were used to simulate quasi-uniform and highly non-uniform electric fields. The radius of the brass sphere electrodes is 25 mm, and the electrode distance is 3 mm. The needle tip part of the needle electrode is about 8 mm with the radius of curvature 0.3 mm. The radius of the plate electrode is 50 mm, and the electrode interval is set to 5 mm.
The electric fields with different uniformities can be characterised by an electric field utilisation coefficient η defined as follows: in which E max is the maximum electric field strength, E av is the average electric field strength. U and d represent the voltage between the electrodes and the distance of the electrode gap. We simulated the electrode distribution of the sphere and needle-plate electrodes (see Fig. 2). The electric field utilisation coefficients of the two electrodes are 0.91 and 0.21, respectively, corresponding to the quasi-uniform and highly non-uniform electric fields [16]. Before the test, the interior of the gas chamber was cleaned using the anhydrous alcohol and the airtightness of the device was detected. Subsequently, the chamber was evacuated and charged with CO 2 (purity of 99.999%) for three times to eliminate the influence of impurity gases. Finally, the C 4 F 7 N/CO 2 gas mixture was injected. Literature [10] points out that at a working temperature of 30°C, the optimum of the C 4 F 7 N mixture is between 5-8% C 4 F 7 N content. Therefore, the mixing ratio (C 4 F 7 N concentration) in this paper was determined to be 0, 2, 4, 6 and 8%.
The pulse current method in this paper was based on the IEC60270 standard [17]. The oscilloscope (Tektronix 7104) has the capability to measure the PD current pulses of real-time sampling rate up to 20 GS/s for frequency up to 1 GHz. The step-stress test method (boost rate about 0.5 kV/s) was used to apply the high voltage to the electrodes.
We first calibrated the PD quantity using the pulsed current source before the test. When the oscilloscope displayed repetitive, clear and stable PD signal, as shown in Fig. 3, the high voltage value was recorded as the PD inception voltage (PDIV). Each group of the PDIV and breakdown tests were repeated five times to avoid accidental errors and the test interval was 3 min.

PD properties of C 4 F 7 N/CO 2 mixture
Due to the obvious polarity effect in highly non-uniform electric field, the negative needle electrode is easy to emit electrons. Then electrons move to the plate electrode quickly and lots of positive space charge are accumulated near the needle electrode, which increases the electric field intensity there and makes it much easier  for PD to occur [18]. Table 2 gives the PDIV of SF 6 and C 4 F 7 N/CO 2 gas mixture at different gas pressure and mixing ratio. Fig. 4 shows the evolution of the PDIV with gas pressure for C 4 F 7 N/CO 2 gas mixture. It can be found that the PDIV of C 4 F 7 N/CO 2 gas mixture with 2-8% C 4 F 7 N increases with the gas mixture but lower than pure SF 6 at the same condition. The relative PDIV (PDIV gas mixture /PDIV SF6 ) of C 4 F 7 N/CO 2 gas mixture also shows increase trend with the growth of gas pressure. For example, the PDIV of gas mixture with 2% C 4 F 7 N under 0.15 and 0.3 MPa reaches 66 and 78% of SF 6 , respectively. The PDIV for 8% C 4 F 7 N/92% CO 2 gas mixture reaches 78 and 86% of SF 6 . This is due to that SF 6 is very sensitive to electric field concerning ionisation coefficient and the value of the effective ionisation coefficient increases with gas pressure.
The PDIV of C 4 F 7 N/CO 2 gas mixture increases with the mixing ratio at the same gas pressure (see Fig. 5). The addition of C 4 F 7 N can significantly increase the PDIV of CO 2 gas. For example, the PDIV of the gas mixture containing 2% C 4 F 7 N at 0.3 MPa is 1.75 times that of pure CO 2 . In addition, the PDIV of C 4 F 7 N/CO 2 gas mixture with C 4 F 7 N content in the range of 2-8% shows linearly increasing trend with the mixing ratio. A linear fit was done to the relationship between PDIV and mixing ratio (R 2 is 0.999) and it was found that the PDIV of the gas mixture increases by about 0.3 kV with the mixing ratio increases 1%. The PDIV of gas mixture with 2, 4, 6 and 8% C 4 F 7 N reach an average value of 78, 81, 83 and 86% of SF 6 at 0.3 MPa and is about 66, 72, 75 and 80% at 0.15 MPa.

Breakdown properties of C 4 F 7 N/CO 2 mixture
3.2.1 Breakdown of C 4 F 7 N/CO 2 mixture in the highly nonuniform field: There are various mechanical structures in the gas insulated electrical equipment, which leads to the uniformity of the internal electric field. The sensitivity of the gas insulating medium to the non-uniformity of the internal electric field directly affects the insulation performance, and thus affects the safety and service life of the equipment. In order to explore the possibility of C 4 F 7 N/CO 2 gas mixture using as a new insulating medium, it is necessary to conduct in-depth research on the insulation performance under different electric field conditions. Table 3 shows the breakdown voltage of SF 6 and C 4 F 7 N/CO 2 gas mixture under highly non-uniform field. Figs. 6 and 7 give the evolution of the AC breakdown voltage of C 4 F 7 N/CO 2 gas mixture with different gas pressure and mixing ratio. According to the test results, the breakdown voltage of C 4 F 7 N/CO 2 gas mixture with pressure lower than 0.2 MPa tends to increase linearly with the gas pressure. The linear fitting results (R 2 is 0.9983) show that the gas breakdown voltage increases by about 7.97 kV when the pressure increases 0.1 MPa. The breakdown voltage shows a saturation increase trend with pressure above 0.2 MPa.
Moreover, the relative breakdown voltage of the gas mixture is relatively stable with the pressure lower than 0.2 MPa. While a downward trend can be found between the relative breakdown voltage and pressure when the gas pressure is higher than 0.2 MPa, indicating that the relative insulation strength of C 4 F 7 N/CO 2 gas under high-pressure condition is inferior to that of low pressure (see Fig. 6b). For example, the breakdown voltage of gas mixture containing 2, 4, 6 and 8% C 4 F 7 N reaches 60, 65, 67 and 71% of pure SF 6 at 0.2 MPa, and this value decreases to 56, 63, 66 and 68% at 0.3 MPa.
The AC breakdown voltage of C 4 F 7 N/CO 2 gas mixture also increases with the mixing ratio in the highly non-uniform field. The relative breakdown strength of C 4 F 7 N/CO 2 gas mixture at 0.1-0.3  MPa shows a linear saturation growth trend with the increase of mixing ratio. Table 4 shows the AC breakdown voltage of SF 6 and C 4 F 7 N/CO 2 gas mixture in quasi-uniform field. Figs. 8 and 9 show the evolution of the AC breakdown voltage of C 4 F 7 N/CO 2 gas mixture with different gas pressure and mixing ratio conditions, respectively. It can be seen that the insulation performance of gas mixture containing 2-8% C 4 F 7 N is inferior to pure SF 6 at the same condition. The breakdown voltage of SF 6 and C 4 F 7 N/CO 2 gas mixture increases linearly with the growth of gas pressure. The relative dielectric strength of C 4 F 7 N/CO 2 gas mixture did not change significantly with the change of gas pressure, indicating that the increase of gas pressure does not enhance the relative insulation properties of C 4 F 7 N/CO 2 gas mixture.

Breakdown of C 4 F 7 N/CO 2 mixture in the quasi-uniform field:
The insulation strength of C 4 F 7 N/CO 2 gas mixture shows a saturation increase trend with mixing ratio in the quasi-uniform field, which is consistent with the test results in literature [6,7]. The insulation strength of C 4 F 7 N/CO 2 gas mixture containing 2, 4, 6 and 8% C 4 F 7 N researches 51.4, 65, 70.9 and 72.8% of pure SF 6 . It can be found that when the mixing ratio is lower than 6%, increasing the C 4 F 7 N content has a better effect on the improvement of insulation performance. This is due to the addition of low content of C 4 F 7 N has been able to trap electrons to form negative ions and then hinder the development of ionisation process in the gas mixture. In addition, the relative dielectric strength of C 4 F 7 N/CO 2 gas mixture also shows a saturation increase trend with the mixing ratio.

Discussion
According to the above experimental results, improving the gas pressure or mixing ratio can effectively enhance the insulation performance of C 4 F 7 N/CO 2 gas mixture. However, the influence mechanism is different.
For the gas insulating medium, the density of the gas mixture increases as the gas pressure increases, then the mean free path of free electron in the gas is shortened and the ionisation process is weakened. Therefore, increasing gas pressure can effectively improve the insulation performance of the gas insulating medium. The breakdown voltage of SF 6 and C 4 F 7 N/CO 2 gas mixture increase linearly with the pressure in the quasi-uniform field. However, the relationship between breakdown voltage and gas pressure changes to a linear saturation growth trend in the highly non-uniform field. This is due to the fact that the non-uniformity of the electric field under high pressure has more obvious effect on the breakdown performance of gas insulating medium than low pressure.
Actually, the non-uniformity of the electric field has a great influence on the insulation properties of SF 6 . The essential reason is that the growth rate of the effective ionisation coefficient α is high, which is also an important feature of SF 6 gas insulation [19]. Literature [9] reports the effective ionisation coefficient results of C 4 F 7 N/CO 2 gas mixture. The growth rate of the effective ionisation coefficient for gas mixture containing 6.7% C 4 F 7 N is close to that of pure SF 6 , and this is the reason why the breakdown voltage of C 4 F 7 N/CO 2 gas mixture shows a linear saturation growth trend with the pressure in the highly non-uniform field. Moreover, due to the larger volume of the C 4 F 7 N molecule itself, the mobility of the particles formed by dissociation of the C 4 F 7 N is low, resulting in a relatively dense space charge near the needle electrode. Thus, it is difficult to form a uniform space charge layer that can improve the electric field distribution near the electrode. Therefore, the breakdown voltage of gas mixture at high pressure appears saturated trend. Fig. 10 shows the main properties of C 4 F 7 N/CO 2 gas mixture. When the content of C 4 F 7 N increases, the composition of the gas mixture is substantially changed. Due to the CN group and F atoms in C 4 F 7 N molecule have strong electronegativity [20], it is easy for gas molecules to capture electrons forming negative ions, which hinders the formation and development of ionisation process. In addition, the mean free path of the free electrons in the gas mixture will be shortened because of the large volume of C 4 F 7 N molecule. The ionisation energy of C 4 F 7 N molecules is relatively high (comparable with SF 6 ) [21]. Moreover, the energy loss of free electrons is increased due to the polarisation process, and the collision ionisation capacity of free electrons is further weakened. Therefore, the insulation performance of gas mixture increases with the increase of C 4 F 7 N content. It should be noted that a low content of C 4 F 7 N could hinder the development of ionisation process effectively. Thus, the relative dielectric strength of  Overall, enhancing the gas pressure and increasing the mixing ratio are two effective ways to improve the insulation performance of C 4 F 7 N/CO 2 gas mixture. In engineering applications, it is necessary to consider the comprehensive demand such as equipment production cost and application environment to further select the appropriate mixing ratio and gas pressure of C 4 F 7 N/CO 2 gas mixture.

Conclusion
In this paper, we tested the PD and power frequency breakdown characteristics of C 4 F 7 N/CO 2 gas mixture under different mixing ratio and gas pressure conditions. The comparison is also made with SF 6 . The effect of gas pressure and mixing ratio on the insulation performance of C 4 F 7 N/CO 2 gas mixture was revealed, and the influence mechanism of two factors was discussed. The main conclusions were obtained as follows: i. The breakdown voltage of C 4 F 7 N/CO 2 gas mixture containing 2-8% C 4 F 7 N in the quasi-uniform electric field increases linearly with the gas pressure, and shows a saturation increase trend with mixing ratio. The insulation performance of 6% C 4 F 7 N/94% CO 2 gas mixture at 0.3 MPa reaches pure SF 6 at 0.21 MPa. The breakdown voltage of 8% C 4 F 7 N/92% CO 2 gas mixture at 0.2 MPa is comparable to pure SF 6 at 0.15 MPa. ii. The PDIV and AC breakdown voltage of C 4 F 7 N/CO 2 gas mixture containing 2-8% C 4 F 7 N in the highly non-uniform electric field shows the saturation increase trend with mixing ratio and gas pressure. The PDIV and AC breakdown voltage of 6% C 4 F 7 N/94% CO 2 gas mixture at 0.3 MPa reach pure SF 6 at 0.17 and 0.15 MPa, respectively. The insulation strength of 8% C 4 F 7 N/92% CO 2 gas mixture at 0.2 MPa is comparable to pure SF 6 at 0.13 MPa. iii. The C 4 F 7 N/CO 2 gas mixture containing 2-8% C 4 F 7 N is sensitive to the non-uniformity of electric field. For engineering application, it is necessary to optimise the structure of equipment to avoid the formation of nonuniformity electric field.

Acknowledgment
The current work is supported by the Science and Technology Project of China Southern Power Grid (no. ZBKJXM20170090).