Introduction

SF6 is regard as the perfect insulating and extinguishing medium applied in high voltage gas insulated switchgear (GIS), gas insulated transmission lines (GIL) and other equipmen1,2. Nevertheless, SF6 has been severely restricted in electrical equipment because of its high global warming potential (GWP) and long atmospheric lifetime. Seeking eco-friendly alternatives to SF6 has been a hot topic of global power industries and academic organizations in recent years3,4,5. There are three alternative solutions, which are natural gases (N2, CO2, dry air), SF6 gas mixtures (SF6/N2, SF6/CO2) and other electronegative gas (CF3I, C4F7N, C5F10O and C6F12O)6,7. Electronegative gases are often mixed with a buffer gas such as CO2, N2, or dry air that reduces the liquefaction temperature.

Many efforts have been conducted on the discharge characteristics of eco-friendly gases. He et al.8 gained the effect of electric field non-uniformity on the positive pulse characteristics in SF6/N2 mixtures, and it is found that the breakdown voltage decreases with the increasing electric field non-uniformity, which also explains the streamer development process of the positive corona discharge. Zhao et al.9 studied the dielectric properties of SF6/N2 mixtures, which considers the liquefaction temperature, saturated vapor pressure and breakdown electric field strength, and the analysis results illustrated that 20%–30% SF6/N2 mixtures were recommended. Meanwhile, Tu et al.10 conducted partial discharge experimental in nonuniform electric field under negative DC voltage and a ±100-kV SF6/N2 gas-insulated transmission line (GIL) prototype was introduced. Although the insulation strength of SF6 mixed gases could be acceptable, it is still considered as a transitional method and cannot eliminate the application of SF6. Another method that received significant attention is C4F7N/CO2 mixtures, which has been regard as the hopeful alternatives of SF6 in recent years. The C4F7N/CO2 is suitable for application as insulation and arc extinguishing medium in gas insulated electrical equipment. Zhang et al.11 investigated the breakdown strength and partial discharge (PD) characteristics of C4F7N/CO2 mixtures, and the 20% C4F7N/CO2 mixtures at 0.1–0.12 MPa constituted an optimal replacement of SF6, which can be used under  − 30 °C without liquefaction. Besides, the decomposition characteristics, breakdown characteristics, surface flashover characteristics and arc characteristics were studied under various conditions12,13,14,15. But 3M company suddenly announced that they would stop generating fluorinated products and the European Union stipulates the prohibition of applying fluorinated gases (GWP>10) in high voltage equipment which brings large uncertainty to future researches and applications of C4F7N/CO2 mixtures. The third method is natural gases at high pressure16,17,18. Inoue et al.19 investigated insulation properties of dry air at 0.1 to 0.6 MPa pressure, and the effect of surface roughness and anodic oxide coating on the insulation properties was analyzed. Jee et al.20 presented the surface discharge characteristics of dry air under a sphere-plane electrodes, and it was clearly observed that two surface discharge paths were revealed with the pressure change in the N2/O2 mixed gas. What’s more, the breakdown characteristics in synthetic air, CO2 and a CO2/O2 mixture in the range of 0.5–10 MPa were obtained, and the surface insulation performance of dry air in GIS was also compared21. The decomposition products detection of PD is conducted to distinguish different defect types by some scholars. Tan et al.22 studied the characteristics gases of metal protrusions partial discharge in 10-kV air-insulated switchgear, and the influence of voltage, air humidity and degree of protrusion on partial discharge are detected by gas composition analysis. Khoury et al.23 analysed the chemical by-products from partial discharges in air, and the generation of by-products reflect the influence of PD duration, power and humidity on PD characteristics.

As mentioned above, SF6 mixed gases would be replaced in the future because of containing SF624,25, while C4F7N/CO2 mixture could also be restricted because of its arc-extinguishing characteristics, gas-solid compatibility and biological safety problems26,27. Natural gases as insulating medium is a preferable choice and gradually receive increasing attentions by power industries and academic institutions again. Siemens company manufactures “Blue” series high voltage productions, which included GIS, tank vacuum circuit breakers and current transformers et al., and they chose dry air as the insulating medium. Pinggao Group also proposed GIS, circuit breaker and wall bushing, and the insulating medium was CO2 or dry air28. Furthermore, it seems that adopting natural gas insulating and vacuum interrupters interrupting could realize the totally eco-friendly application and have the great developing prospect of GIS and circuit breakers. However, breakdown characteristics of dry air and CO2 are usually investigated under 0.1~0.6 MPa, where the insulation properties is much lower than SF6 under the same gas pressure, and the experiments electrodes are often one or two types, which could not compare the relative insulation strength and insulation configuration with SF6 comprehensively. Although there are studies of eco-friendly gases on partial discharge (PD) decomposition products detection, the insufficient defect types and unsuitable characteristic gases still needs further investigations.

In this paper, the AC breakdown characteristics of C4F7N/CO2 mixtures, CO2 and dry air under various gas pressure and electrodes are gained, and 0.6 MPa 7% C4F7N/93% CO2, 0.9 MPa dry air, 0.9 MPa CO2 are recommended as eco-friendly insulating mediums in GIS. On the other hand, the partial discharge experiments of dry air is conducted, and the decomposition products are detected, which is explained by the decomposition products generation mechanism. Finally, the specific insulation strength of eco-friendly gases under various electrodes types are obtained, and the partial discharge defects are distinguished according to the decomposition products. The discharge characteristics and analysis of dry air would promote the development of eco-friendly gas insulated switchgear.

Experiment

Experimental platform

Fig. 1 shows the experiment circuit for obtaining the discharge characteristics of eco-friendly insulating gases29. The experimental circuit includes a voltage regulator, a 100-kV AC experimental transformer, a current-limiting protective resistor, a capacitive voltage divider and the experimental device. The voltage regulator provides the input voltage of experimental voltage transformer, which transfers the voltage from 0 to 100 kV. The capacitance of capacitive voltage divider is 1000 pF, and the voltage ratio is 1000:1. Based on the multiple sealed construction, the capacity of experimental device could withstand the maximum pressure 1.0 MPa and leak less than 0.001 MPa every 24 h, and experimental electrodes in red dashed box are adjusted according to various experimental conditions. Three eco-friendly insulating gases, such as C4F7N, CO2 and dry air, are selected for the investigation. Meanwhile, the on-line infrared gas analyzer TY-6300 is used to measure the volume concentration of CO, CO2, NO and NO2 gases, which adopts electrochemical sensors.

Figure 1
figure 1

Experimental circuit.

Full size image

Experimental procedure

Fig. 2 illustrates the experimental procedure. Firstly, the electrodes are polished by 3000-mesh sandpaper and cleaned by anhydrous alcohol, which could avoid the effect of particulate. Then experimental electrodes or defects are installed according to various experimental demands. The experimental device is evacuated below 10 Pa using a vacuum pump, and it is filled with CO2 or dry air to 0.1 MPa. The above process is repeated three times to eliminate the influence of other gases and moisture. According to Dalton's Law, different mixing ratios are calculated before gas mixture process. Then, the lower content partial component gases is filled into the test chamber, and the higher contend partial component gases is filled after that. Completing the gas mixture process, the test chamber must be static for more than 12 h to ensure the gas mixtures are well mixed. The AC voltage is applied to the experimental device, and the breakdown experiment or partial discharge experiment is conducted30. Furthermore, the experimental voltage rise at a voltage ramp rate of 0.5 kV/s and the time interval between two gas breakdown processes is 5 min. The breakdown experiments are repeated five times, and the average value is considered as the breakdown voltage. It is worth noting that the mixing ratio is equal to the ratio of partial pressure. For the C4F7N/CO2 mixture, C4F7N is filled in first up to the desired partial pressure and then CO2 is topped up as the carrier gas to guarantee the adequate mixture. In terms of partial discharge experiments, the decomposition products detection is accomplished under different discharge times and applied voltage amplitudes, while the sampling is recorded every 4 h with 2 min gas circulate by on-line infrared gas analyzer TY-6300.

Figure 2
figure 2

Experimental procedure.

Full size image

Results and analysis

Dielectric strength characteristics

In order to simulate the electrical field of different locations in GIS, four experimental electrodes are designed in Fig. 3. The ball-plate electrodes and coaxial electrodes are designed to simulate the slightly non-uniform electric fields, which are also the mostly common electrical field types in practical GIS. Compared with ball-plate electrodes, coaxial electrodes are closer to the parallel electrode structures. Considering the effect of sharp protrusions and metal particles, needle-plate electrodes are designed to simulate the extremely non-uniform electric fields. In contrast, the uniform electric fields are simulated by plate-plate electrodes. The ball electrode is a hemispherical electrode with a radius of 15 mm, while the plate electrode has a 102 mm diameter. The radius of needle electrode tip is 1 mm, and the external grounding electrode of coaxial electrode possesses a bowl structure with a diameter of 42 mm in the bottom, which ensure the AC breakdown appearing in the coaxial area. By adjusting the gap distance h between electrodes, the dielectric strength characteristics are obtained under different electric field uniformity.

Figure 3
figure 3

AC breakdown experimental electrodes.

Full size image

Figure 4a shows the relationship between AC breakdown voltages and gas pressure under 5 mm coaxial electrodes. On the whole, the AC breakdown voltages of C4F7N/CO2 mixtures possess linearly increasing trend with increasing C4F7N mixing ratio and pressure. Compared with CO2 and dry air, the AC breakdown voltages of C4F7N/CO2 mixture is significantly improved after adding a certain amount of C4F7N. Less than 0.3 MPa, the growth rate of C4F7N/CO2 mixture is similar, while 7% C4F7N/93% CO2 mixture firstly appears the saturation trend at 0.4–0.6 MPa. At 0.4 MPa, the AC breakdown voltage of 7% C4F7N/93% CO2, 13% C4F7N/87% CO2 and 20% C4F7N/80% CO2 are 50.78 kV, 55.32 kV and 62 kV, which are 0.75, 0.82 and 0.92 compared with SF6. The AC breakdown voltage of 7% C4F7N/93% CO2 is 67.1 kV, which possess approximate insulation strength of 0.4 MPa SF6 under coaxial electrodes. As for dry air and CO2, the stable increasing trend appears at 0.5–0.8 MPa, and the saturation trend appears gradually beyond 0.8 MPa. It is worth noting that the breakdown voltage of dry air and CO2 at 0.9 MPa are 49.4 kV and 44.62 kV, which occupy 0.73 and 0.66 of 0.4 MPa SF6.

Figure 4
figure 4

AC breakdown voltages under slightly non-uniform electric fields.

Full size image

The AC breakdown voltage of SF6 under coaxial electrode is regard as the similar values with the ball-plate electrode, and the 20% C4F7N/80% CO2 could not be widely applied in the practical GIS or GIL because of its relatively high liquefaction temperature (above − 5 °C)11,14. Considering the practical application requirement and unnecessary consumption, the AC breakdown voltages of SF6 and 20% C4F7N/80% CO2 are not conducted under the ball-plate electrode. In terms of 4 mm ball-plate electrodes, the overall values and variation trends of AC breakdown voltage are similar to coaxial electrodes, while the saturation trend appears more obviously. The AC breakdown voltages between 7% C4F7N/93% CO2 and 13% C4F7N/87% CO2 get more difference, which indicates that adding more C4F7N brings the more increase in breakdown voltages of C4F7N/CO2 mixtures under the ball-plate electrodes. The electric field nonuniform coefficients f are 1.34 and 1.1 for coaxial electrodes in Figure 4a and ball-plate electrodes in Figure 4b. Due to the slightly difference in electric field nonuniform coefficients, the variation trend of C4F7N/CO2 mixtures in Figure 4a and in Figure 4b possess a little difference in AC breakdown voltage values, but the the whole variation trend are similar, which indicates that the C4F7N/CO2 mixtures are insensitive to the variation of electric field nonuniform coefficients. As for dry air and CO2, both the variation trend and AC breakdown voltage values are affected by the electric field nonuniform coefficient, and the crossover phenomenon appears between dry air and CO2 around 0.7 MPa. It can be derived that the relative insulation strength between dry air and CO2 may be opposite according to the different electric field nonuniform coefficients and gas pressure, which indicates that dry air and CO2 are more sensitive to the application conditions.

Although C4F7N has excellent insulation properties, it always needs to be mixed with buffer gases to avoid the high liquefaction temperature disadvantage. In addition, the high cost and poor arc extinguishing capability cause the limited application, and it still contains F atoms which cannot completely solve the environmental problems. Some countries and regions, such as 3M company and European Union, have issued a series of policies on the control of fluorine gases to regulate their applications. Therefore, the AC breakdown voltages experiments under uniform electric fields and extremely non-uniform electric fields are conducted with dry air and CO2.

As Figure 5a shows, the AC breakdown voltage of dry air and CO2 increases approximately linearly with gas pressure. Under the same condition, the AC breakdown voltage of dry air is higher than CO2. When the gap distance is 3 mm, both dry air and CO2 show a slightly saturation trend. Specifically, the AC breakdown voltage of dry air and CO2 are 56 kV and 47.7 kV at 0.9 MPa, while they are 57.5 kV and 49.2 kV at 1.0 MPa. Under the uniform electric fields, the increasing gap distance bring the more increase in breakdown voltage compared with the gas pressure. What’s more, the AC breakdown voltage of 0.7 MPa dry air and 0.8 MPa CO2 could reach the near insulation properties of 0.4 MPa SF6. When the electric field is extremely non-uniform as shown in Figure 5b, the AC breakdown characteristics of dry air and CO2 under the needle-plate electrodes with 3 mm, 5 mm and 10 mm gap distance are presented. When the electric field is extremely non-uniform, there is still a phenomenon that the AC breakdown voltage of dry air is higher than CO2. Furthermore, the AC breakdown voltage between dry air and CO2 at high gas pressure has the greater difference, which is 6.7 kV at 0.8 MPa gas pressure of 5 mm gap distance. Overall, 0.9 MPa dry air and CO2 under 10 mm gap distance of extremely non-uniform electric fields could reach 0.67 and 0.64 of 0.4 MPa SF6.

Figure 5
figure 5

AC breakdown voltages under uniform electric fields and extremely non-uniform electric fields.

Full size image

Based on the above experimental results, the insulating performance of C4F7N/CO2 mixture is the best among three eco-friendly gases. The increase of gas pressure or C4F7N mixing ratio both improves the AC breakdown voltage of C4F7N/CO2 mixture. Besides, CO2 and dry air at high gas pressure also have the potential to replace SF6. CO2 has the worst insulating performance, and its breakdown voltage is much lower than C4F7N/CO2 mixture. Even if the gas pressure is increased, the improvement could be limited. The insulating performance of dry air is between the C4F7N/CO2 mixture and CO2, but it still has the saturation phenomenon when the gas pressure is too high. At the same time, high gas pressure also brings challenges to the airtightness of GIS. Although the GWP value of C4F7N/CO2 mixture has been greatly reduced, it still contains F atoms, which is in conflict with the trend of environmental protection. Therefore, it seems that 0.9 MPa dry air is recommended to be the eco-friendly insulating gas in GIS to substitute SF6, and the higher requirements of withstand pressure needs more attention.

Partial discharge characteristics

The unavoidable insulation defects during manufacturing, transportation, and assembly can cause various kinds of insulation defects in GIS, such as metal protrusion defects, air gap defects, and metal contamination defects, which would eventually lead to insulation breakdown or flashover. Considering the future application of dry air as insulating medium in GIS, the investigation of PD characteristics and decomposition products detection are crucial for safe eco-friendly GIS operation.

Decomposition products generation mechanism

In fact, the decomposition products of dry air is generated through collision ionisation, thermal ionisation and other processes under partial discharge, and the various generated particles would be further reacted and restored to gas molecules or other decomposition products, which lead to make changes in the components of insulation medium. The basic chemical properties of dry air are calculated by density functional theory31,32, and the decomposition paths of different elements are analysed by thermodynamic parameters. As shown in Fig. 6, the energy variation and products generation in the process are illustrated, which also contains the main reactions through partial discharge process. The initial process of the reaction is under the action of the local electric field exciting charged particles or free electrons, which accelerate the collision of O2 and N2 molecules and make single O and N atoms. O atoms are easy to achieve many reactions with other elements, and O3 would be produced O atoms and O2 molecules reacted. On the other hand, N atoms react with O, O2 and O3 under the oxidative reaction to produce NO and NO2, while NO possesses strong chemically reactive, and the reaction products are mainly forming to NO2. When the organic materials or solid insulating materials involves in partial discharge process, the C elements excited by strong ionized discharge would also combine with the O atom to form CO and CO2. Due to the strong oxidising property of O3, CO is not the final product under the reaction process, and the content of CO2 product is higher than CO. Therefore, CO, CO2 and NO2 could exist stably in the dry air, and are closely related to the characteristics of partial discharge, which would be used as characteristic gases of partial discharge detection.

Figure 6
figure 6

Energy variation in the process of decomposition product generation.

Full size image

PD experiments

As shown in the Fig. 7, PD is monitored by the pulse current method in the experiment. The circuit is composed of a detection impedance in series with a coupling capacitor, transmitting the pulse signal to the oscilloscope through a coaxial cable. According to IEC 60270–2000 and Chinese GB/T7354-2018 standard, number of pulses and voltage amplitude are monitored during the PD experiment, which could be statistically calculated as the average of apparent charge (Qavg) and repetition rate N of each group of experiments. Since PD is a phenomenon that can exist for a long time, our experiment obtains the Qavg in each power frequency cycle (20 ms).

Figure 7
figure 7

Schematic of the calibration circuit.

Full size image

Fig. 8 illustrates the PD signal calibration procedure, and PD calibration is monitored by the pulse current method in the experiment. The typical partial discharge waveform is transmitted to the oscilloscope through the coaxial cable, and the relationship between partial discharge and applied voltage is as Figure 6b shown22,33,34. There are three common types of partial discharge defects in GIS, which are metal protrusion defect, air gap defect, and metal contamination defect. The stable partial discharge decomposition products of dry air mainly include NO2, CO2 and CO, which is detected by the on-line infrared gas analyzer. As Fig. 9 shows, the experimental defects are designed to simulate the common partial discharge defects.

Figure 8
figure 8

PD signal calibration procedure.

Full size image
Figure 9
figure 9

Experimental partial discharge defects.

Full size image

For the air gap defect, the decomposition products detection is carried out by selected four voltage amplitudes to simulate different partial discharge strengths and different components are represented. As Figure 10a indicated, the CO2 concentration increases with the increasing partial discharge times and voltage amplitudes. The higher voltage amplitude is, the more significantly CO2 concentration presents, which also indicates that the increase of voltage amplitude plays an important role in promoting CO2 production. At 26 kV or 27 kV applied voltage, the CO2 concentration shows a linear growth trend during the first 14 h, and then the saturation phenomenon occurs. Due to the insufficient discharge intensity, the C atoms contained in the insulator cannot be continuously excited, and CO2 is decomposed into CO under the action of discharge. Thus, the production rate and decomposition rate of CO2 are equal, which makes the CO2 concentration reaches saturation. When the applied voltage is 28 kV or 29 kV, the CO2 concentration shows an approximately linear growth trend within the discharge cycle, and the saturation phenomenon is not obviously, which indicates that the high energy electrons are excited under the action of higher applied voltage. For Figure 10b, the trend of CO concentration is shown. It can be observed that the increasing trend of CO concentration with discharge time and applied voltage is more obviously. One reason is C atoms generated by a large number of free electrons colliding to the insulator can directly combine with O atoms to generate CO. Another reason is CO2 could be decomposed into CO under the action of discharge. As for Figure 10c, High-speed electrons collide with N2 and O2 molecules, which generates N and O atoms to combine into NO2 molecules. Unlike the CO concentration, NO2 concentration shows a linear increase in the middle time stage and the slow growth occurs in the beginning and final time stage. The reason for this phenomenon is that O3 produced by the decomposition of dry air has strong oxidizing properties, which oxidizes NO2 into NO3 and N2O5. However, NO3 and N2O5 are extremely unstable, and will further decompose into NO2. The oxidizing process and NO2 generation process both determine the variation trend of NO2.

Figure 10
figure 10

Decomposition products detection under air gap defect.

Full size image

As shown in Figure 11a, CO2 concentration is positively related to the applied voltage and discharge time. The metal contamination defect is located along the surface of the epoxy resin insulator, and the C elements is easily excited by strong ionized discharge, which also combine with the O atom to form CO and CO2. On the other hand, CO reacts with the sufficient strongly oxidizing O3 to form CO2, which also causes the increase of CO2 concentration. In addition, the partial discharge generated by metal contamination defect occurs along the surface of epoxy resin insulator, which provides continuous C elements under the high voltage field, thus the saturation phenomenon is not obvious. As for CO concentration in Figure 11b, it has a faster growth rate at the initial stage and a decreasing growth rate at the later stage, which gradually approach saturation stage. Moreover, CO2 and CO contamination under metal contamination defect on the insulator surface are much lower than those under air gap defects. The partial discharge under contamination defect only occurs on the surface of epoxy resin, while the partial discharge under air gap defect develops through the entire epoxy resin, which leads to more severe deterioration of insulation materials and excitation of more C atoms. As shown in Figure 11c, the variation trend of NO2 concentration also follows the same growth trend as the air gap defect, but it is higher than that under air gap defects. The gas production rate of NO2 shows a nearly linear increase in the first 4 h. In the initial stage, the volume fraction of NO2 is approximately proportional to the partial discharge time because of the stable partial discharge. The N2 and O2 molecules are collided with the accelerated charged particles, while N atoms and O atoms are generated. Then, N atoms react with O, O2 and O3 under the oxidative reaction to produce NO and NO2. With the partial discharge time increasing, NO2 would be oxidized and consumed by enough O3, which generates NO3 and N2O5, and the gas production rate of NO2 also begins to decrease due to the saturation of the volume fraction. Although NO3 and N2O5 are chemically unstable and a part of NO3 and N2O5 would further decompose into NO and NO2, the gas consumption rate of NO2 keeps on increasing. When the gas production rate and gas consumption rate reaches the balance, the NO2 concentration shows a trend of saturation growth.

Figure 11
figure 11

Decomposition products detection under metal contamination defect.

Full size image

Figure 12 illustrates the NO2 concentration under metal protrusion defect. The decomposition products is detected under the metal protrusion defect, which represents by the needle and plate electrode. The electrodes are made of aluminium alloy, and there is no epoxy resin between electrodes, which means little C element involves directly in the partial discharge reaction. Therefore, CO2 and CO concentration under the metal protrusion defect are extremely low and could not be detected. On the other hand, the production rate of NO is slow under the partial discharge where NO is continually oxidized by O3 into NO2. Therefore, it can be observed that only NO2 concentration was detected during the experimental process. Compared to other defects, NO2 concentration under metal protrusion defect is higher. Considering the electric field distribution under metal protrusion defect is extremely nonuniform, and a large number of free electrons are more easily excited. Additionally, chemical bonds of gas molecules are also more easily to breakdown near the needle tip, which results in more N and O atoms and higher NO2 concentrations.

Figure 12
figure 12

Decomposition products detection under metal protrusion defect.

Full size image

Based on the above result, the defect identification could be accomplished. NO2 is formed by the recombination of N and O atoms produced by electron collision with N2 and O2, and its concentration is directly related to the electric field distribution under different defect types. Therefore, c(NO2) can directly reflect the total decomposition of dry air. On the other hand, The C atoms in CO2 and CO generated by partial discharge under air gap defect and metal contamination defect on insulator surfaces, while c(CO2 + CO) represents the decomposition degree of insulation materials. Considering the difference between the decomposition of dry air and the decomposition degree of insulation material under different defect types, c(CO2 + CO)/c(NO2) ratio and c(CO2)/c(CO) ratio are used as characteristics for diagnosing and evaluating defects which is illustrated in Fig. 13. Under air gap defect, c(CO2)/c(CO) has the relative concentrated distribution, which is mainly in the range of 1 to 4. As for c(CO2 + CO)/c(NO2), it commonly exceeds 9. Under metal contamination defect, c(CO2)/c(CO) and c(CO2 + CO)/c(NO2) are in the range of 1 to 13 and 1 to 9, respectively. Besides, the distribution area possesses the vertical growth along the c(CO2 + CO)/c(NO2) axis for the air gap defect, while it is the horizontal growth along the c(CO2)/c(CO) axis for the metal contamination defect. Metal protrusion defects would be diagnosed by the small quantity near coordinate origin. It can be observed that the selection of two characteristic quantities has the good effect on the distinction of different defect types. The intensity of partial discharge may cause a certain degree of dispersion in the distribution under the same defect type. Adopting two characteristics through the ratio method eliminates the influence of single absolute numerical value, which better reflects the relative changes in decomposition products under different defect types.

Figure 13
figure 13

Relationship between defect types and characteristic ratio [c(CO2 + CO)/c(NO2), c(CO2)/c(CO)].

Full size image

Conclusion

This paper investigates the AC breakdown and decomposition detection characteristics of eco-friendly insulating medium in gas insulated switchgear. The influence of pressure, mixing ratio of C4F7N, gap distance on AC breakdown voltage is discussed, and the decomposition products of dry air under various partial discharge conditions are compared, which provides a good foundation for the research of eco-friendly gas as an insulation substitute for SF6. The following conclusions are drawn.

  1. (1)

    In generally, the AC breakdown voltages of C4F7N/CO2, dry air and CO2 all increase with the increasing pressure and gap distance. The C4F7N/CO2 mixtures possess linearly increasing trend with increasing C4F7N mixing ratios and pressures. Besides, CO2 and dry air at high gas pressure also have the potential to replace SF6, but the breakdown voltage improvement could be limited by increasing pressure, and dry air and CO2 beyond 0.8 MPa have the obviously saturation phenomenon.

  2. (2)

    Among these eco-friendly insulating gases, the C4F7N/CO2 mixtures possess highest breakdown voltage, and dry air has the better insulating property than CO2. Under the slightly non-uniform electric fields, 0.6 MPa 7% C4F7N/93% CO2, 0.9 MPa dry air and 0.9 MPa CO2 occupy 0.99, 0.73 and 0.66 compared with 0.4 MPa SF6, which are also suggested to be the insulating medium in 126 kV GIS.

  3. (3)

    Partial discharge characteristics of dry air are also investigated through air gap defect, metal contamination defect and metal protrusion defect. The stable decomposition products under partial discharge of dry air mainly include NO2, CO2 and CO. When 1 < c(CO2)/c(CO) < 4 and 9 < c(CO2 + CO)/c(NO2), it is thought to be air gap defect. While 1 < c(CO2)/c(CO) < 13 and 1 < c(CO2 + CO)/c(NO2) < 9, the metal contamination defect could be diagnosed. As for metal protrusion defect, c(CO2)/c(CO) and c(CO2 + CO)/c(NO2) both have the small value. The partial discharge decomposition products detection of dry air promotes the development of eco-friendly GIS.