Clarifications on the Behavior of Alternative Gases to SF6 in Divergent Electric Field Distributions under AC Voltage

Negative and positive partial discharge inception voltages and breakdown measurements are reported in a needle-plane electrode system as a function of pressure under AC voltage for natural gases (N2, CO2, and O2/CO2), pure NovecTM gases (C4F7N and C5F10O) and NovecTM in different natural gas admixtures. For compressed 4% C4F7N–96% CO2 and 6% C5F10O–12% O2–82% CO2 gas mixtures, the positive-streamer mode is identified as the breakdown mechanism. Breakdown and negative partial discharge inception voltages of 6% C5F10O–12% O2–82% CO2 are higher than those of 4% C4F7N–96% CO2. At 8.8 bar abs, the breakdown voltage of 6% C5F10O–12% O2–82% CO2 is equal to that of 12.77% O2–87.23% CO2 (buffer gas). Synergism in negative partial discharge inception voltage/electric field fits with the mean value and the sum of each partial pressure individually component for a 20% C4F7N–80% CO2 and 6% C5F10O–12% O2–82% CO2, respectively. In 9% C4F7N–91% CO2, the comparison of partial discharge inception electric fields is Emax (CO2) = Emax(C4F7N), and Emax (12.77% O2–87.23% CO2) = Emax(C5F10O) in 19% C5F10O–81%(12.77% O2–87.23% CO2). Polarity reversal occurs under AC voltage when the breakdown polarity changes from negative to positive cycle. Polarity reversal electric field EPR was quantified. Fitting results show that EPR (CO2) = EPR(9% C4F7N–91% CO2) and EPR(SF6) = EPR (22% C4F7N–78% CO2). EPR (4% C4F7N–96% CO2) = EPR (12.77% O2–87.23% CO2) and EPR (6% C5F10O–12% O2–82% CO2) < EPR (4% C4F7N–96% CO2) < EPR (CO2).


Introduction
In compressed SF 6 [1][2][3][4][5], the propagation mechanism (leader criterion) is the breakdown mode in the presence of conducting particles fixed/floating on conductors or insulators. In the presence of invasive pollutants, the streamer inception level is lower than the leader inception level. The conventional partial discharge (PDs) measurement technique constitutes a means to detect the presence of defects without conducting destructive tests.
On the other hand, for C 4 F 7 N/CO 2 mixtures [6], the PDs activity under AC voltage varies as a function of pressure and the magnitude of the electric field, including breakdown polarity reversal (PR), when the AC breakdown polarity changes from negative half-cycle to positive half-cycle with the increase of pressure. At the reversal critical pressure (PR pressure), breakdowns occur on positive or negative half-cycles. Before PR, PDs are observed during the negative half-cycle, and current pulses occur on the positive cycle. For the same electric field distribution, increasing the pressure leads to an increase of the partial discharge inception voltage (PDIV) and an increase of the intensity of the maximum electric field on the tip of the needle. The latter causes inception of a denser PDs activity on the positive cycle and an attenuation of the PDs on the negative cycle. For pressures above PR, negative PDs activity decreases until being hidden by the background noise of the current sensing circuit. In this case, only higher positive polarity PDs activity is detectable, and the positive PDIV is close to breakdown. It was reported that the breakdown voltage is equal under positive and negative polarities under lightning impulse voltage waveform in quasiuniform fields and at PR pressure [7,8]. The PR pressure increases linearly with the increase of field utilization factor η given by Equation (1). It should be noted that η = E mean /E max , E max being the maximum electric field for a given electrode configuration; it is computed using finite element method simulation using COMSOL Multiphysics TM software.; and E mean is the mean electric field given by E mean = V/d where V is the applied voltage and d is the electrodes gap. The linear increase is characterized by a constant polarity reversal electric field E PR calculated according to Equation (1), where V i is the positive PDIV for divergent electric fields and the breakdown voltage for quasi-uniform electric fields, d is the electrode gap. Table 1 reports E PR for different gas/gas mixtures [6][7][8].
(E PR or E max ) = V i d · n (1)  [6] 64 2.5 0.0385 3.7% C 4 F 7 N-96.3% CO 2 [8] 47 9.5 0.33 10% C 4 F 7 N-90% CO 2 [7] 66 5.5 0.22 The present work investigates both the negative/positive-inception and breakdown voltages of a classic defect in GIS (fixed particle on the live conductor) under AC voltage waveform for different gases and gas mixtures. Natural gases, such as N 2 , CO 2, and O 2 /CO 2 , are also assessed since they constitute the buffer gas(es) when mixed with the fluorinated SF 6 replacement candidates (C 4 F 7 N or C 5 F 10 O). The main purpose of adding natural gases is to lower the boiling point of the final fluorinated/natural gas mixture to reach operating temperatures similar to SF 6 for indoor and outdoor high voltage applications. In addition, pure C 4 F 7 N and C 5 F 10 O were studied at the sub-atmospheric pressure level without liquefaction. Then, including minimal working temperature and its corresponding concentrations [7][8][9], the addition of highly electronegative gases to natural gases is investigated, and the synergism of the constituents is quantified. Inception electric field was calculated in positive/negative polarities for pure/gas mixtures, and synergism laws are deducted. The polarity reversal electric fields are calculated for the C 4 F 7 N/CO 2 and C 5 F 10 O/O 2 /CO 2 gas mixtures.

Materials and Methods
To study pure gas/gas mixture pre-breakdown and breakdown characteristics under non-uniform fields, a needle/plane defect model was utilized as described in previous work [6]. The needle was placed between two plane electrodes of Bruce profile to provide homogeneous background electric field conditions. The plane electrodes were made of aluminum and the 100 µm tip radius needle from tungsten of 99.99% purity. The electrode set was placed inside a vessel made of stainless steel (volume = 12 L, max pressure = 12 bar abs). The high voltage needle electrode that was connected to the bushing was fixed and was connected to a high voltage AC source (3.75 kVA) with a maximum output voltage of 50 kV rms. The high voltage bushing installed on the test vessel was rated at 38 kV AC rms. The grounded plane electrode, however, was vertically moveable within the test chamber so that the desired gap length can be achieved with an accuracy of 0.1 mm. Current sensing was performed by coupling a high-frequency current transformer (HFCT), of 500 MHz upper cutoff frequency, to the lower (grounded) plane electrode, while the recording of the generated signals was performed through a 500 MHz, 2 GS/s oscilloscope (see Figure 1). chamber so that the desired gap length can be achieved with an accuracy of 0.1 mm. Current sensing was performed by coupling a high-frequency current transformer (HFCT), of 500 MHz upper cutoff frequency, to the lower (grounded) plane electrode, while the recording of the generated signals was performed through a 500 MHz, 2 GS/s oscilloscope (see Figure 1). Figure 2 shows the field utilization factor η given by Equation (1) of the experimental arrangement as a function of gap distance. Such an electrode configuration gives a constant field utilization factor η = 0.0385 for gaps between 10 mm and 20 mm. For a 60 kV peak AC applied voltage, η = 0.0385 and d = 10 mm, the maximum electric field Emax is ≈156 kV/mm.  The different investigated gas/gas mixtures are listed in Table 2. A Dilo (mini-series C5 and mini-series C4 ) was used to circulate the Novec TM 5110 (C5F10O) and Novec TM 4710 (C4F7N) mixtures in the gaseous state. For a 6% C5F10O/12% O2/82% CO2 gas mixture [9], a premixed 12.77% O2/87.23% CO2 gas mixture was used for the preparation of the gas mixture and representing the buffer gas.   Figure 2 shows the field utilization factor η given by Equation (1) of the experimental arrangement as a function of gap distance. Such an electrode configuration gives a constant field utilization factor η = 0.0385 for gaps between 10 mm and 20 mm. For a 60 kV peak AC applied voltage, η = 0.0385 and d = 10 mm, the maximum electric field E max is ≈156 kV/mm. AC rms. The grounded plane electrode, however, was vertically moveable within the test chamber so that the desired gap length can be achieved with an accuracy of 0.1 mm. Current sensing was performed by coupling a high-frequency current transformer (HFCT), of 500 MHz upper cutoff frequency, to the lower (grounded) plane electrode, while the recording of the generated signals was performed through a 500 MHz, 2 GS/s oscilloscope (see Figure 1). Figure 2 shows the field utilization factor η given by Equation (1) of the experimental arrangement as a function of gap distance. Such an electrode configuration gives a constant field utilization factor η = 0.0385 for gaps between 10 mm and 20 mm. For a 60 kV peak AC applied voltage, η = 0.0385 and d = 10 mm, the maximum electric field Emax is ≈156 kV/mm.  The different investigated gas/gas mixtures are listed in Table 2. A Dilo (mini-series C5 and mini-series C4 ) was used to circulate the Novec TM 5110 (C5F10O) and Novec TM 4710 (C4F7N) mixtures in the gaseous state. For a 6% C5F10O/12% O2/82% CO2 gas mixture [9], a premixed 12.77% O2/87.23% CO2 gas mixture was used for the preparation of the gas mixture and representing the buffer gas.  The different investigated gas/gas mixtures are listed in Table 2. A Dilo (mini-series C5 and mini-series C4 ) was used to circulate the Novec TM 5110 (C 5 F 10 O) and Novec TM 4710 (C 4 F 7 N) mixtures in the gaseous state. For a 6% C 5 F 10 O/12% O 2 /82% CO 2 gas mixture [9], a premixed 12.77% O 2 /87.23% CO 2 gas mixture was used for the preparation of the gas mixture and representing the buffer gas.
The inception voltage of pre-breakdown discharges, which originated from the needle tip, was investigated optically in the visible and ultraviolet light emission spectrum. The optical detection system consisted of a UV image intensifier, incorporating an S20 photocathode, mounted on the front of an 8-bit high-speed camera with a maximum resolution of 1024 × 1024 px. A chromatically corrected Nikon UV-105, 105 mm f/4.5 lens was installed on the optical input of the intensifier. Observations were performed through a UV-grade fused silica side viewport installed on the pressure vessel. The entire camera system was aligned to and focus adjusted towards the installed electrode configuration. A UV bandpass filter mounted on the front of the UV lens was used to absorb the visible spectrum light emitted by glow discharge. While current detection was difficult and sometimes impossible in the high-pressure range (at pressure levels between 5 and 10 bar abs), using a high-frequency current transformer (500 MHz) of high gain ratio (5 V/A), combined with the camera system described above, consisted an effective method to detect PDIV in the UV and VIS spectral range. Figure 3 shows an example of a negative-streamer light emission image of N 2 at 5 bar abs.  The inception voltage of pre-breakdown discharges, which originated from the needle tip, was investigated optically in the visible and ultraviolet light emission spectrum. The optical detection system consisted of a UV image intensifier, incorporating an S20 photocathode, mounted on the front of an 8-bit high-speed camera with a maximum resolution of 1024×1024 px. A chromatically corrected Nikon UV-105, 105 mm f/4.5 lens was installed on the optical input of the intensifier. Observations were performed through a UV-grade fused silica side viewport installed on the pressure vessel. The entire camera system was aligned to and focus adjusted towards the installed electrode configuration. A UV bandpass filter mounted on the front of the UV lens was used to absorb the visible spectrum light emitted by glow discharge. While current detection was difficult and sometimes impossible in the high-pressure range (at pressure levels between 5 and 10 bar abs), using a high-frequency current transformer (500 MHz) of high gain ratio (5 V/A), combined with the camera system described above, consisted an effective method to detect PDIV in the UV and VIS spectral range. Figure 3 shows an example of a negativestreamer light emission image of N2 at 5 bar abs. A new needle was used for each gas/gas mixture to prevent deformation in the radius of the needle and biased results. No erosion effect on needle tips was observed after partial discharge inception voltage (PDIV) measurements. Initially, the PDIV was investigated, followed by breakdown voltage (BDV) measurements. No difference in BDV results was observable between new and used needles up to a certain limit; however, the needle was replaced frequently (see Figure 4). In contrast, one single BDV caused a decrease in PDIV. A new needle was used for each gas/gas mixture to prevent deformation in the radius of the needle and biased results. No erosion effect on needle tips was observed after partial discharge inception voltage (PDIV) measurements. Initially, the PDIV was investigated, followed by breakdown voltage (BDV) measurements. No difference in BDV results was observable between new and used needles up to a certain limit; however, the needle was replaced frequently (see Figure 4). In contrast, one single BDV caused a decrease in PDIV.  For breakdown measurements, an AC voltage ramp was increased at a rate of 2 kV/s up to 90% of the voltage level that initiated electrical discharges, and then 0.1 kV/s was used. For each breakdown event, measurement series of 10 individual discharges were  For breakdown measurements, an AC voltage ramp was increased at a rate of 2 kV/s up to 90% of the voltage level that initiated electrical discharges, and then 0.1 kV/s was used. For each breakdown event, measurement series of 10 individual discharges were performed. Three minutes were kept in between the individual electrical discharges. The test rig bushing flashover limit was 60 kV peak. The ramp method was employed to identify the inception voltage of partial discharge. The voltage was increased with the rate of 1 kV/s until the first pulse, or a light emission image (UV-vis spectrum), was detected.

Natural Gases
N 2 , CO 2 , O 2 /CO 2 mixtures are interesting because they are perfectly ecologically compatible, and they constitute a major part of the atmosphere. The pure nitrogen used contains around 0.01% oxygen. Here, negative and positive-inception voltages and AC breakdown measurements are reported for N 2 at various pressures up to 8.8 bar abs (see Figure 5). The effect of needle condition (non-arced vs. arced) was investigated. After breakdown measurements (arced needle), a change in negative-inception voltage was observed at high pressure, and a saturation trend occurs. To prevent this deviation, inception voltage was measured first, followed by breakdowns for all the following measurements. For each gas/gas mixture, a new needle was used. The negative-inception voltage increases linearly as the gas pressure increases. An example of negative-inception voltage-current characteristics and its visual proof (UV vs. UV + VIS) at 8.8 bar abs is shown in Figure 6. The captures show that it contained combined ultraviolet and visible emitted light. The glow discharge is detectable across the pressure range (see Figure 7). Figure 5 shows a deviation between breakdown and negative-inception voltages at 5 bar abs, corresponding to an electric field of 67 mkV/mm according to Equation (1). By increasing the pressure, deviation increased. Positive-current pulses were detectable for pressures above 6 bar abs (see Figures 5 and 8).     Negative-inception electric field in the visible +ultraviolet light and ultraviolet radiation along the electromagnetic spectrum, as a function of gas pressure (N 2 , d = 10 mm, G I = 60% is the relative luminous gain of the image recording system).
The breakdown voltage of N 2 showed a linear dependence on increasing gas pressure in contrast to electronegative gas/gas mixtures that present nonlinear tendency for the same needle-plane configuration, such as CO 2 and 4% C 4 F 7 N-96% CO 2 [6]. The breakdown occurred exclusively under a negative AC voltage peak for the pressure range. According to the pre-breakdown current measurements (see Figure 8) and the optical observations  (see Figures 3 and 8), the negative glow streamer mode was identified as the breakdown mechanism for compressed N 2 . These findings agree with previous investigations [10][11][12][13][14][15].

Figure 7.
Negative-inception electric field in the visible +ultraviolet light and ultraviolet radiation along the electromagnetic spectrum, as a function of gas pressure (N2, d = 10 mm, GI = 60% is the relative luminous gain of the image recording system).  Figure 9 summarizes the negative and the positive-inception voltage and AC breakdown measurement for 12.77% O2-87.23% CO2 at various pressures up to 10 bar abs. The negative-inception voltage increased linearly with the increase of gas pressure. An example of negative and positive partial discharge voltage-current characteristics and its emission image (UV + VIS) at 8.8 bar abs are shown in Figure 10. The intensity of negative partial discharge current decreased with an increase of pressure (see Figure 11a). The breakdown characteristic of 12.77% O2-87.23% CO2 showed a nonlinear behavior when the gas pressure increased. The breakdown occurred under the negative AC voltage peak for pressures below 1.6 bar abs and on the positive AC voltage peak above. The polarity reversal (PR) cross point at 1.6 bar abs corresponds to a positive-inception electric field of 50 kV/mm, calculated according to Equation (1). At pressures below PR, only negative PDs were detectable up to breakdown. At PR pressure, similar partial discharge activities occurred on positive and negative cycles (see Figure 10a). The luminosity of the positive cycle was lower than that of the negative cycle (see Figure 10b,c). At pressures above PR [3,6], the needle tip was covered with a cloud of charge carriers, which were stabilized by negative ions formed by the attachment of electrons. Consequently, the negative space  The negative-inception voltage increased linearly with the increase of gas pressure. An example of negative and positive partial discharge voltage-current characteristics and its emission image (UV + VIS) at 8.8 bar abs are shown in Figure 10. The intensity of negative partial discharge current decreased with an increase of pressure (see Figure 11a). The breakdown characteristic of 12.77% O 2 -87.23% CO 2 showed a nonlinear behavior when the gas pressure increased. The breakdown occurred under the negative AC voltage peak for pressures below 1.6 bar abs and on the positive AC voltage peak above. The polarity reversal (PR) cross point at 1.6 bar abs corresponds to a positive-inception electric field of 50 kV/mm, calculated according to Equation (1). At pressures below PR, only negative PDs were detectable up to breakdown. At PR pressure, similar partial discharge activities occurred on positive and negative cycles (see Figure 10a). The luminosity of the positive cycle was lower than that of the negative cycle (see Figure 10b,c). At pressures above PR [3,6], the needle tip was covered with a cloud of charge carriers, which were stabilized by negative ions formed by the attachment of electrons. Consequently, the negative space charges generate a reduction of the electric field on the side of the tip. This causes an increase in the breakdown voltage. For high pressure (8.8 bar abs), one positive-current pulse was detectable before the breakdown; the positive-streamer channels transited to leader channels and arrived at the opposite electrode instantly. The space charge near the needle tip resulted in a reduced electric field. The electrode was virtually elongated, the gap width was reduced, and the electric field was enhanced in the remaining gap, resulting in a reduction in breakdown voltage. It established an electrical conduction path between the two electrodes. Heating and expansion of the conductive channel occurred, and multiple branches were created (see Figure 11c). The negative partial discharge inception voltage (PDIV) was identified with current and emission light images. The negative partial discharge inception electric field (PDIEF) E max was calculated according to Equation (1) for different natural gases as a function of pressure. The results are shown in Figure 12. Along the pressure range investigated, PDIEF of N 2 and 12.77% O 2 -87.23% CO 2 exhibits a linear trend. On the other hand, PDIEF of CO 2 evolved linearly and presented a saturation trend at higher pressure levels (>5 abs). As a result, at the highest and lowest pressures (8.    (c) Figure 11. Voltage-current waveform for measuring partial discharge and breakdown voltages in 12.77% O2-87.23% CO2 (P = 8.8 bar abs, d = 10 mm): (a) negative partial discharge (PD), (b) breakdown on the positive cycle and (c) associated gaseous discharge with an expanding plasma channel as a function of time.

Figure 12.
Negative-inception electric field in the visible +ultraviolet light as a function of gas pressure,12.77% O2-87.23% CO2 vs. CO2 vs. N2 (d = 10 mm, GI = 60% is the relative luminous gain of the image recording system).

C4F7N Gas Based
C4F7N or Novec TM 4710 was combined with a simple dilution natural gas/gas mixture (CO2, O2 and/or N2). For quasi-homogeneous and divergent electric field distributions, the dielectric strength of the 20% C4F7N/80% CO2 mixture was equivalent to that of pure SF6 at equivalent pressure [7,8]. In practical applications, a reduction in the concentration of C4F7N was required to enable an increase in the filling pressure of the apparatus without liquefaction. Figure

C 4 F 7 N Gas Based
C 4 F 7 N or Novec TM 4710 was combined with a simple dilution natural gas/gas mixture (CO 2 , O 2 and/or N 2 ). For quasi-homogeneous and divergent electric field distributions, the dielectric strength of the 20% C 4 F 7 N/80% CO 2 mixture was equivalent to that of pure SF 6 at equivalent pressure [7,8]. In practical applications, a reduction in the concentration of C 4 F 7 N was required to enable an increase in the filling pressure of the apparatus without liquefaction. Figure 13 summarizes the measured negative and the positive-inception and AC breakdown voltages for 20% C 4 F 7 N-80% CO 2 at various pressures up to 5 bar abs. The negative-inception voltage increased linearly with the increase of gas pressure. The breakdown characteristic of 20% C 4 F 7 N-80% CO 2 showed a nonlinear behavior over the gas pressure levels. The breakdown occurred under a negative AC voltage peak for pressure below 1.6 bar abs and on a positive AC voltage peak. The polarity reversal (PR) cross point at 1.6 bar abs corresponded to a positive-inception electric field of 84 kV/mm, calculated according to Equation (1). Below PR, negative and positive PDs were detectable until breakdown occurs. An example of negative and positive partial discharge voltagecurrent characteristics and its emission image (UV vs. UV + VIS) at 1.3 bar abs are shown in Figure 14.
Negative PDIV detection was possible via current measurement (current transformer 5-1) and via emission image. For high pressure (5 bar abs), multiple pulses around the positive cycle peak were detectable before the breakdown (see Figure 15). While negative partial discharge amplitude decreases with pressure increases, the intensity of positive partial discharge current increases. Increasing the applied voltage level induced denser current activity and consequently increased light emission (see Figures 14a and 15a); in turn, this generated a sufficient critical space charge avalanche to create a discharge. The positive glow streamer mode was identified as the breakdown mechanism for compressed 20% C 4 F 7 N-80% CO 2 . The findings agreed with previous investigations on a 4% C 4 F 7 N-96% CO 2 gas mixture [6]. The streamer criterion was proven to fit the breakdown values for different electric field distributions [16]. The critical space charge size was given by the natural logarithm of the number of electrons N critical in the critical avalanche; Table 3 summarizes Ln (N critical ) for different C 4 F 7 N based gas mixtures. Increasing C 4 F 7 N content led to an increase in the breakdown critical space charge and a larger gap between positive PDIV and breakdown, leading to a higher rate of gas molecules decomposition.      Table 3. Streamer criterion mechanism: critical breakdown space charge for different C 4 F 7 N based gas mixtures [17].

Gas/Gas Mixture Ln (N critical )
3.7% C 4 F 7 N-96.3% CO 2 6.228768 20% C 4 F 7 N-80% CO 2 8.448 C 4 F 7 N 14.56 To clarify the synergism between components of 20% C 4 F 7 N-80% CO 2 gas mixture, negative PDIV was identified for each pure component separately, i.e., C 4 F 7 N up to 1 bar abs (20% of the mixture), CO 2 up to 4 bar abs (80% of the mixture) and the final mixture of them up to 5 bar abs. The negative partial discharge inception electric field (PDIEF) E max was calculated for each component according to Equation (1), reported to the final mixture pressure and plotted in Figure 16. For all the pressure ranges, PDIEF (C 4 F 7 N) was higher than PDIEF (20% C 4 F 7 N-80% CO 2 ), which in its turn was higher than PDIEF (CO 2 ). Synergism on PDIEF of 20% C 4 F 7 N-80% CO 2 gas mixture fits with the mean value of each individual component according to Equation (2). Based on PDIEF data of pure CO 2 (up to 8.8 bar abs, Figure 12) and pure C 4 F 7 N (up to 1 bar abs, Figure 15), three different variations with the increase of C 4 F 7 N content in a C 4 F 7 N-CO 2 gas mixture were identified. Table 4 summarizes the comparison between PDIEF (E max ) of CO 2 and C 4 F 7 N separately in C 4 F 7 N-CO 2 gas mixtures. At 9± 1 0 % C 4 F 7 N concentration, E max (CO 2 ) = E max (C 4 F 7 N). When C 4 F 7 N concentration < 9± 1 0 , Equation (2) was no longer applicable. Concerning polarity reversal, when the breakdown polarity changed from negative to positive halfcycle, the polarity reversal electric field E PR was calculated according to Equation (1) for CO 2 , SF 6, and different C 4 F 7 N-CO 2 gas mixtures and plotted in Figure 17. Fitting results show that E PR (CO 2 ) = E PR (9% C 4 F 7 N-91% CO 2 ) and E PR (SF 6 ) = E PR (22% C 4 F 7 N-78% CO 2 ). At the same time, the pressure of polarity reversal increased linearly with the increase of field utilization factor η. Increasing C 4 F 7 N content improved E PR and polarity reversal pressure at the same time at a constant field utilization factor η (see Table 5). This variation was governed by a constant E PR intrinsic characteristic for each gas/gas mixture.

C5F10O Gas Based
The vaporization of C5F10O or Novec TM 5110 was obtained above 26.9 °C. C5F10O could only be used as an additive at a pressure below the saturated vapor pressure and could not be used in a pure state for insulation applications where the minimum operating temperature of an HV apparatus may drop to −25 °C or below. A mixture with a low concentration of a C5F10O (4-6%) and a buffer gas (O2/N2/CO2) resulted in an improvement of the dielectric strength [9]. Figure 18 reports AC breakdown measurements for 12.77% O2-87.23% CO2, 6% C5F10O-12% O2-82% CO2, and 20% C4F7N-80% CO2 at various pressures and for a gap distance of 5 mm. All the gas mixtures show nonlinear behavior as a function of pressure and with different peak values. At 8.8 bar abs, the breakdown voltage of 6% C5F10O-12% O2-82% CO2 was equal to that of 12.77% O2-87.23% CO2. A saturation trend of the breakdown voltage was observed for 12.77% O2-87.23% CO2 and 6% C5F10O-12% O2-82% CO2 above 8.8 bar abs, while the increase for 20% C4F7N-80% CO2 was linear. At 5 bar abs, the breakdown voltage of 20% C4F7N-80% CO2 was 2 times higher compared to that of 6% C5F10O-12% O2-82% CO2 at 8.8 bar abs.   Figure 16. Synergistic effect on negative partial discharge inception electric field (PDIEF) as a function of gas pressure (20% C 4 F 7 N-80% CO 2 vs. partial pressure (CO 2 ) vs. partial pressure (C 4 F 7 N), d = 10 mm).

C5F10O Gas Based
The vaporization of C5F10O or Novec TM 5110 was obtained above 26.9 °C. C5F10O could only be used as an additive at a pressure below the saturated vapor pressure and could not be used in a pure state for insulation applications where the minimum operating temperature of an HV apparatus may drop to −25 °C or below. A mixture with a low concentration of a C5F10O (4-6%) and a buffer gas (O2/N2/CO2) resulted in an improvement   Figure 17. Synergistic effect on polarity reversal electric field of C 4 F 7 N-CO 2 gas mixtures compared to pure CO 2 and SF 6 .

C 5 F 10 O Gas Based
The vaporization of C 5 F 10 O or Novec TM 5110 was obtained above 26.9 • C. C 5 F 10 O could only be used as an additive at a pressure below the saturated vapor pressure and could not be used in a pure state for insulation applications where the minimum operating temperature of an HV apparatus may drop to −25 • C or below. A mixture with a low concentration of a C 5 F 10 O (4-6%) and a buffer gas (O 2 /N 2 /CO 2 ) resulted in an improvement of the dielectric strength [9]. Figure 18 reports AC breakdown measurements for 12.77% O 2 -87.23% CO 2 , 6% C 5 F 10 O-12% O 2 -82% CO 2 , and 20% C 4 F 7 N-80% CO 2 at various pressures and for a gap distance of 5 mm. All the gas mixtures show nonlinear behavior as a function of pressure and with different peak values. At 8.8 bar abs, the breakdown voltage of 6% C 5 F 10 O-12% O 2 -82% CO 2 was equal to that of 12.77% O 2 -87.23% CO 2 . A saturation trend of the breakdown voltage was observed for 12.77% O 2 -87.23% CO 2 and 6% C 5 F 10 O-12% O 2 -82% CO 2 above 8.8 bar abs, while the increase for 20% C 4 F 7 N-80% CO 2 was linear. At 5 bar abs, the breakdown voltage of 20% C 4 F 7 N-80% CO 2 was 2 times higher compared to that of 6% C 5 F 10 O-12% O 2 -82% CO 2 at 8.8 bar abs.
The breakdown occurred under a negative AC voltage peak for pressures below 0.5 bar abs and on a positive AC voltage peak. The polarity reversal (PR) cross point at 0.5 bar abs corresponds to a positive-inception electric field of 36 kV/mm, calculated according to Equation (1). In terms of partial discharge activity, the 6% C5F10O-12% O2-82% CO2 exhibited a similar behavior compared with that of 12.77% O2-87.23% CO2 or 20% C4F7N-80% CO2. Unlike 12.77% O2-87.23% CO2 and below PR, negative and positive PDs were detectable up to breakdown for the 6% C5F10O-12% O2-82% CO2. To explain the latter behavior, pure C5F10O at 0.54 bar abs, which corresponds to a 6% molar fraction of 9 bar abs total pressure, was evaluated separately, and the results are shown in Figure 19. High amplitude and positive polarity current pulses were detected during the positive cycle (see Figure 19a) before breakdown on the negative cycle (see Figure 19b), and a positiveinception electric field of 60 kV/mm was calculated according to Equation (1). On the other hand, C4F7N at 0.352 bar abs, which corresponds to a 4% molar fraction of 8.8 bar abs total pressure, exhibits similar behavior to C5F10O with less dense positive polarity discharge activity (see Figure 19c). The corresponding positive-inception electric field was 70 kV/mm calculated according to Equation (1). The breakdown voltage of the C5F10O (0.54 bar abs) was 1.09 times of C4F7N at 0.352 bar abs.
Above PR, the intensity of negative partial discharge current decreased with the increase of pressure. For high pressure (7 bar abs, see Figure 20), negative PDIV detection was not possible via current measurement (current transformer 5-1); however, the detection was performed via emission image. Before breakdown at high pressure (p = 6 bar abs), positive partial discharge appears in multiple pulsed within the positive AC halfcycle (see Figure 21a and Figure 22). Increasing the applied voltage levels induced more current activity and a brighter emission image. The breakdown on the positive cycle was led by a positive-streamer sufficient space charge. 6% The 6% C5F10O-12% O2-82% CO2 presents denser positive partial discharge activities before the breakdown compared to the 4% C4F7N-96% CO2 [6].  The breakdown occurred under a negative AC voltage peak for pressures below 0.5 bar abs and on a positive AC voltage peak. The polarity reversal (PR) cross point at 0.5 bar abs corresponds to a positive-inception electric field of 36 kV/mm, calculated according to Equation (1). In terms of partial discharge activity, the 6% C 5 F 10 O-12% O 2 -82% CO 2 exhibited a similar behavior compared with that of 12.77% O 2 -87.23% CO 2 or 20% C 4 F 7 N-80% CO 2 . Unlike 12.77% O 2 -87.23% CO 2 and below PR, negative and positive PDs were detectable up to breakdown for the 6% C 5 F 10 O-12% O 2 -82% CO 2 . To explain the latter behavior, pure C 5 F 10 O at 0.54 bar abs, which corresponds to a 6% molar fraction of 9 bar abs total pressure, was evaluated separately, and the results are shown in Figure 19. High amplitude and positive polarity current pulses were detected during the positive cycle (see Figure 19a) before breakdown on the negative cycle (see Figure 19b), and a positive-inception electric field of 60 kV/mm was calculated according to Equation (1). On the other hand, C 4 F 7 N at 0.352 bar abs, which corresponds to a 4% molar fraction of 8.8 bar abs total pressure, exhibits similar behavior to C 5 F 10 O with less dense positive polarity discharge activity (see Figure 19c). The corresponding positive-inception electric field was 70 kV/mm calculated according to Equation (1). The breakdown voltage of the C 5 F 10 O (0.54 bar abs) was 1.09 times of C 4 F 7 N at 0.352 bar abs. Above PR, the intensity of negative partial discharge current decreased with the increase of pressure. For high pressure (7 bar abs, see Figure 20), negative PDIV detection was not possible via current measurement (current transformer 5-1); however, the detection was performed via emission image. Before breakdown at high pressure (p = 6 bar abs), positive partial discharge appears in multiple pulsed within the positive AC half-cycle (see Figures 21a and 22). Increasing the applied voltage levels induced more current activity and a brighter emission image. The breakdown on the positive cycle was led by a positivestreamer sufficient space charge. 6% The 6% C 5 F 10 O-12% O 2 -82% CO 2 presents denser positive partial discharge activities before the breakdown compared to the 4% C 4 F 7 N-96% CO 2 [6].
To study the synergism between components of 6% C5F10O-12% O2-82% CO2 gas mixture, negative PDIV was identified for each pure component separately, i.e., C5F10O up to 0.6 bar abs (6% of the mixture), 12.77% O2-87.23% CO2 up to 9.4 bar abs (94% of the mixture), and the final mixed gas up to 10 bar abs. The negative partial discharge inception electric field (PDIEF) Emax was calculated for each component according to Equation (1), To study the synergism between components of 6% C 5 F 10 O-12% O 2 -82% CO 2 gas mixture, negative PDIV was identified for each pure component separately, i.e., C 5 F 10 O up to 0.6 bar abs (6% of the mixture), 12.77% O 2 -87.23% CO 2 up to 9.4 bar abs (94% of the mixture), and the final mixed gas up to 10 bar abs. The negative partial discharge inception electric field (PDIEF) E max was calculated for each component according to Equation (1), reported to the final mixture pressure and plotted in Figure 25. For all the pressure ranges, PDIEF (6% C 5 F 10 O-12% O 2 -82% CO 2 ) was higher than PDIEF (12.77% O 2 -87.23% CO 2 ), which, in its turn, was higher than PDIEF (C 5 F 10 O). Synergism on PDIEF of 6% C 5 F 10 O-12% O 2 -82% CO 2 gas mixture fits with the sum of each negative partial discharge inception electric field E max individually component according to Equation (3).    Based on PDIEF data of pure 12.77% O 2 -87.23% CO 2 (up to 9.4 bar abs) and pure C 5 F 10 O (up to 0.6 bar abs), three different variations with the increase of C 5 F 10 O content in a C 5 F 10 O-(12.77% O 2 -87.23% CO 2 ) gas mixture were identified. Table 6  Regarding polarity reversal, when the breakdown polarity changes from negative to positive cycle, the polarity reversal electric field E PR was calculated according to Equation (1) for CO 2 , 12.77% O 2 -87.23% CO 2, and 6% C 5 F 10 O-12% O 2 -82% CO 2 gas mixtures (see Table 7). At a constant field utilization factor η, decreasing the concentration of CO 2 reduces final E PR and polarity reversal pressure. A linear trend was observed between polarity reversal pressure and final polarity reversal electric field E PR . Comparing data in Tables 5 Energies 2021, 14, 1065 18 of 23 and 7, it could be observed that E PR (4% C 4 F 7 N-96% CO 2 ) = E PR (12.77% O 2 -87.23% CO 2 ) and E PR (6% C 5 F 10 O-12% O 2 -82% CO 2 ) was less than E PR (4% C 4 F 7 N-96% CO 2 ).

Discussion
Compared to CO 2 , the addition of O 2 (12.77% O 2 -87.23% CO 2 ) increased the critical reduced electric field strength from 82 Td to 90 Td (1 Td = 10 −21 V.m 2 ) with ≈10% improvement [18,19]. N 2 -O 2 gas mixtures exhibited better critical reduced electric field strength than CO 2 -O 2 for oxygen content higher than 8% (see Figure 26). At 15 Td, the mixture 16% O 2 -84% CO 2 exhibited the same value of reduced attachment Townsend coefficient as pure O 2 [18]. Compared to N 2 or N 2 -O 2 gas mixtures, CO 2 or CO 2 -O 2 mixtures had better arcbreaking capabilities. Oxygen was selected to be part of the gas mixture (C 4 F 7 N/CO 2 /O 2 ) to reduce the generation rate of both carbon monoxide (main decomposition product) and fluorinated byproducts during arc quenching [20]. The analysis of the effective ionization coefficients in C5F10O/O2/CO2 [21,22] and C4F7N/CO2 [23] gas mixtures showed a strong dependence of the effective ionization coefficient on the gas density. C5F10O is characterized by an intrinsic dielectric strength highly dependent on the pressure, i.e., the increase of pressure increases the limiting dielectric strength. For a stationary value E/N of density reduced electric field (see Equation (4)), where: • V: DC applied voltage; • d: Insulation distance; and • N: gas density, increasing the pressure decreases the applied voltage V, resulting in a decrease of the velocity and the energy of the emitted electrons.
The decrease of the energy of the swarm inhibits the generations in fractional negative ions result of dissociative attachment. The pure C5F10O and C4F7N exhibit PDs activities on the AC positive half-cycle when the breakdown occurs on the negative half-cycle (see Figure 19b,c). The pure C5F10O shows denser PDs activity than that of pure C4F7N. Moreover, for similar breakdown voltages, negative partial discharge inception electric field of C4F7N at 0.352 bar abs is 1.75 times higher than that of C5F10O at 0.54 bar abs, as shown in Figure 23. The positive PDs incept by dissociative attachment of electrons in C5F10O or C4F7N. Thynne and Harland [24] reported negative ions formed by low energy electron impact and attachment to C4F7N, using a Bendix time of flight spectrometer. Multiple dissociative attachment ions are formed (F − , CN − , CF3 − , C2F5 − and C3F7 − ) in the range of appearance potential 0.3-12 eV. The appearance potential is defined as the energy required to produce the ion and its accompanying neutral fragment from a given molecule. The attachment cross-section of C4F7N is continuous and higher than SF6 in the energy range between 0.1 eV and 1 eV [25]. Hösl et al. [23] proved the existence of three types of anions (F − or CN − and/or C4F7N − ) at low energies and developed a model, including the anions electron detachment to fit their experimental breakdown data. Ranković et al. [26] detected and identified the C4F7N fragments (F − , CN − , C3F4N − , C4F6N − ) by the electron energy loss spectroscopy and the dissociative electron attachment spectroscopy created at The analysis of the effective ionization coefficients in C 5 F 10 O/O 2 /CO 2 [21,22] and C 4 F 7 N/CO 2 [23] gas mixtures showed a strong dependence of the effective ionization coefficient on the gas density. C 5 F 10 O is characterized by an intrinsic dielectric strength highly dependent on the pressure, i.e., the increase of pressure increases the limiting dielectric strength. For a stationary value E/N of density reduced electric field (see Equation (4)), where: • V: DC applied voltage; • d: Insulation distance; and • N: gas density, increasing the pressure decreases the applied voltage V, resulting in a decrease of the velocity and the energy of the emitted electrons.
The decrease of the energy of the swarm inhibits the generations in fractional negative ions result of dissociative attachment. The pure C 5 F 10 O and C 4 F 7 N exhibit PDs activities on the AC positive half-cycle when the breakdown occurs on the negative half-cycle (see Figure 19b,c). The pure C 5 F 10 O shows denser PDs activity than that of pure C 4 F 7 N. Moreover, for similar breakdown voltages, negative partial discharge inception electric field of C 4 F 7 N at 0.352 bar abs is 1.75 times higher than that of C 5 F 10 O at 0.54 bar abs, as shown in Figure 23. The positive PDs incept by dissociative attachment of electrons in C 5 F 10 O or C 4 F 7 N. Thynne and Harland [24] reported negative ions formed by low energy electron impact and attachment to C 4 F 7 N, using a Bendix time of flight spectrometer. Multiple dissociative attachment ions are formed (F − , CN − , CF 3 − , C 2 F 5 − and C 3 F 7 − ) in the range of appearance potential 0.3-12 eV. The appearance potential is defined as the energy required to produce the ion and its accompanying neutral fragment from a given molecule. The attachment cross-section of C 4 F 7 N is continuous and higher than SF 6 in the energy range between 0.1 eV and 1 eV [25]. Hösl et al. [23] proved the existence of three types of anions (F − or CN − and/or C 4 F 7 N − ) at low energies and developed a model, including the anions electron detachment to fit their experimental breakdown data. Ranković et al. [26] detected and identified the C 4 F 7 N fragments (F − , CN − , C 3 F 4 N − , C 4 F 6 N − ) by the electron energy loss spectroscopy and the dissociative electron attachment spectroscopy created at energies around~1 eV.
The attachment cross-section C 5 F 10 O [25] is similar to that of SF 6 for electron energies below 0.7 eV, and that it is substantially larger than that of SF 6 in the region between 0.7 eV and 10 eV. The formation of anions is governed by dissociative attachment to C 5 F 10 O. Dissociative electronic attachment leads to fragmentation of the C 5 F 10 O molecules; different chemical species appear in the gas phase. The species generated in the gas phase cause a destabilization of the molecular texture and a weakening of the dielectric behavior of gas or gas mixture.
The SF 6 presents different behaviors of the electron attachment cross-section [27] in the incident energy of the electron E i between 0 and 10 eVas shown in Figure 27. Electrons with low-energy < 0.6 eV and low-speed are generally absorbed by SF 6 molecules to form SF 6 − anion in a stable state. For unstable SF 6 − anions, the electrons auto-detaches after 68 ± 0.2 µs [28]. Dissociative attachment of electrons in SF 6 is dependent on the incident energy of the electrons E i . For E i between 0.6 eV and 2 eV, SF 5 − anions are produced by fragmentation of the parent SF 6 molecule. For E i between 2 eV and 3.5 eV, no dissociative attachment occurs. For E i between 3.5 eV and 10 eV, SF 4 − , SF 3 − and SF 2 − anions are produced. The previous dissociative attachment patterns in SF 6 result in multiple polarity reversal (at least 2) (+)/(−) and (−)/(+) as a function of the maximum electric field given by Equation (1) [7,8,14,29,30]. Dissociative electronic attachment leads to fragmentation of the C5F10O molecules; different chemical species appear in the gas phase. The species generated in the gas phase cause a destabilization of the molecular texture and a weakening of the dielectric behavior of gas or gas mixture.
The SF6 presents different behaviors of the electron attachment cross-section [27] in the incident energy of the electron Ei between 0 and 10 eVas shown in Figure 27. Electrons with low-energy < 0.6 eV and low-speed are generally absorbed by SF6 molecules to form SF6 − anion in a stable state. For unstable SF6 − anions, the electrons auto-detaches after 68 ± 0.2µs [28]. Dissociative attachment of electrons in SF6 is dependent on the incident energy of the electrons Ei. For Ei between 0.6 eV and 2 eV, SF5 − anions are produced by fragmentation of the parent SF6 molecule. For Ei between 2 eV and 3.5 eV, no dissociative attachment occurs. For Ei between 3.5 eV and 10 eV, SF4 − , SF3 − and SF2 − anions are produced. The previous dissociative attachment patterns in SF6 result in multiple polarity reversal (at least 2) (+)/(−) and (−)/(+) as a function of the maximum electric field given by Equation (1) [7,8,14,29,30]. Only one polarity reversal happens in C4F7N/CO2 mixtures and pure CO2. The breakdown voltage in positive polarity becomes constraining and goes below the voltage in negative polarity when the pressure increases and/or the field tends to be divergent. Only one polarity reversal happens in C 4 F 7 N/CO 2 mixtures and pure CO 2 . The breakdown voltage in positive polarity becomes constraining and goes below the voltage in negative polarity when the pressure increases and/or the field tends to be divergent.
For compressed 4% C 4 F 7 N-96% CO 2 and 6% C 5 F 10 O-12% O 2 -82% CO 2 gas mixtures under non-uniform electric field distribution, the positive-streamer mode is identified as the breakdown mechanism. Breakdown and negative partial discharge inception voltages of 6% C 5 F 10 O-12% O 2 -82% CO 2 are higher than those of 4% C 4 F 7 N-96% CO 2 . For high pressure, negative PDIV detection is not possible via current measurement; the detection is feasible via emission image. At 8.8 bar abs, the breakdown voltage of 6% C 5 F 10 O-12% O 2 -82% CO 2 is equal to that of 12.77% O 2 -87.23% CO 2 (buffer gas). At 5 bar abs, the breakdown voltage of 20% C 4 F 7 N-80% CO 2 is 2 times higher compared with that of 6% C 5 F 10 O-12% O 2 -82% CO 2 at 8.8 bar abs.
For 20% C 4 F 7 N-80% CO 2 gas mixture, synergism in negative partial discharge inception voltage/electric field fits with the mean value of each individual partial pressure component (C 4 F 7 N and CO 2 ). Based on partial discharge inception voltage/electric field data of pure CO 2 (up to 8.8 bar abs) and pure C 4 F 7 N (up to 1 bar abs), three different variations with the increase of C 4 F 7 N content in a C 4 F 7 N-CO 2 gas mixture was identified. At 9 ± 1 0 C 4 F 7 N concentration, partial discharge inception electric field E max (CO 2 ) = E max (C 4 F 7 N). When C 4 F 7 N concentration < 9 ± 1 0 , the mean value of each partial pressure individual component (C 4 F 7 N and CO 2 ) is no longer applicable.
For 6% C 5 F 10 O-12% O 2 -82% CO 2 gas mixture, synergism in negative partial discharge inception voltage/electric field (E max ) fits with the sum of each individual partial pressure component (C 5 F 10 O and 12.77% O 2 -87.23% CO 2 ). Based on partial discharge inception voltage/electric field data of 12.77% O 2 -87.23% CO 2 (up to 9.4 bar abs) and pure C 5 F 10 O (up to 0.6 bar abs), three different variations with the increase of C 5 F 10 O content in a C 5 F 10 O-(12.77% O 2 -87.23% CO 2 ) gas mixture was identified. At 19 ± 1 0 C 5 F 10 O concentration, E max (12.77% O 2 -87.23% CO 2 ) = E max (C 5 F 10 O).
Polarity reversal occurs under AC voltage when the breakdown polarity changes from negative to positive cycle. Polarity reversal electric field E PR was quantified. Fitting results show that E PR (CO 2 ) = E PR (9% C 4 F 7 N-91% CO 2 ) and E PR (SF 6 ) = E PR (22% C 4 F 7 N-78% CO 2 ). The pressure of polarity reversal increases linearly with the increasing field utilization factor η. At a constant field utilization factor η, increasing C 4 F 7 N content improves E PR and polarity reversal pressure. This variation is ruled by a constant E PR intrinsic characteristic for each gas/gas mixture. The addition of O 2 in 12.77% O 2 -87.23% CO 2 and C 5 F 10 O in C 5 F 10 O-(12.77% O 2 -87.23% CO 2 ) reduces final E PR and polarity reversal pressure, E PR (4% C 4 F 7 N-96% CO 2 ) = E PR (12.77% O 2 -87.23% CO 2 ) and E PR (6% C 5 F 10 O-12% O 2 -82% CO 2 ) < E PR (4% C 4 F 7 N-96% CO 2 ) < E PR (CO 2 ).