There is increasing interest in developing aircraft that are wholly electric or utilize hybrid-electric architectures to deliver a reduced carbon footprint. The higher operating voltage is necessary to deliver electrical machines with an acceptable power density, which will increase the risk of partial discharge (PD), especially in the low-pressure environment at high altitude. This paper reviews the opportunity to use high dielectric strength gases to improve the PD performance of electrical machines for aerospace applications. PD characteristics of machine winding wires are tested in air, CO ₂ , SF ₆ , CF ₃ I, C ₃ H ₂ F ₄ , and C ₃ F ₇ CN. Mixture ratios of 60% C ₃ H ₂ F ₄ / 40% CO ₂ and 30% C ₃ F ₇ CN / 70% CO ₂ can remain gaseous under atmospheric pressure at a temperature of -30 °C, with the partial discharge inception voltage (PDIV) of machine winding wires approximately three times higher in these mixtures than air at 0.2 bar absolute. The test results have been applied to demonstrate that a machine using a high dielectric strength gas as part of its insulation system could tolerate a higher DC bus voltage while still being considered as Type I (PD-free). Additionally, endurance tests using insulating gases demonstrate the prospect of an improved time to failure for Type II insulation systems.

It has been shown that noise emissions from HV overhead line conductors can be reduced under rain conditions by making their surfaces superhydrophobic. The working environment makes ensuring the longevity of any treatment a major challenge. The degradation of various superhydrophobic surfaces generated by applying a superhydrophobic coating and patterning microscale channels is assessed under an AC electric field (18 kV/cm) with continuous water spray. By examining the droplet distribution on the surfaces during the degradation and the surface roughness before and after degradation, the authors demonstrate that no water droplets were found on the microscale patterned surface, but droplets were formed on the coated surface after degradation. The surface roughness reduction of the coated surface and microscale patterned surface was 29.8% and 11.3%, respectively, indicating that the microscale patterned surface has better durability than the superhydrophobic coating under the AC electric stress.

Sulphur hexafluoride (SF ₆ ) is a potent greenhouse gas used in high voltage accelerators. As a promising alternative to SF ₆ , the C ₃ F ₇ CN/CO ₂ gas mixture and its by‐products are of great interest to ensure the safe operation of accelerators that will adopt any SF ₆ ‐free solution. This work experimentally examines the electrical ageing characteristics of a 20% C ₃ F ₇ CN/80% CO ₂ gas mixture tested using spark gaps under a pressure of 7.2 bar (abs.). Gas samples were collected after 1000 DC breakdowns and analysed using gas chromatography mass spectrometry (GC‐MS) with an estimated toxicity value of 54,459 ppm _v , which indicates the aged mixture to be non‐toxic. Subsequent investigation was conducted on the gas‐solid interface after 500 breakdowns for both SF ₆ and the 20% C ₃ F ₇ CN/80% CO ₂ gas mixture. Aged electrodes were analysed using X‐ray photoelectron spectroscopy (XPS) and X‐ray diffraction (XRD) techniques. Electrode surface analysis revealed the formation of metal fluorides on the electrode surface tested using the 20% C ₃ F ₇ CN/80% CO ₂ mixture, whereas metal fluorides and sulphides were detected for electrodes tested with SF ₆ . The findings provide a reference on the toxicity and gas‐solid interaction of the electrically aged 20% C ₃ F ₇ CN/80% CO ₂ gas mixture for potential retro‐fill application in high voltage accelerators.

There is an increasing scrutiny on the use of sulfur hexafluoride (SF6) due to its high global warming potential (GWP) with numerous alternative gases and mixtures being researched. This work proposes a single protrusion streamer criterion model to predict the DC breakdown voltage for SF6 alternatives in coaxial geometries close to the optimal ratio as found in practical equipment. DC breakdown characteristics of SF6 and CO2 gases and mixtures of 7% C3F7CN, 20% C3F7CN and 4% C5F10O with CO2 are experimentally analyzed for pressures of 1-9 bar absolute, conductor/enclosure diameters of 8/30, 10/30, 15/30, 15/45 and 20/60 mm, and maximum surface roughness (Rz) between 8-121 μm. Results show that, 20% C3F7CN / 80% CO2 mixture and SF6 have comparable DC breakdown characteristics in the investigated coaxial field geometries and surface roughness. The majority of the experimental and calculated results are within ±5% for different coaxial sizes, pressures (>1 bar), gas mediums, DC polarities and conductor surface roughness. The key advantage of this modeling approach is the ability to provide a reasonable estimation of DC breakdown voltage over a range of representative test conditions, which optimize the selection process of new SF6 alternatives without having to perform significant level of experimental activity.

Manufacturing or assembly defects in gas-insulated equipment can introduce field enhancements that could lead to partial discharge (PD). This paper examines the PD characteristics of SF₆ alternatives considered for potential application to retro-filling existing SF₆-designed equipment. The PD performance of the C₃F₇CN/CO₂ gas mixture and SF₆ were characterised adopting the ultra-high frequency (UHF) method and investigated for different defect configurations, pressures, and gas mediums. Hemispherical rod-plane and plane-to-plane configurations with needle on the high-voltage (HV) and ground electrodes were used to mimic conductor and enclosure protrusion defects, respectively. The results demonstrate that with a needle length of 15 mm, the 20% C₃F₇CN/80% CO₂ gas mixture had almost half the partial discharge inception and extinction voltages (PDIV/EV) of SF₆. For less divergent fields, the 20% C₃F₇CN/80% CO₂ gas mixture demonstrated a comparable PDIV/EV performance as SF₆. The phase-resolved PD patterns of the 20% C₃F₇CN/80% CO₂ gas mixture demonstrated a 3-stage transition phase that was not observed with SF₆, which could be due to the discharge mechanism of the weakly attaching CO₂ gas used within the mixture.

SF6 alternatives with significantly lower environmental impact have been extensively researched. This paper investigates two electrode configurations and test pressures to mimic the conditions as found in MV/HV equipment to systematically compare the breakdown performance of $\mathrm{SF}_{6}, \mathrm{C}_{3} \mathrm{H}_{2} \mathrm{F}_{4}$ and a mixture of 30 % CF3 I/ 70 % CO2 under $\mathbf{AC}$ and LI stresses. SF6 possesses comparatively better breakdown performance than the other alternatives investigated in this study. C3 H2 F4 shows “first breakdown” behavior and only the stable data series is considered for evaluation of 50 % breakdown voltage, U50. The lower boiling points of C3 H2 F4 and CF3 Ilimit its potential application to medium-voltage equipment with a low operating pressure. Due to pyrolysis, there is formation of soot and iodine post-breakdown in C3 H2 F4 and 30 % CF3 I/ 70 % CO2 respectively. This will reduce their dielectric performance and could pose health hazard to maintenance personnel.