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The proliferation of urbanization and the integration of new energy sources have stimulated the development of gas-insulated transmission lines and switchgear (GIL/GIS). In particular, the compact DC GIS in offshore converter platforms will significantly reduce footprints for DC switchyards, exhibit exceptional climatic resistance, and facilitate the cost-effective connection of remote offshore wind farms and submarine links. Nevertheless, insulators used in GIS/GIL always suffer from surface charge accumulation under DC stress, which could distort and enhance the local electric field and thus trigger a flashover at the gas–solid interface if it exceeds certain magnitude levels. This susceptibility becomes a major concern affecting the reliability of DC gas-insulated systems. Beyond these engineering-related challenges lie fundamental physics problems involving mechanisms of charge accumulation and charge-induced flashover which still require exploration. To this end, this paper presents an overview of recent advancements on this topic whilst highlighting relevant issues to be addressed. Specifically, the surface charge accumulation phenomena under DC fields are reviewed, and the charging mechanisms are summarized from macroscopic to microscopic perspectives. Further, the correlation between surface charge and surface flashover is discussed. Moreover, recent developments in tailoring methods for surface charging are also presented. Finally, perspectives are given on current research progress and future needs. [ABSTRACT FROM AUTHOR]

HFO-1336mzz(E) is proposed as a novel alternative to SF6 due to its low greenhouse effect and high insulation strength. Typically, it is mixed with CO2 or Air to lower its boiling point to meet the minimum operating temperature. To better understand the thermophysical properties of the gases' arcing plasma, the composition, thermodynamic properties, transport properties, and net emission coefficient of 30% molar fraction of HFO-1336mzz(E) mixed with CO2 or Air at temperatures from 300 K to 30 000 K at 0.12 MPa are calculated. It is found that HFO-1336mzz(E) mixtures have similar turbulent energy dissipation and thermal interruption capability to the pure SF6. Analysis of radiation characteristics demonstrates that the HFO-1336mzz(E) mixtures at 0.12 MPa exhibit stronger radiation emission compared to SF6 at 0.1 MPa, which indicates good arc radiation dissipation capabilities within such mixtures. This study reports the properties of thermal plasma of HFO-1336mzz(E) mixtures for the first time. These findings not only provide fundamental data for further magneto-hydro-dynamic calculations for arcing process but also put forward the potential application of these mixtures as arc interruption medium in medium voltage switchgears. [ABSTRACT FROM AUTHOR]

During the arc breaking process of high-voltage circuit breakers, the eco-friendly SF6-alternative gases will inevitably decompose and generate various decomposition products. In some cases, this will contain solid by-products such as solid carbon, which will have a deterioration effect on the electrical insulation performance of the equipment. It has been found that adding a proper amount of O2 can effectively inhibit the formation of solid carbon. In this paper, based on the improved Gibbs free energy minimization method, a calculation model considering the solid decomposition products was established, and the arc plasma composition of CO2/O2 mixtures with the new eco-friendly gases, such as C4F7N, C5F10O, HFO-1234ze(E) and HFO-1336mzz(E), in local thermodynamic equilibrium state was calculated. The change of decomposition products with the initial O2 ratio is studied, and the criterion expression of inhibiting solid carbon formation is obtained. We also applied the method to the calculation of other SF6-alternative gases containing sulfur atoms such as NSF3 and CF3SO2F. Finally, we showed that solid carbon can be inhibited when a proper molecule formula is satisfied. This work may provide new ideas for further exploring the potential of SF6-alternative gases. [ABSTRACT FROM AUTHOR]

The perfluoronitrile C4F7N is considered a promising SF6-alternative in high-voltage gas-insulated apparatus, thanks to its high dielectric strength and low global warming potential. However, a complete and consistent set of electron-neutral collision cross-sections of C4F7N is still lacking, which hinders relevant plasma modeling. In this contribution, the available electron-neutral collision cross-sections of C4F7N are first compiled and assessed. The initial cross-sections are adjusted iteratively by the electron swarm method to determine a complete and self-consistent cross-section set of C4F7N for the first time. The set is validated by a systematic comparison of electron swarm parameters between Boltzmann equation analysis and experimental measurements in pure C4F7N as well as C4F7N/N2 and C4F7N/Ar mixtures. The proposed cross-section set of C4F7N will be made available to the community in the LXCat database. It will be of particular importance for applications with an emphasis on the discharge mechanisms of this novel gas. [ABSTRACT FROM AUTHOR]

Underwater electrical wire explosion (UEWE) is an efficient source of underwater shock waves (SWs). In order to efficiently simulate the interaction between the UEWE SW and structures, a coupled model that includes the electric circuit, the exploding wire and the surrounding water is established based on user-subroutines provided by the commercial explicit dynamics software ANSYS AUTODYN. The modeling starts from room temperature by using the tabular wide-range metal equation of state (EOS) and conductivity data. Experimental validation is performed with copper wires exploded by a μ s-timescale pulsed discharge. The numerical results show satisfactory consistency with experiments in terms of the current and voltage waveforms, the wire expansion trajectory, the evolution of SW front, the interaction between SW and electrodes and the SW pressure profiles. The main discrepancy lies in the SW amplitude that is ∼20% higher in the calculation and the possible reasons are discussed in detail. Based on this approach and with proper modifications to the metal EOS and conductivity data, the interaction between UEWE SWs and structures can be efficiently modeled in 2D and 3D for practical applications. [ABSTRACT FROM AUTHOR]

Recently, perfluoroketone C5F10O has been seen as a promising alternative gas for sulphur hexafluoride (SF6), of which the global warming potential (GWP) is extremely high. C5F10O has a good insulation property and the GWP value is only 1, thus can be used as insulation medium in gas-insulated switchgears. In this study, the decomposition pathway of C5F10O and the recombination processes of free radicals are firstly explored. Secondly, the rate constants of the reactions are calculated based on the variational transition state theory, which then are fitted in the NIST form. The results not only provide the basic data for the chemical kinetics calculations, but also supplementary data for the NIST database. Finally, the chemical kinetic calculations are performed to study the decomposition characteristics at 400 °C and 700 °C. Only 4% C5F10O decomposes at 400 °C, while 96% decomposes at 700 °C. The stable products like CO, C2F4, C2F6, C3F6 are formed. The mole fraction of CF4 is above 10–10 at 400 °C. A certain amount of C2F2 is produced at 700 °C. Besides, precise chemical kinetic composition results at steady state during the thermal decomposition of C5F10O are obtained. By analyzing the energy change of reactions, the formation of products is explained theoretically. The results not only reveal the decomposition mechanism of C5F10O and recombination mechanism of the stable products, but also provide the guidance for electrical engineering. [ABSTRACT FROM AUTHOR]

Reduced critical electric field strength (E/N)cr is an important indicator for the evaluation of dielectric breakdown of SF6 and its mixtures. This paper aims to analytically investigate the dielectric breakdown properties of SF6-N2 mixtures. First, (ɑ-η)/N and (E/N)cr of SF6-N2 mixtures at room temperature, and then (E/N)cr of hot SF6 at several pressures are calculated and compared with other studies. The results confirmed the validity of the present calculation method and parameters. In addition, the electron energy distribution function, reduced ionization coefficient ɑ/N and attachment coefficient η/N of SF6-N2 mixtures are obtained at different proportions and under 1 atm in the gas temperature range 300-3000 K. Then (E/N)cr can be determined when the effective ionization coefficient (ɑ-η)/N = 0. The results show that in a certain temperature range, due to the effect of N2 on reduction in the proportion of high-energy electrons, (E/N)cr of SF6-N2 mixtures will be enhanced by increasing N2. [ABSTRACT FROM AUTHOR]

This paper provides theoretical calculations that predict the dielectric breakdown properties of carbon dioxide (CO2) and its mixtures with 50% O2 and 50% H2 for a gas temperature range of 300–3500 K at 0.1 MPa. CO2 is one of the most likely candidates for an environment-friendly arc-quenching medium to replace SF6 in high-voltage circuit breakers. Initially, the electron energy distribution function (EEDF) is derived by solving the Boltzmann equation under the zero-dimensional two-term spherical harmonic approximation. Then the reduced ionization and attachment coefficients are obtained, based on the calculated EEDF. Finally, the critical reduced electric field strength (E/N)cr, which is defined as the value for which total ionization reactions are equal to total attachment reactions, is obtained and analysed. The results demonstrate the superior breakdown properties of a 50% CO2–50% O2 mixture to those of both pure CO2 and 50% CO2–50% H2. Nearly no deviation in (E/N)cr is found in a 50% CO2–50% O2 mixture for gas temperatures up to 2500 K, and although there is clear reduction as the gas temperature is increased further to 3500 K, the value remains higher than that of pure CO2. [ABSTRACT FROM AUTHOR]