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This paper aims at providing a reliable algorithm to identify photovoltaic (PV) series arc faults regardless of complex fault-like interferences. Through conducting various arc fault experiments with different PV current levels, arc gap lengths, and load types, PV series arc fault features have been understood comprehensively. To avoid unwanted nuisance tripping, fault-like conditions are analyzed to confirm the unique arc fault features. Based on the loop current signature, a greater unstable fluctuation in the time domain and extra arc noises in the time-frequency domain are chosen as identification features. By quantificational evaluations, optimal detection variables with the Hamming window and the proper time resolution have been established to achieve the best identification results. By building fusion coefficients, two variables are arithmetically fused to achieve the arc fault discovery. The algorithm could also classify fault-like into normal and adjust the threshold value dynamically to fit different normal current levels. Its validity has been verified by experimental results on the simulated platform.

SF6 has been employed as insulation and arc-quenching medium in high voltage apparatus since the 1960s. However, due to the high global warming potential (GWP) of SF6 gas, the alternative to SF6 is required. R12 and R134 gases offer good dielectric properties to be good replacements of SF6 as insulation medium considering the environmental concerns. In order to further investigate the ionization processes in those gases, the electron-impact ionization cross sections are calculated by Deutsch-Mark (DM) and its modified methods. The molecular structures and the molecular orbital compositions of R12 and R134 were determined by ab initio calculation. The results give important reference for the two SF6 substitute gases.

Perfluoroketone C5F10O has drawn much attention in recent years as a promising environmental-friendly gas to substitute SF6 in power equipment. In this paper, the decomposition characteristics of C5F10O and C5F10O/air mixture under pyrolysis and partial discharge conditions were explored by experiments using a gas chromatography/mass spectrometer. Besides, the compatibility of decomposition products with the copper and aluminum was evaluated based on the density functional theory calculation. We found that C5F10O has good thermal stability at 300 ℃. At 500 ℃, C5F10O starts to decompose and it produces CO, CO2, C2F6, C2F4, C3F8, C3F6, C4F10, C4F8 and CHF3, among which CO2, C3F6, and C4F10 are dominant. For C5F10O/air mixtures, the pyrolysis products are mainly saturated fluorocarbons and more CO2 is produced. Partial discharges in C5F10O/air mixture mainly produce CO2, saturated fluorocarbons and a unique compound C3F6O3. It is found that CF4, C3F6, CF2O, CHF3, and C3HF7 have good compatibility with Al (1 1 1) and Cu (1 1 1) surfaces, while the interactions between C2F6O3 and Al (1 1 1), Cu (1 1 1) surfaces are relatively strong, which are chemisorption. The identified by-products of C5F10O and C5F10O/air mixture under different conditions as well as their compatibility with metal materials is essential for online fault-diagnostic and long-term maintenance in engineering applications.

Recently, C4F7N has been considered as one of the most promising candidates to replace SF6 due to its excellent insulation performance and environmental-friendly property. Metal zinc is widely used as a coating material in gas-insulated electrical equipment. However, its compatibility with C4F7N has never been explored before. In this paper, the adsorption properties of C4F7N on Zn(0 0 0 1) and ZnO(1 0 1 ¯ 0) surfaces were theoretically studied by a first-principles calculation. Adsorption energy, total charge transfer, density of states, and electron density difference were calculated to evaluate the compatibility of C4F7N with Zn and ZnO. The results demonstrated that the interaction between C4F7N and ZnO(1 0 1 ¯ 0) is stronger than that of Zn(0 0 0 1). The dissociation pathway of C4F7N after adsorption on the ZnO(1 0 1 ¯ 0) surface was also determined, which shows that ZnO(1 0 1 ¯ 0) has a catalytic effect on the decomposition process of C4F7N. Compared with Al(1 1 1) and Cu(1 1 1), the dissociation of C4F7N on ZnO(1 0 1 ¯ 0) has a higher energy barrier, indicating that the reaction is more difficult to occur. As a result, Zn and ZnO have certain compatibility with C4F7N, which was also verified by experiments. Therefore, we infer that Zn shows potentials to be used as anti-corrosion coating in C4F7N gas-insulated electrical equipment.

Environmental considerations are increasingly taking a front seat in all arenas of our daily lives. Among the major concerns are greenhouse gases and the resulting global warming problem. Although sulfur hexafluoride (SF6) is the most widely used insulation and arc-interruption medium in electrical power equipment, it is an extremely potent greenhouse gas with a very high global warming potential (GWP). Hence, the global power industry has been actively seeking SF6-alternative gases for decades. In recent years, much progress has been made in developing promising new gases that have dielectric properties similar to, or even better than, SF6 but with much lower GWPs. Many engineering-oriented tests have been performed on the technical performance of these gases and some manufacturers have announced their application in pilot projects. However, their large-scale application still seems premature, in particular because of the lack of some critical fundamental physicochemical information for these relatively novel compounds. To provide a better understanding of these SF6-alternative gases, this paper reviews the recent advances in obtaining their fundamental physicochemical properties, emphasizing basic data calculations and fundamental experiments. The most important properties are analyzed comprehensively. These include the basic physical properties of arc plasmas formed from the gases, decomposition characteristics, materials compatibilities, swarm parameters, cross-sections and some dielectric properties. Finally, perspectives on current research and future research directions are presented.

Recently, the C4F7N/CO2 mixture has been highly considered as a promising SF6 alternative due to its low greenhouse effect and excellent electrical performance. Apart from its dielectric strength and arc-quenching capability, the decomposition characteristics of C4F7N/CO2 also play an important role in its future applications in gas-insulated electrical equipment. Therefore, decomposition experiments of the C4F7N/CO2 mixture were carried out under multiple AC discharge breakdowns by using a needle-to-plate electrode configuration. The compositions of the gas mixture before and after breakdowns were detected by using a gas chromatography-mass spectrometer. The results show that a trace amount of C3HF7 exists in the C4F7N product; the decomposition of the C4F7N/CO2 mixture mainly produces CO, CF4, CO2, C2F4, C2F6, C3F8, CF3CN, C3F6, C4F6, C4F10, C2F5CN, C2N2, HCN, C2F3CN, and HF. As the number of breakdowns increases, the content of most decomposition products increases significantly, while the content of C3HF7 decreases. The possible formation pathways of the decomposition products were determined, and the corresponding reaction enthalpies were calculated by quantum chemistry calculation. Relevant results not only reveal the decomposition characteristics of C4F7N/CO2 under AC breakdowns but also help us to better understand the corresponding physicochemical mechanisms, which will provide a good reference for stable operation and safe maintenance in applications.

Iso-C4 perfluoronitrile (C4F7N) is one of the most promising alternatives to SF6 for use in power equipment, such as high-voltage circuit breakers, due to its excellent electrical properties and environmentally friendly characteristics. The use of SF6 is being reduced because of its high global warming potential. To describe the physical and chemical processes occurring in the arc plasma in circuit breakers, both local thermodynamic equilibrium (LTE) and nonlocal chemical equilibrium (LCE) conditions have to be considered. The plasma composition of the arc core region can be calculated under the assumption of LTE by the method of minimization of the Gibbs free energy. The plasma composition of the arc boundary region or during the arc ignition period is not in LTE or LCE, so the use of a chemical kinetic model that considers the effects of the energy barrier in chemical reactions is required. Calculations for both conditions are presented for C4F7N. To develop the chemical kinetic model, the complete decomposition pathway and transition states were first reexamined and further developed. Based on the decomposition pathway, the rate constants of reactions were obtained according to the variational transition state theory method. The results obtained for the two cases provide a reference for the systematic understanding of the decomposition characteristics of C4F7N gas and for related engineering applications.

SF6 has been designated as a greenhouse gas by the Kyoto Protocol because of its high global warming potential. C5-perfluorinated ketone (C5F10O or C5-PFK) has excellent insulation properties and environmental characteristics, and has been chosen as an alternative gas to SF6. Calculation of thermal decomposition is usually based on local thermodynamic equilibrium (LTE) and local chemical equilibrium assumptions. However, the composition may not reach chemical equilibrium, particularly at lower temperatures and high rates of temperature increase. In order to compare the chemical non-equilibrium and local thermodynamic equilibrium compositions, a chemical kinetic model is used to calculate time evolution of composition; this considers the effects of the energy barrier in chemical reactions. To obtain the required data for the model, which is necessary because of the limited research on reaction rates in C5-PFK decomposition, the methods of computational chemistry are used to calculate transition states and reaction rates. The LTE composition is calculated using Gibbs free energy minimization. Calculations are presented for two possible C5-PFK thermal decomposition products, C4F7O and C3F4O. The result indicates that chemical kinetics is required to calculate the dissociation of C5-PFK at low temperatures, such as occur during arc ignition and near the arc boundary.

Both fluoroketone C5F10O and fluoronitrile C4F7N are promising substitute gases for SF6. The electron-impact ionization cross sections for these two gases are calculated using the Deutsch–Mark (DM) formula and its modified method. The necessary molecular geometry optimization and electron population were determined by ab initio calculation, which was performed with quantum chemistry code. The level of calculation, including the theoretical method and basis-set, are carefully determined. To eliminate the drawbacks of the DM formula, a modified DM formula is set in this paper. The modified DM formula, of which the weighting factors are changed, has a better agreement with the experimental data on both the peak and shape of the cross-section curves. The results calculated by DM formula and modified DM formula are given as references to fill in gaps in further research into C5F10O and C4F7N.