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C4F7N/CO2 mixture has been widely studied as an environmental-friendly insulating gas expected to replace sulfur hexafluoride (SF6) in high-voltage equipment. In high-voltage power systems, the medium dielectric characteristics under switching impulse (SI) voltage are an important factor in insulation coordination. Nevertheless, few studies on this aspect of C4F7N/CO2 mixture have been reported. This article investigates the surface flashover of a cylindrical insulator in a C4F7N/CO2 mixture under switching voltage. The effects of molar fraction, gas pressure, creepage distance, and insulator materials on surface flashover voltage are studied in a uniform field. The result shows that the increase of flashover voltage indicates a saturation trend with the increase of C4F7N molar fraction and pressure, respectively. The dielectric strength of 13% C4F7N/87% CO2 relative to SF6 can reach at least 84%. The limiting factors of the flashover voltage increase under high pressure are preliminarily discussed. Additionally, the empirical formula on the critical flashover electrical field has been obtained, which can provide a reference for insulation design for environment-friendly gas insulation equipment. [ABSTRACT FROM AUTHOR]

This paper investigates the external circuit effect in corona discharge by means of numerical simulation, while the effect is usually disregarded in previous plasma simulations. A general numerical solution method of serial external circuit, which can compute surface charges on any shape of electrodes adaptively and solve electrode potential and circuit current simultaneously, is developed to support the investigation, due to lack of theoretical solution in irregular electrode systems. Brief description and validation of the scheme are presented in section 2. In general, the accuracy of numerical solution improves with mesh refinement in orthogonal mesh, due to stair step approximation. Simulations are performed in our self-developed 3D PIC-MCC (particle-in-cell, Monte Carlo collision) code with photoionization and external circuit. The simulation model including recently developed photoionization model is described briefly in section 3. Then the simulation results in a needle-plate electrode in air, such as spatiotemporal evolution of corona discharge and waveforms of electrode voltage and circuit current, are presented. Physical mechanisms of formation of branched corona discharge and self-consistent adjustment of electrode voltage and circuit current are discussed. The waveforms of voltage and current in this simulation are generally consistent with experimental result. The external circuit has negative feedback effect on the discharge, and can adjusts electrode voltage self-consistently. The application of external circuit improves the applicability of particle simulation method to the simulation of discharge phenomenon in actual devices. [ABSTRACT FROM AUTHOR]

Plenty of solid products will be produced when the arc discharge fault occurs in SF6 gas-insulated electrical equipment, which will extremely threaten the internal insulation properties of the equipment. Therefore, it is necessary to carry out research on the solid products formed by the reaction of SF6 with metal electrode under electric arc, aiming to study their effect on the operation of the equipment. First of all, the formation characteristics of the solid products with different metal electrodes and different pressures are studied. Then, the composition of the solid products is determined. Besides, the quantitative relationship between the mass of the solid products and the discharge energy is analyzed as well. [ABSTRACT FROM AUTHOR]

This paper reports on an investigation into the surface flashover of an insulator in a C4F7N/CO2 mixture which is used as an alternative to SF6. To date, no studies on this aspect have been reported in the literature. This study was done for a uniform field using a 220 kV AC experimental platform. The influence of molar fraction, gas pressure and creepage distance is reported and compared to that in SF6. The results show that small quantities of C4F7N mixed in CO2 can increase the surface flashover voltage but, as the fraction of C4F7N increases, saturation of the surface flashover occurs. The relative surface flashover voltage of 13.0% C4F7N/87.0% CO2 is approximately 80% that of SF6. The empirical formulae on the critical flashover electrical field has been obtained, which may be referenced in the design of environment friendly high voltage equipment. [ABSTRACT FROM AUTHOR]

To monitor fast pulsed voltage in the transmission line of the fast linear transformer driver, a compact and self-integrating capacitive sensor is developed. A printed circuit board (PCB) with multi-layer ceramic capacitors (MLCCs) serves as a low-voltage arm. Given the reflow structure of the PCB and a parallel arrangement of MLCCs, the residual inductance of the PCB low-voltage arm can be effectively reduced. Moreover, its capacitance can be easily adjusted. The capacitive sensor is designed using a coaxial structure, suitable for installation on the inner high voltage electrode of the secondary water-insulated transmission line. Three matching schemes for the measuring cable are discussed, showing that two-stage division with matching at the end of the cable is more suitable for fast pulsed voltage measurement due to its high dividing ratio and flat frequency response. A square wave with a rise time of ∼1.5 ns and a pulse width of ∼200 ns is used to verify the properties of the capacitive sensor. The results show that the rise time of the designed capacitive sensor can reach 1.5 ns when a short cable is used. Increasing capacitance of the low-voltage arm or resistance at the cable entrance will increase the time constant of the capacitive sensor, which improves its low-frequency properties. A long cable will significantly reduce the high-frequency response and the magnitude of the signal from the capacitive sensor. [ABSTRACT FROM AUTHOR]

In this paper, a study concerning self-breakdown voltage characteristics, electrode erosion, and degradation of the insulator, of a two-electrode spark-gap switch has been presented based on experiments. A 6 ?F pulse capacitor and a DC high voltage source (set at ?40 kV) were utilized to achieve ?0.1 pulses per second repetitive arc discharge between two fixed Cu-W (Cu30/W70) alloy electrodes in the 1.2 L airtight chamber of the switch. With average transfer charge of 2.2 C per shot, this switch was tested for 5800 shots altogether. The results demonstrated the electrode erosion rate of the cathode was 6.3434?10?6 cm3/C whereas that of anode was 5.4691?10?6 cm3/C. Besides, by analyzing electrode surfaces' morphology and PTFE insulator surfaces' compositions after 5800 shots with the help of a scanning electron microscope, a dynamic 3D aspheric metrology instrument, an X-ray photoelectron spectrometer, a metallographic microscope, and a Mettler-Toledo analytical balance, some evidence of physical processes and chemical reactions between plasma channel, electrode materials, and insulation materials could be observed. Moreover, the chamber was opened and inspected for 10 times during the whole procedure, thus providing an overall description of the variation of the switch. It should also be pointed out that owing to complex mechanisms during the aging process, the self-breakdown voltage followed a mixed Normal distribution other than any other simple distributions. [ABSTRACT FROM AUTHOR]

Experiments concerning the self-breakdown voltage characteristics have been done and a simplified model has been proposed to describe the phenomena in this study. A test platform has been established to investigate the Cu-W (90 wt%) two-electrode switch working in a small airtight cavity (1.2 L). The switch was tested under the conditions of frequency of ?0.1 Hz, self-breakdown voltage of ?30 kV, and peak current of ?60 kA for more than 20000 shots. Inevitably, the process is accompanied with the insulation and electrode's aging in the switch, including the variation of gas components and electrode surface states, which eventually alters the self-breakdown voltage. The results of the experiment demonstrated that the lifetime of the switch could be divided into three stages. In the third stage, this switch showed a satisfactory reliability and stability of the self-breakdown voltage: mean of 32.4 kV and standard deviation of 2.0 %. Besides, from the view of statistics, calculation results proved that the 20000 shots' self-breakdown voltage obeyed mixed Normal distribution, whereas a continuous small sample of that simply followed Normal distribution or Weibull distribution. Moreover, it could be anticipated from the results that the Cu-W spark gap switch could have a lifetime more than 20000 shots (total charge transferred was 13259 C). However, in fact, the lifetime was terminated in advance by insulation degradation in the switch. [ABSTRACT FROM AUTHOR]

Corona and spark are two typical discharge types in gas-insulated switchgear (GIS). Studies on the SF6 decomposition characteristics under corona and spark are significant for the safe operation of the GIS. In this paper, the discharge energy/charge and SF6 decomposition products under ac corona with point-to-plane electrode defect and spark with floating potential defect were investigated under different electrode dimensions, applied voltages, SF6 gas pressures, adsorbents and moisture contents. The upper limit of the formation rates, feature extraction, and the amount of wear of the GIS were also discussed. The results show that the influences of gas pressure and moisture content on the discharge energy and SF6 decomposition products under two different defects are distinguishable. In addition, the concentration ratios of (SOF2+SO2)/(SO2F2) under corona and spark lie in the ranges, 0-1, and 1?5, respectively. The formation rates of SF6 decomposition products calculated from experimental results under corona discharge lie in the range, 0.1-5 μmoles/J. However, the formation rates under spark are usually in the range, 0.02-0.04 μmoles/J, which are far lower than that under corona. Theoretically the upper limit of the formation rates under corona and spark are estimated to be in the ranges, 67-540 μmoles/J, and 188- 376 nmoles/J respectively. Besides, the concentration ratios and formation rates can be used to distinguish the discharge types and the amount of wear of the GIS can be estimated. [ABSTRACT FROM AUTHOR]