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Numerical simulation of discharge mechanism under nanosecond pulse voltage assists in the insulation performance optimization of N2-insulated switches that are widely used in the pulse power devices. This article presents a fluid dynamic model for the positive streamer propagation in N2 based on a needle-plate discharge system. The effects of the operating conditions, such as voltage amplitude, rising edge, gap width, and needle radius, are studied. Simulating results indicate that the streamer might be evoked only when the maximum electric field exceeds the critical value that is closely influenced by the needle radius and gap width, e.g., $1.3\times 10^{7}$ V/m based on a needle-to-plate electrode configuration with a 0.4-mm tip radius and 5-mm gap. The maximum electric field occurs at the head of streamer and it drops suddenly by nearly two orders of magnitude in the back of the head, which makes the streamer moves forward rapidly. Certainly, it needs a high enough voltage to ensure continuous propagation of the streamer, for example, more than 20 kV needed as the tip radius and gap set to 0.4 and 5 mm, and the electric field strength and migration speed increase with the increasing voltage amplitude. The radius of the discharge region is smaller when the rising edge time increases from 10 to 40 ns. However, if the rising edge time is too low (e.g., 2 ns), the streamer will cease before it propagates to the plate electrode and the radius of the discharge region shrink at the same time. In addition, the peak electric field and the averaged electron energy at the same position away from the needle decrease slightly with the gap width. Finally, there is an obvious decrease of the initial peak electric field as the needle radius increases. [ABSTRACT FROM AUTHOR]

Due to the greenhouse effect of SF6, the search for new environmentally friendly substitute gases has become an urgent task for the electrical industry. In this article, we have employed a 2-D fluid model to study the characteristic of streamer discharges in the C4F7N-N2 mixture. The discharge process and insulation performance of the C4F7N-N2 mixture with different C4F7N content are compared under the same conditions. In a5.07% C4F7N-N2 mixture, the streamer expands continuously and reaches the top boundary quickly. In 7% and 13.1% C4F7N-N2 mixture, the streamer is gradually stagnated in the simulation domain. The streamer radius shows a decreasing trend, and a low electron density area appears behind the streamer head as the streamer moves forward. The streamer discharge simulation in 50% SF6–50% N2 is also performed to compare its insulation performance to the C4F7N-N2 mixture. The results show that the discharge is suppressed more heavily in 13.1% C4F7N-N2 than in 50% SF6–50% N2. The influence of buffer gas is studied by comparing the discharge process in C4F7N-N2 and C4F7N-CO2 with 13.1% C4F7N content. It is found that the discharge is more easily to a breakdown in C4F7N-CO2 than that in C4F7N-N2 in our simulation configuration. The simulation provides a way to optimizing the buffer gas ratio of new environmentally friendly gases from a microscopic perspective. [ABSTRACT FROM AUTHOR]

An experimental and testing platform was established to comparative study the decomposition characteristic of SF6 under different voltage waveform (negative direct current (DC) voltage, positive DC voltage, and alternating voltage), and investigate the influence of pressure and adsorbent. Content variation of typical products under different conditions and the influence mechanism were observed and analysed in detail. The results indicate that SF6 decomposition characteristics are closely related to a voltage waveform, pressure, and adsorbent. Concentrations of by‐products under AC voltage are higher than those under DC voltage and SF6 decomposes more easily under positive voltage than negative voltage, which can be explained by different discharge energy and distribution characteristic of space charges. The values of typical products show an obvious downward trend with pressure due to falling discharge energy, volume of active region and dissociation rate of SF6. Concentrations of typical products including SOF2, SO2F2, and SO2 decrease dramatically after adding adsorbents because of physical absorption and chemisorption. [ABSTRACT FROM AUTHOR]

Detection of decomposition products is a widely studied method for condition diagnosis of SF6 insulated equipment because of its anti-electromagnetic interference and high sensitivity. A 90 L discharge chamber with adjustable defect was designed to simulate typical failures including metal protrusion defect, floating potential defect and insulator defect. Primary products of both the former two defects are SOF2, SO2F2, CO2, and SO2. Concentration ratios including Φ(SOF2+SO2F2+SO2)Φ(CO2) and Φ(SOF2+SO2)/Φ(SO2F2) could be used to distinguish these two defects which have different structures and discharge energy. Influence of essential conditions such as applied voltage, pressure, and moisture content has been investigated under metal protrusion defect. For the insulator defect, several extra carbon containing by-products, namely, CF4, CO, and CS2, are found to be critical characteristic decomposition products. Moreover, a triangle method utilizing composition of decomposition products is proposed. It is proved to be a graphical method which helps to follow the faults more easily and more precisely. [ABSTRACT FROM AUTHOR]

Detection of decomposition products is a newly-emerging method for the condition diagnosis of SF6-insulated equipment. Based on the two grading structures that are commonly adopted in SF6-insulated current transformers (CT), three models consisting of the needle-to-plane, the ball-to-plane, and the plane-to-plane were established. Gas decomposition was investigated in a 90 L discharge chamber under operating conditions. Products were analyzed with gas chromatography (GC) and X-ray photoelectron spectroscopy (XPS). Typical gas decomposition products are CF4, SO2F2, CS2, SO2, and S2OF10. For the needle-to-plane with a gap of 30 mm, near-linear growths with time are observed in the concentrations of SO2F2, SO2, and S2OF10 when the applied voltage is set as 33 kV. Addition of the adsorbent suppresses the concentrations of all three products, especially SO2. Besides, CF4 and CS2 are also found to be the characteristic components of partial discharge involving polyester films. And their concentrations are higher when the electric field lines are parallel to the surface of the polyester. As for the polyester films, the atomic ratio of C decreases, the atomic ratio of S increases, and the atomic ratio of F increases significantly as the deterioration becomes more serious. Moreover, the concentration ratio of S2OF10/SO2F2 in the needle-to-plane is mostly within the range of 0- 2, whether with the addition of adsorbent or not. And the ratio is much higher (2-10) when polyester is involved in the discharge process. It could be used to diagnose the insulation condition of CT effectively. [ABSTRACT FROM AUTHOR]