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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]

The polymer film dielectric has been widely used in high voltage equipment for bulk insulation owing to the excellent insulation performance. Martin’s empirical formula for bulk breakdown is frequently used to predict the breakdown field strength of multilayer thin film with different electrode areas. In this article, based on the academic consensus that the cumulative breakdown probability of single-layer film obeys the Weibull distribution, the relationship between the cumulative breakdown probability of film dielectric under different electrode areas and the applied electrical field is deduced. Moreover, the relationship between the cumulative breakdown probability of film dielectric with various numbers of film layers and the applied electrical field is obtained and simplified. On these bases, combined with the experimental results of the average breakdown field of film dielectric with smaller electrode areas and fewer layers, a prediction model of the average breakdown field strength of film dielectric with different electrode areas and film layers is established. To obtain the solution of the prediction model concerning the number of film layer, an effective approximate solution method is proposed to solve the calculation problem with ultralarge and ultrasmall data, which significantly reduces the complexity of the solution process. A coaxial film capacitor is developed and tested to verify the prediction model with the “Chenguang” accelerator. The experimental breakdown field strength of 54 layers of 15 $\mu \text{m}$ biaxially oriented polypropylene (BOPP) film with an electrode area of 218.1 cm2 is 355.6 V/ $\mu \text{m}$ , which is comparable to the predicted result of 374 V/ $\mu \text{m}$. The prediction model presented in this article is of certain reference significance for the insulation design of film dielectrics. [ABSTRACT FROM AUTHOR]