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The surface flashover characteristics of BOPP (biaxial-oriented polypropylene) films in SF 6 under nanosecond-pulsed voltages are experimentally investigated in this research by using finger electrodes and using the front-edge flashover method. The test factors include the gas pressure, electrode distance, pulse voltage steepness, pulse polarity and applied field direction. The flashover voltage first increases with gas pressure and then saturates at a certain value, such as 0.4 MPa for a 9 mm gap. The more serious the field distortion is, the lower the saturation gas pressure. The increase in the pulse voltage steepness enhances the flashover voltage in general, but a frap phenomenon of the flashover voltage occurs, and the flashover voltages under pulses with steeper fronts are lower at some pressures. Generally, the negative flashover voltage is higher than a positive voltage by using finger-cuboid electrodes, especially at high pressures. The flashover voltage on the film surface with a normal electrical field component is higher than that with a parallel component at low pressures and lower at high pressures. Moreover, a qualitative avalanche model based on the accumulation and emission of surface charges on the films and the field distortion effect of space charge behaviors is employed to explain the above experimental results. [ABSTRACT FROM AUTHOR]

A polypropylene (PP) film is usually employed as the main insulation of the peaking capacitor, which is important in an electromagnetic pulse simulator to generate high-amplitude nanosecond pulse. The flashover voltage of PP films under nanosecond pulse limits the highest working voltage of a peaking capacitor. Surface fluorination was employed to increase the flashover voltage of PP. The flashover voltages of PP samples with different fluorination times (0, 4, and 6 h) were tested in an oven with nanosecond pulse voltage. The results showed that the flashover voltage of the PP film with 6-h surface fluorination was increased about 15% than the untreated film in the air. To clarify the influential mechanism, the physicochemical properties of PP film samples were tested and characterized. The results of thermally stimulated depolarization current (TSDC) showed that the trap density of the PP film increased obviously after fluorination treatment. With the increase in treatment time, the trap depth and density were increased obviously. The surface conductivity of PP films slightly decreased with the increase in fluorination time. The mechanism of surface fluorination on the influence of the flashover performance of the PP film under nanosecond pulse is discussed. It is concluded that surface fluorination significantly increases the carbon–fluorine (C–F) bond of the PP film, resulting in the increase in the depth and density of the surface trap, and then, a large amount of homocharges trapped in the surface trap reduces the electric field near the electrode. [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]