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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]

In the practical operation of an electromagnetic pulse (EMP) simulator, the flashover phenomenon on the edge of polypropylene dielectrics dominates the insulation failure mode of a peaking capacitor. To analyze the characteristics of polypropylene film flashover voltage under high-pressure SF6, an experimental platform was established, and the pulse generator could output a pulse with a rise time of approximately 27 ns. The experiments were performed on finger electrodes, and the SF6 pressure was set from 0.1 to 1.0 MPa. The experimental results show that the relationship of flashover voltage and gas pressure could be divided into two regions: in low-pressure regions, the flashover voltage increases with the gas pressure linearly and in high-pressure regions, the growth of flashover voltage gradually slows down and approaches a saturation value. As the gap distance increases, the saturation point of gas pressure decreases, and even when close to 0.4 MPa, with a 9-mm gap distance, this unconventional phenomenon cannot be ignored, particularly in practical EMP simulators. When the high surface resistivity and the charge trap distribution on the polypropylene film are considered, the accumulation and emission of surface charges on the film might be the primary reason for the saturated phenomena. [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]

Due to its low-loss and high dielectric strength, polypropylene is widely used as electrical insulation material in pulse capacitors. In order to study its breakdown characteristics for nanosecond voltage pulses, an experimental setup was designed to generate voltage pulses with a 10%–90% rise time of 38 ns and a full width at half maximum of about 90 ns. Tests of the three polypropylene films with thicknesses of 12, 15, and $18~\mu \text{m}$ were performed. In addition, hundreds of nanoseconds pulse and dc voltage were also applied for comparison. The experimental results reveal that most of the breakdown occurs during the front edge of the pulse for tens of nanoseconds pulse, and the breakdown strengths of the three kinds of polypropylene films are almost identical for different applied voltages, which show a weak correlation between the thickness of the dielectric and the breakdown field. The expected breakdown strength of tens of nanoseconds pulse defined in this paper is not significantly higher than the mean value of the hundreds of nanoseconds pulse and the dc condition, suggesting that the breakdown of a thin polypropylene film is mainly determined by the electrical field but not the voltage-time cumulative effect. All these observations strengthen the conclusion that the breakdown of a thin polypropylene film is an intrinsic phenomenon. [ABSTRACT FROM AUTHOR]