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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]

For nanosecond pulsed gas discharge under hundreds of kV/cm in sulfur hexafluoride (SF6), the electric field emission (EFE) effect may be the mechanism of the initial electrons. Electrode surface micro-protrusions (ESMPs) generated after discharging, would affect the discharge process under a high electric field. To investigate the effect of ESMPs on the breakdown time delay and jitter, the breakdown characteristics of different electrode materials and surface roughness are determined in this paper. Three types of electrode materials include stainless 304L, EDM-AF5 graphite, and CuW70 W-Cu alloy. The influences of stainless 304L electrodes with surface roughness coefficients of 1.6 and 0.8 on the breakdown delay and jitter are compared. After 700 discharge shots, the morphology characteristics and distributions of ESMPs are observed by a contourgraph. Based on the statistical results of ESMPs and EFE theory, the relationship between the electric field of EFE and ESMPs can be calculated. In addition, the effective electron rates of negative ion detachment (NID) are estimated. The experimental and calculation results indicate that the initial electrons are mainly produced by the EFE effect. When the pressure is below 0.5 MPa, the NID effect is limited and the breakdown time delay and jitter are dependent on the height to radius ratio and distributions of ESMPs. Once the pressure exceeds 0.5 MPa, the NID effect will be significant and the influence of ESMPs can be neglected. [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]