A document by Timothy Wong. Click on the document to view its contents.

Power-centric research conducted in the past few decades has identified many influencing factors that affect and contribute to the development of high voltage surface flashover. Among them, the surface condition—morphology, roughness, and texture—is one such physical characteristic of gas-solid interfaces that is believed to be exploitable, relatively inexpensively, as a method to improve surface flashover strength. High voltage pulsed power systems and equipment face similar challenges, but there presently exists limited literature focused on impulse-driven flashover and the effects of surface roughness. In this work, the impulsive flashover strengths of five polymers relevant to pulsed power system design (PVC, Delrin, Ultem, Torlon, Perspex) are reported under two different (∼20 nanosecond and ∼100 microsecond risetime) impulse waveforms in atmospheric air. Samples of different surface conditions— “as received” and “machined”—were subjected to flashover tests, complemented with surface profilometry measurements to evaluate the effects of various roughness characteristics on the impulse-driven breakdown strengths and times-to-breakdown. The results obtained indicate a general enhancement of the impulsive flashover strength with increased roughness with a corresponding prolongation of the time-to-breakdown. Rougher “machined” surfaces were therefore found to outperform the smoother “as received” surfaces. A correlation analysis between the measured surface roughness parameters and the breakdown data suggests that the short-wavelength components of the surface profile contributes more towards the enhancement of the flashover strength compared to longer-wavelength “waviness” components. The consistency of this result with the theory of increased streamer path length and streamer inhibition is discussed, as are the potential consequences to insulator surface modification for flashover mitigation.

With the ever-increasing requirements placed on current and future pulsed power systems in terms of voltage, power, and compactness; solid insulation is a strong candidate for the development of novel insulation systems capable of meeting these specifications. However, the issue of solid-solid interfaces under non-standard and fast-rising impulses must firstly be addressed, as the failure to do so may pose significant risk of electrical breakdown due to reduced dielectric strength across interfacial contacts. In this work, the impulsive breakdown characteristics across dry-mate solid interfaces formed between PVC (polyvinylchloride), Torlon (polyamide-imide), Delrin (polyoxymethylene), Perspex (polymethylmethacrylate), and Ultem (polyetherimide) has been investigated in atmospheric air and under two different impulsive waveforms rising at ∼2400 kV/µs and ∼0.35 kV/µs. The statistical treatment of the obtained impulsive breakdown voltages and time to breakdowns are presented, alongside an analysis of the post-breakdown surfaces and discharge channel morphologies. The results indicate that under low mating pressure conditions (10’s of kPa), the interfacial breakdown strength may be below that of only an air gap with no dielectrics. A correlation between the estimated asperity aspect ratio and the interfacial breakdown strength has been observed. This suggests that under the present experimental conditions, field enhancement around surface asperities may be a dominating factor which defines the breakdown strength of the interface, since the surface asperities do not deform sufficiently to form strong interfacial contact spots, and thus reducing the interfacial tracking resistance. This therefore provides little to impede the development of interfacial discharges. The widths of post-breakdown traces left by plasma channels on the contacting surfaces have also been shown to be dependent on the rate of voltage rise, dV/dt, and on the material forming the interface. The results arising from this work may aid in the future development of high voltage solid insulating systems for power and pulsed power systems.

A document by Timothy Wong. Click on the document to view its contents.