Fast Pulse-Widths of Partial Discharges Using Needle-Plane Electrodes With Different Tip Radius Over Polyimide Film

Partial discharge (PD) has been analyzed for many years; however, it is still difficult to understand clearly because it strongly depends on experimental conditions. To understand PD characteristics better, we directly measure fast-PD current waveforms on the order of nanoseconds instead of PD charge amounts. These fast-PD current peak measurements may provide us the deep understanding of the PD mechanism under different electric field strengths with different tip-radiuses and discharge gaps. Recent advances in high-speed oscilloscopes now enable nanosecond-scale pulse measurements under appropriate conditions. In this study, the PD measurement circuit is minimized to ensure accurate peak acquisition, reducing the total length of the circuit-flow path to less than 150 mm. A simple needle-plane electrode is used with discharge gaps ranging from 0 to 150 μm. The needle electrode has a diameter of 1 mm with several tip radii of 30, 100, 300 and 500 μm. An AC voltage with peak amplitudes of 2 kV at 50–1000 Hz is applied, and a 100-μm-thick polyimide film is employed as the insulating specimen. The measured new PD characteristics are the peak time that is the time interval between pulse detection to pulse peak and the half-maximum pulse widths of the fast PD current pulse. The peak time shows relatively small variation, between 4 ns and 6 ns across all electrode conditions. In contrast, pulse widths are approximately 5-15 ns, depending on conditions such as dischargegap length and needle tip radius. Moreover, the dependence of these times on AC voltage frequency is consistently small, suggesting that accelerated testing using higher AC frequencies up to 1000 Hz may be effective for estimating insulation lifetime under PD stress for this electrode configuration.