Nanosecond Current Waveforms With Small Discharge Gaps in Needle-Plane Electrode Over Three Kinds of Polymer Film

Abstract

The nanosecond partial discharge (PD) currents have become increasingly important with the recent development of large power modules used in automobile drive mechanisms and other high-temperature equipment. Those modules are sometimes operated at frequencies higher than normal power frequencies, making it crucial to analyze the PD characteristics at these elevated frequencies. Measuring PD characteristics with a small gap, less than $100~\mu $ m, has also become essential due to system compactization. PD currents typically flow in a few tens of nanoseconds; hence, the wire length of the current measurement circuit should be sufficiently short. The electrode system employed in this study consists of a simple needle-plane electrode with small discharge gaps of 0, 12.5, 25, 50, 75, 100, and $150~\mu $ m. The needle diameter is 1 mm with a tip radius of $300~\mu $ m. The total length of the PD current-flow circuit is set to be less than approximately 150 mm. We measure nanosecond PD pulse current waveforms using a digital oscilloscope with an ac peak voltage of 2000 V at 50–1000 Hz. PD current forms were observed over polyimide (PI), polyethylene terephthalate (PET), and polyethylene naphthalate (PEN) films, widely used as insulating films. The PD current waveforms over PI, PET, and PEN films exhibited approximately 5 ns peak time for both positive and negative pulses, with a pulsewidth time at half height of the peak value ranging from about 5–15 ns. These times vary depending on the material kind and discharge gap length. The frequency dependence of those times showed little change over applied voltage frequencies. Moreover, new time-domain NMR analyses correlate the current waveform differences with the dipole movement affected by the trapped charges.