Nanosecond Current Waveforms With Small Discharge Gaps Over Polyimide Films

Nanosecond partial discharge (PD) current has been very important with recent development of large power modules used in an automobile drive mechanism. Those modules are operated at higher frequency than normal power frequencies and therefore to analyze the PD characteristics of recent excellent polymers at higher frequencies than power frequencies have been very important. For using small gap distances to measure PD characteristics are also very important to determine the PD resistance of polymers in power equipment. Among the polymer insulating materials, we selected polyimide (PI), which is widely used for circuit mounting. We measure the nanosecond PD pulse current waveforms with the
$\varphi $
-q-n characteristic, which has been widely investigated in prior PD studies. We measured the PD waveforms under an ac peak voltage of 2 and 3 kV at 50–1000 Hz to clarify the PD waveforms, which may give us better understanding of PD characteristics. The electrode system used in this study is a simple needle-plane electrode with small discharge gaps of 0, 12.5, 25, 50, 75, 100, and
$150 \mu \text{m}$
. The needle diameter is 1 mm. We observe that the positive current magnitudes are almost the same as those of the negative currents for the thinner film cases, which are 25 and
$50 \mu \text{m}$
at almost all the applied voltages and frequencies, where the positive current is defined as the current from the needle to ground. Conversely, the positive pulse magnitudes are almost twice those of the negative ones at a film thickness of
$75 \mu \text{m}$
.