High-bandwidth measurement of low-level prebreakdown currents in liquid dielectrics

The development of a measurement system optimized for making low-noise, high-bandwidth measurements of prebreakdown currents in liquid dielectrics under DC stress is described. The system consists of a 90-MHz bandwidth transimpedance amplifier with a 5000-V/A gain, two wideband buffer amplifiers, a 200-Msample/s transient digitizer, and waveform reconstruction software. The system has an overall bandwidth of 65 MHz, including the digitizer, and a minimum detectable current of 300 nA for a signal-to-RMS noise ratio of five. Waveform reconstruction software is used to quadruple the effective sampling rate from 5 ns per point to 1.25 ns per point. Initial experimental results from the use of this measurement system are presented. Prebreakdown currents were measured in hexane using a point-plane electrode geometry with the needle connected to the preamplifier input and either positive or negative high voltage applied to the plane. Pulses with extremely short durations (<8 ns full width at half-maximum amplitude) and amplitudes ranging from tens of microamperes down to near the noise level of the measurement system were observed. The usual pulse shape appears to be the system impulse response, which implies that the current pulses are very fast indeed