Partial discharges characterization in spherical voids using ultra-short x-ray pulses

This work aims to an improved characterization of partial discharges (PDs) in spherical voids. Start electrons are created by an ultra-short (50 ns) X-ray pulse to eliminate the statistical time lag and the individual PDs are detected with an ultra-wideband detection circuit and a photomultiplier tube. The minimum PD inception field strength is determined by triggering at different phase angles of the applied ac voltage. From this, also the gas pressure inside the void can be calculated. The observed PD characteristic (pulse height, charge magnitude, rise-time, and pulse width) are in agreement with the theory of streamer-like discharges. The measurement setup allows the exact determination of the phase angle of each PD related to the ac phase and an improved modeling of the PD activity can be made. Further unknown parameters (residual field strength, charge decay time constant) can be determined from comparison of the measurements to PD modeling.