The objective of this article is to characterize partial discharge (PD) inception voltages and patterns at sinusoidal and square voltages in insulating ceramic substrates with generic manufacturing defects. PD tests were conducted on active metal brazed (AMB) aluminum nitride (AlN) substrates either with cavities in the polymer coating or sharp protrusions of the metal brazing. The substrates were tested in silicone liquid instead of silicone gel. Light sensitive imaging was used to localize the PD sites on the surface. PDs at square voltage with rise time of $1.6 ~\mu \text{s}$ were measured with a high-frequency current transformer and a photomultiplier. In both substrate types, the peak–peak value of the PD inception voltage (PDIV) was higher at sinusoidal voltage compared to peak-peak value of the square wave. This difference was more pronounced in uncoated substrates, where discharges incepted at sharp edges surrounded by silicone liquid. For both types of defects, comparable PDIVs were measured at bipolar and unipolar square waves. Results for unipolar pulses are discussed considering the role of charge memory effect.

This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under negative polarity stress. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. Compared to positive polarity, there are less differences in streamer behaviour in the oils examined under negative polarity. Breakdown voltages and acceleration voltages are higher for negative streamers than for positive ones, while their propagation velocities are lower. While propagation modes for positive voltages are quite distinct, the mode changes for negative ones are more gradual. The behaviour of both positive and negative streamers is in line with the hypothesis that the propagation is governed by electron avalanches and quantum chemical properties of liquid components.

This paper describes measurements of conduction currents in both neat and commercial insulating liquids. Conduction in thermodynamic equilibrium has been measured by applying a low ac voltage in a uniform field. High-field pre-discharge currents have been measured in a needle-to-plane configuration stressed with respectively high voltage ac and steep step high voltage pulses. There was significantly increased conduction for high field compared to low field conditions. Notably, measurements at high fields in needle-plane configurations indicate space charge limitation of currents with polarity differences. The high field conduction exhibited considerable variation across different liquids. At high applied fields, electronic processes are probably governing the charge formation. Simulations indicate that space charges may reduce the local field in front of needle tips at time scales relevant for propagating streamers.

This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under positive polarity stress. It is of interest to improve knowledge on how functional properties of dielectric liquids vary, as new brands arrive in the market, and existing standards are unsuited for documenting the dielectric functional parameters of these new liquids. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with a 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. There are clear differences in streamer stopping lengths and mode occurrence and mode velocities between these liquids. The differences seem to be influenced by molecular sizes governing evaporation energy for streamer formation and by concentration of aromatics which can be coupled to electron avalanche processes in the streamer heads.

This paper summarizes experiments that were carried out on insulating ceramic substrates and a needle-plane arrangement with the purpose to characterize the inception of partial discharges (PD) in the insulation of high-voltage semiconductors. PD measurement results obtained on substrates are correlated with the high-field pre-inception currents measured in a needle-plane electrode arrangement in liquids and gel. The tested substrates were embedded in either silicone liquid, silicone gel, or mineral oil. The PD inception was measured at sinusoidal and switched voltage of different polarities and rise times. For all test objects, the PD inception voltage (PDIV) is significantly lower under switched voltage compared to the sinusoidal voltage. Silicone gel and silicone liquid as insulating media surrounding the substrate are similar in terms of the PDIV. The PDIV is lower for substrates placed in mineral oil, particularly under switched voltage. Silicone gel and liquid are characterized by very low pre-inception currents measured at a high electric field. In contrast, a higher pre-inception current is observed in mineral oil. These results are consistent with the hypothesis that the high pre-discharge current in the more conductive mineral oil leads to space charge build-up in the high-field region that, by deforming the electric field, affects the PD inception probability.