Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

Abstract

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.

Pressure-dependent propagation of streamers under step voltage in a long point-plane gap in transformer oil

“…Negative streamers are strange: streamer shape for a given mode is close to the same through all liquids tested in [16], and here it is found that increased pressure has virtually no effect on streamer shape or velocity, either. Breakdown voltage only increased a factor of 2 for negative streamers compared to 5 for the positive ones over the investigated pressure range.…”

“…Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works." There are lots of current pulses during negative 1 st mode, and they have corresponding light pulses [16]. While the typical "reillumination" is from 0.5 A and upwards, most of these pulses are much smaller.…”

“…In the oil used in this study, the shapes of the different modes are also very distinct. The mode distinctions are less clear for negative streamers [16], both in shape and velocity, but the streamers can similarly be forced into a corresponding mode classification with typical velocities 0.05 -0.2, 0.4 -2, 3 -20 and 30 -70 km/s with a distinct shape transition between 2 nd and 3 rd mode. It has been found that more pressure is required to suppress the faster modes than the slower modes [6].…”

“…With increasing voltage, different propagation modes appear, with different appearance and increasing velocity that is liquid dependent, and several modes can appear at different times during propagation of one streamer [14][15][16]. The four modes commonly found with positive streamers have typical velocities 0.1, 0.7 -3, 3 -20 and 50 -400 km/s when counting from 1 st to 4 th mode [15].…”

Prebreakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 1: behaviour at positive polarity

“…This frequent filtering may not have been necessary for positive streamers, but certainly was for negative ones. This is described in detail in part 2 of this paper [32], which focuses on negative streamers.…”