Abstract
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.
Highlights
Electric insulating liquids are used in various types of electric power apparatus, with transformer applications dominating totally in terms of market volume
The present study focuses on hydrocarbon liquids; from pure cyclohexane via isoparaffinic liquids synthetized from gases and white oils to commercial inhibited transformer oils
One way is the average propagation velocity of a streamer which may vary in speed through time due to mode shifts
Summary
Electric insulating liquids are used in various types of electric power apparatus, with transformer applications dominating totally in terms of market volume. Since there is a background current, and simultaneously a continuous light exists, movement of the charges separated during continuous ionization is expected This is very small compared to the current amplitudes registered during reilluminations. The faster a streamer grows and the wider the frontal area it has towards the plane, the higher should this current be This would be a fitting explanation on why the current is higher in Nytro 10 XN than in Marcol 52. There could possibly have been a similar step in Primol 352 if measurements had been extended to higher voltages This hypothesis of a moving charge layer will be in line with a streamer model where propagation is driven by the field created by the space charge itself, something that agrees with the fact that in Marcol 52, 2nd mode light and dark channels move fast
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