Compact, Pulsed Power - Liquid Dielectric
Objectives
Since the beginnings of pulsed power, liquids have played a central role as dielectric material in energy storage, pulse forming, and switching. The latter utilizes liquid breakdown at high fields, thus going beyond just the insulating property of liquids. However, in most cases (energy storage, pulse forming) breakdown is detrimental to the proper functioning of a pulsed power system. It is also obvious that the size of a pulsed power system has always been a concern, and the effort of making existing systems even more compact will have to deal with the higher electric fields that come with the inherently smaller distances of a compact system.
Though an abundance of experimental data is available for breakdown in various insulating liquids, such as water, cyclohexane, and the noble gases, the basic physics of liquid breakdown has remained unclear. The two classic physical models are the crack propagation, which was recently transferred from solid breakdown, and the older bubble mechanism. It is the objective of this experimental investigation to further our understanding of liquid breakdown on a nanosecond timescale, thus paving the way to developing breakdown models with a-priori prediction capabilities.
Status of Effort
Two manuscripts have been generated in 2006 that describe the progress in research effort (1 conference contribution, 1 manuscript accepted, 1 submitted waiting for review). Another graduate student, who was 100% supported by this MURI, has received his PhD in 2006.
The past year's efforts have been focused on identifying and quantifying the differences between cathode initiated and anode initiated breakdown. Many researchers have observed differences between these two breakdown processes, but we have made an effort identifying the physical mechanisms between the two. We have also conducted several experiments where the conditions of breakdown are "artificially" (i.e. pressure, temperature, etc) enhanced for one process or the other to show exaggerated changes in breakdown proecess.
Accomplishments/New Findings
A summary of the experimental results for each experiment is shown below in the following figures. Figure 1 illustrates the dependence of pressure on breakdown voltage. The negative needle (Blue) shows a strong dependence on pressure while the positive needle case shows little or no dependence on pressure. This helps support the theory of bubble formation for cathode initiated breakdown and an electronic process for anode initiated breakdown.
Figure 2 is more qualitative and shows high speed images of shadowgraphy with overlaid luminosity of DC Breakdown in Univolt 60 oil for positive and negative needles. Important observations to make in these images is the general shape of the breakdown channels for both breakdowns (cathode initiated is "bushy" and has less branching, while the anode is narrow and has more branching. More interesting is the increased light emission from the cathode initiated breakdown. In contrast, the total light output of the anode initiated breakdown is many times less.
The final two graphs shown in Figure 3A&B show the time-to-breakdown for pulsed voltage application of positive and negative needles at atmosphere and partial vacuum. Again, the data shows a strong dependence on pressure for the cathode initiated breakdown and little dependence for the anode initiated breakdown.
Figure 1 Breakdown voltage vs pressure for Positive (Red) and Negative (Blue) Needle in Univolt 60 Oil
Figure 2. Shadowgraphy and Overlaid Luminosity – of DC Breakdown in Univolt 60 Oil for Positive and Negative Needles.
(A)
(B)
Figure 3. Dependence of Time to Breakdown for Positive and Negative Needle
Texas Tech University
Journal Papers
M. Cevallos, M. Butcher, J. Dickens, A. Neuber, H. Krompholz, "Imaging of Negative Polarity DC Breakdown Streamer Expansion in Transformer Oil due to Variations in Background Presssure", IEEE Transactions on Plasma Science, vol. 33, 494-495, 2005.
M. Butcher, A. Neuber, M. Cevallos, J. Dickens, H. Krompholz "Conduction and Breakdown Mechanisms in Transformer Oil", IEEE Transactions on Plasma Science, Volume 34, Issue 2, Part 3, April 2006 Page(s):467 - 475.
J. Qian, R. P. Joshi, J. Kolb, and K. H. Schoenbach, J. Dickens, A. Neuber, M. Butcher, M. Cevallos, and H. Krompholz, E. Schamiloglu and J. Gaudet, "Microbubble-based model analysis of liquid breakdown initiation by a submicrosecond pulse" J. Appl. Phys. 97, 113304, 2005.
M. Cevallos, M. Butcher, J. Dickens, A. Neuber, H. Krompholz, "Bubble Dynamics and Channel Formation for Cathode Initiated Discharges in Transformer Oil" (in review)
M. Cevallos, M. Butcher, J. Dickens, A. Neuber, H. Krompholz, "Composite
Shadowgraphy and Luminosity Images of Self Breakdown Discharge Channels in
Transformer Oil" submitted to IEEE Transactions (Feb 2006)
Papers in Conference Proceedings
M.D. Cevallos, M.D. Butcher, J.C. Dickens, A.A. Neuber, H.G. Krompholz, "Bubble Dynamics and Channel Formation for Cathode Initiated Discharges in Transformer Oil", to be published in Proceedings of the 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005.
J. Qian, R.P. Joshi, J.F. Kolb, K.H. Schoenbach, J. Dickens, A. Neuber, M. Butcher, M. Cevallos, H. Krompholz, E. Schamiloglu, J. Gaudet, "Simulation Studies of Liquid Water Breakdown by a Sub-Microsecond Pulse", to be published in Proceedings of the 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005
M.D. Cevallos, M.D. Butcher, J.C. Dickens, A.A. Neuber, H.G. Krompholz, "Composite Shadowgraphy and Luminosity Images of Self Breakdown Discharge Channels in Transformer Oil", to be published in Proceedings of the 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005.
M. Butcher, M. Cevallos, A. Neuber, H. Krompholz, J. Dickens, "Investigation of Charge Conduction and Self-Breakdown in Transformer Oil", to be published in Proceedings of the 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005.
J. Dickens, A. Neuber, M. Kristiansen, "Pulsed and DC Breakdown in Liquids", to be published in the Proceedings of The First Euro-Asian Pulsed Power Conference to be held on September 18-22, 2006.
Presentations
M.D. Cevallos, M.D. Butcher, J.C. Dickens, A.A. Neuber, H.G. Krompholz, "Bubble Dynamics and Channel Formation for Cathode Initiated Discharges in Transformer Oil", 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005.
M.D. Cevallos, M.D. Butcher, J.C. Dickens, A.A. Neuber, H.G. Krompholz, "Composite Shadowgraphy and Luminosity Images of Self Breakdown Discharge Channels in Transformer Oil", 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005.
M. Butcher, M. Cevallos, A. Neuber, H. Krompholz, J. Dickens, "Investigation of Charge Conduction and Self-Breakdown in Transformer Oil", 15th Int. IEEE Pulsed Power Conference, Monterey, CA, June 13-17, 2005.
