Initiation from a Point Anode in a Dielectric Liquid

Many previous studies of electrical breakdowns in dielectric liquids in point-plane geometry have examined the relationships among the breakdown structure (or speed), the electrode geometry (point radius, gap length), and/or voltage. A paper, “Initiation from a Point Anode in a Dielectric Liquid” by Minkyu Kim and Robert Hebner, which was published in the December 2006 issue of the IEEE Transactions on Dielectrics and Electrical Insulation, explores the hypothesis that, for streamers initiating from a point anode, the critical volume model used for similar geometry in gaseous dielectrics is useful in liquids. The assumption of the critical volume is shown to be consistent with experimental data. Specifically, a critical volume of 0.4 – 1.0 μm3 is consistent with the location of streamer initiation, with the independence of the initiation voltage for the 2nd anode mode from the tip radius for sharp tips, and the measured free paths of electrons in cyclohexane for the energies of interest. For further information, please contact Robert Hebner.

Some benefits of pulsed alternators as electromagnetic gun power supplies in power systems for future electric ship

An electromagnetic gun provides a new component to be integrated into the power system of an electric ship.  Electromagnetic guns have been fired successfully using either capacitors, inductors, or rotating machines to store the energy for the shot.  Because capacitors trail rotating machines by more than an order of magnitude in the stored energy density, fielded systems will likely use rotating machines even though other approaches are used in laboratory experiments.  An interesting topology for the power supply is a set of pulsed alternators with sufficient energy stored in the kinetic energy of the rotors to power a sequence of shots.  In this configuration, when no shot is required, which historically has been most of the life of the system, the machine topology is analogous to that of a flywheel battery used to provide ride-through capability and improve power quality in land-based power systems or to provide load leveling in hybrid vehicles.  This similarity raises the possibility that the power supply could be designed to serve multiple roles under conditions of ship operation.  The conceptual design of such a power supply of eight alternators storing a total of 800 MJ has been completed.  Megawatt-level motor-generators operating at speeds up to 15,000 rpm have been designed, constructed, and tested.  Simulations of future electric ship power systems show the utility of this storage for load leveling.  In addition, these components provide storage for an uninterruptible power supply for critical ship loads in the case of the loss of a generator.  An important unresolved problem is the design of bearings that will operate at high speeds reliably for long periods and survive the high levels of shock and vibration that will be encountered.  The paper, “Some benefits of pulsed alternators as electromagnetic gun power supplies in power systems for future electric ships,” by R. E. Hebner , J. A. Pappas, J. R. Kitzmiller, K. R. Davey, J. D. Herbst, A. Ouroua, and J. H. Beno was presented at the IMAREST Engine as a Weapon II International Symposium in London, UK, in December 2006. For further information, please contact Robert Hebner.

Higher Frequency Performance of Stress-grading Systems for HV Large Rotating Machines

The paper “Higher Frequency Performance of Stress-grading Systems for HV Large Rotating Machines,” by R. Hebner, H. El-Kishky, M. Abdel-Salam, and F. Brown and presented at the IEEE Conference on Electrical Insulation and Dielectric Phenomena in October 2006, presents the results of numerical simulation and analysis of the characteristics of stress-grading systems of high voltage rotating machines against frequency.  The stress-grading systems were optimally designed to minimize the local electric field enhancement at a discrete number of frequencies.  The performance characteristics of each systems were then generated and studied within a given frequency range.  The potential distribution characteristics as well as the local electric stress, surface resistance, and the power dissipation characteristics were generated and studied against frequency For further information, please contact Robert Hebner .

Examination of the Influence of Streamer Growth Criteria on Morphology

Work by Fowler, Daveny, and Hagedom showed that the morphology of an anode streamer could be modeled as stochastic growth of a branching fractal tree in point-plane geometry.  In their work, the experimentally observed range of fractal densities, from sparse to bushy, was modeled by using two assumptions.  One was that the growth was driven by En, where E is the local electric field and n is 1, 2, 3, or 4.  The other assumption is that there was a threshold (cutoff) electric field strength for streamer growth in a particular direction.  The investigation published in the paper, “Examination of the Influence of Streamer Growth Criteria on Morphology,” by Minkyu Kim, Robert E. Hebner, and Gary A. Hallock and presented at the IEEE Conference on Electrical Insulation and Dielectric Phenomena in October 2006, first reproduced the results of the earlier study to demonstrate that the model and its software implementation yielded the previous results.  The model was then modified to operate under a different set of assumptions.  In this case, only linear electric field dependence was assumed, and the number density of available electrons was used as a parameter to match the observed data.  In addition, rather than assume a sharp cutoff of the threshold field strength, the cutoff was assumed to be a function of electron density.  Under these assumptions, it was also possible to simulate the experimentally observed behavior of anode streamers.  The current assumptions are consistent with those that have proven to be useful in an earlier investigation. For further information, please contact Robert Hebner .

Minimization of Local Field Enhancement along Stress-grading systems of HV Large Rotating Machines

The paper “Minimization of Local Field Enhancement along Stress-grading systems of HV Large Rotating Machines,” by H. El-Kishky, R. Hebner, M. Abdel-Salam, and F. Brown and presented at the IEEE Conference on Electrical Insulation and Dielectric Phenomena in October 2006, presents an approach to the design of nonlinear stress-grading systems of high voltage rotating machines at different frequencies.  The parameters of stress-grading systems were selected to minimize the local electric field enhancement at a set of discrete operating frequencies.  The nonlinear stress-grading resistances were updated in response to the dynamic change of the local surface field compared to the desired average field.  This development allowed fast and accurate design and analysis of nonlinear stress-grading systems with the deviation of the highest local field from the desired average field kept very low over a frequency range of up to 300 Hz.  Simulation examples and several optimal stress-grading system characteristics were also presented.

For further information, please contact Robert Hebner .