Electric power system concepts for integration of advanced sensor and pulsed loads in the DDG-51 class ships

Advanced weapons and sensors increase demand on the electric power systems of Navy surface combatants, driving the need for fully Integrated Power Systems (IPS) such as those found in the DDG-1000 Zumwalt class of ships. The goal of this paper is to introduce novel power system configurations that could potentially be integrated into future flights of the DDG-51 class to support expanded electric power system capability at reasonable cost. Two concepts are presented: the first addresses the need for additional power for advanced sensor systems and the second addresses the need for a more significant increase in capacity to support higher power electric loads.Center for Electromechanic

High speed induction motor and inverter drive for flywheel energy storage

The use of flywheels to store energy is a technology which is centuries old. The confluence of several modern technologies has resulted in flywheels becoming a viable solution for the needs of the transportation, electric utility, and aerospace industries. This paper discusses a high-speed induction motor and its associated inverter drive which were developed for the Federal Railroad Administration’s “Advanced Locomotive Propulsion System.” The design of the induction motor provided several significant challenges. A megawatt rated, 12,000 rpm motor operating at a rotor surface velocity speed of 230 m/s required a unique mechanical configuration to withstand the centrifugal forces as well as an electromagnetic design, which produced a high efficiency at 200 Hz. Extending the design practices used in smaller motors would not achieve the goals required for a megawatt size machine. Similarly, the inverter was developed using a soft switching technique in order to meet the demands of high power output in a compact package. Application requirements, electrical and mechanical features of the motor, design strategy for the inverter, and test results are all presented in this paper.Center for Electromechanic

MVDC and HFAC Electric Power System Architectures for the Transformable Sea Base Connector (T-Craft)

The paper presents high frequency ac (HFAC) and medium voltage dc (MVDC) power system architectures for a notional T-Craft concept design and provides a qualitative comparison of relevant performance parameters. In particular the following items are discussed as worthy of attention in view of the large potential benefits they could produce: Risk mitigation in operating multiple series and parallel connected power conversion modules for both rectification and inversion at multi-megawatt power levels and high peak operating frequency. Synchronous drive topology offering the potential benefit of eliminating the need for multi-megawatt power conversion without compromising capabilities or performance: this would significantly reduce risk by eliminating much of the power electronics and the attendant size, weight and cost. Circuit protection strategies and components, for either HFAC or MVDC, for a flexible architecture suitable for fault management and reconfiguration. Integration of compact, lightweight gearboxes, either conventional planetary or advanced magnetic gear type, with existing designs for high-speed motors at the power level required by T-Craft. Critical technical issues for each power system architecture are identified and proposed simulation and technology development activities are described.Center for Electromechanic

Intelligent Microgrid Demonstrator

The Center for Electromechanics at the University of Texas at Austin has a microgrid demonstrator capable of operating at power levels up to 2 MW. The microgrid is designed to interface with the commercial power system and other distributed sources of electric power, both ac and dc. It incorporates local prime movers that drive generators, both conventional 60 Hz and high frequency ac (800 Hz), as well as energy storage units. The microgrid is designed as a flexible system, easily expandable in power and re-definable in terms of voltage levels and system configuration. Several grid architectures have been studied and simulated, including a variety of pulsed and intermittent duty loads, as they can be found on US Navy ships. One problem under investigation is that of dc bus stability under the influence of constant power loads: these loads are known to induce instability at kW power levels but experimental data is very limited at MW power levels which are more representative of the situation on a ship. This paper describes the microgrid architecture, simulation results and data collected to date, as well as plans to demonstrate critical issues like stability, reconfigurability, fault management, integration of renewables, and model validation.Center for Electromechanic

Modeling and Simulation Roadmap to Enhance Electrical Energy security of U.S. Naval Bases

The enhancement of the electrical energy security of Navy bases within the continental U.S. has been studied using an approach based on modeling and simulation, with the intent to achieve real time control and energy management. The commercially available software packages are reviewed and the most suitable ones are indicated. A notional base was developed and used for this study. Its format is general enough to be able to be used as a template for each specific Navy installation. Several preliminary simulations using commercial software have been performed on the power system of this notional base addressing a variety of operating scenarios including islanded mode and the impact of wind and solar power sources. Results of steady state and transient operation are also reviewed. The results of these simulations indicate that existing software packages available today, with proper adaptations, can provide a well validated and consistent process for evaluating power system architectures and technologies and, therefore, can become a valuable tool for the implementation of the described plan for Navy bases. The study concludes giving the details of the roadmap to move forward in the process of energy security enhancement of U.S. Navy bases through modeling and simulation.Center for Electromechanic

Analytical Description of a Series Fault on a DC Bus

The solution of the equations of a dc circuit containing an arc is given and compared with experimental data. The arc is modeled according to its classical equivalent circuit and the adequacy of this model is discussed. The analytical solution for the circuit with an opening gap is given for the case of a constant gap and the results are extended to the cases of a gap opening with uniform velocity and a gap opening with constant acceleration, under the assumption of a quasi-static approximation for which the limits of applicability are estimated. Voltage and current evolutions in time are derived, including an estimate of the arc duration and quenching time. The results are compared to experimental data. Also provided is a generalized view of the transient behavior of an arc in a circuit that extends the description commonly used, in terms of only a voltage-current relationship, by also including inductive effects.Center for Electromechanic

Challenges in the Design of a 100 kW Induction Motor for a PHEV Application

This paper summarizes some design challenges encountered in the development of a high speed induction motor/generator for a plug-in hybrid electric vehicle (PHEV). The traction system motor/generator was developed for integration into a high performance full-sized passenger car being developed by a major automotive manufacturer. The paper summarizes the traction motor performance requirements and presents the electromagnetic, mechanical and thermal design issues of the high speed 100 kW peak power induction machine.Center for Electromechanic

Summing up the perturbation series in the Schwinger Model

Perturbation series for the electron propagator in the Schwinger Model is summed up in a direct way by adding contributions coming from individual Feynman diagrams. The calculation shows the complete agreement between nonperturbative and perturbative approaches.Comment: 10 pages (in REVTEX

Low-mass pre--main-sequence stars in the Magellanic Clouds

[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar stars form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in star-forming regions. The low-mass stars in such regions are still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature of these objects and the contamination of their samples by the evolved populations of the Galactic disk impose demanding observational techniques for the detection of complete numbers of PMS stars in the Milky Way. The Magellanic Clouds, the companion galaxies to our own, demonstrate an exceptional star formation activity. The low extinction and stellar field contamination in star-forming regions of these galaxies imply a more efficient detection of low-mass PMS stars than in the Milky Way, but their distance from us make the application of special detection techniques unfeasible. Nonetheless, imaging with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of such objects are identified as the low-mass stellar content of their star-forming regions, changing completely our picture of young stellar systems outside the Milky Way, and extending the extragalactic stellar IMF below the persisting threshold of a few solar masses. This review presents the recent developments in the investigation of PMS stars in the Magellanic Clouds, with special focus on the limitations by single-epoch photometry that can only be circumvented by the detailed study of the observable behavior of these stars in the color-magnitude diagram. The achieved characterization of the low-mass PMS stars in the Magellanic Clouds allowed thus a more comprehensive understanding of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4 figures. Accepted for publication in Space Science Review

The composition of the protosolar disk and the formation conditions for comets

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today. This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.Comment: To appear in Space Science Reviews. The final publication is available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-