The central element of a ship power system model is typically a circuit model. This level of modeling has been valuable in evaluating architectures for future electric ships as it provides initial power flow and stability information as well as performance specifications, perhaps most notably overall efficiency. This circuit model is typically the middle layer in what is a three layer approach. At the more basic level, the physics of the components and processes is captured. In a power system, there are interactions among the electromagnetic, mechanical, and thermal behaviors. The third level is even more approximate than the circuit level. It includes such models as cost-of-ownership models, models of physical layout and integration of the power system with the balance of plant. These models are critical in the design process and depend on the circuit model to specify components and their interconnections. Incorporating breakdown physics demonstrates the linkage between the basic physics and the circuit models. Rotor dynamics provides examples of phenomena that cannot be captured in a circuit model. The design of insulation systems is an example of a field in which the circuit modeling may lead to less costly electrical systems for future electric ships.
For the past decade, significant advances have been made in the modeling and simulation of the power system of electric ships. This growth is leading to an operational definition of what is meant by modeling of an electric ship power system. That progress in the field will be aided by an examination of this evolving definition is a premise of the paper, “Modeling of Electric Ship Power Systems,” coauthored by Abdelhamid Ouroua, Brian Murphy, John Herbst, and Robert Hebner and presented at the July 2009 Grand Challenges in Modeling and Simulation conference (part of the 2009 International Simulation Multiconference) in Istanbul, Turkey.
Schematic diagram of dc microgrid. The entire grid can be operated at 680 V dc. If the lower half is removed, the operating voltage can be increased to 1100 V dc.
Modeling and Simulation of Electric Propulsion Concepts for a Multimodal Prototype Demonstrator
The Office of Naval Research (ONR) is planning a prototype demonstration of a novel craft envisioned to have three modes of operation:
Fuel-efficient, good sea keeping mode for open ocean transits
High-speed, shallow water mode
Amphibious mode to enable “feet dry on the beach” capability
The disparate operating modes impose discrete requirements on the various elements of the ship propulsion system, opening a wide range of potential solutions. To meet these requirements, each of the contractor teams involved in the T-Craft program is investigating the potential use of advanced electric drive technologies, including medium voltage dc distribution systems. The Electric Ship Research and Development Consortium (ESRDC) conducted a technical evaluation of three electric drive options. The paper, “Modeling and Simulation of Electric Propulsion Concepts for a Multimodal Prototype Demonstrator, coauthored by John Herbst, Angelo Gattozzi, and John Uglum (UT), J. S. Chalfant and C. Chryssostomidis (MIT), and J. Langston, M. Steurer, and M. Andrus (Florida State University), and presented at the July 2009 Grand Challenges in Modeling and Simulation conference (part of the 2009 International Simulation Multiconference) in Istanbul, Turkey, discusses the modeling and simulation activities conducted during the initial technical evaluations of the electric drive concepts, as well as challenges and potential solutions for higher fidelity modeling of the complex propulsion power systems.
High temperature superconductor motor using bulk trapped field magnet.
Simplified Morton Effect Analysis for Synchronous Spiral Instability
It has been known for many years that thermal temperature gradients across a rotor can alter the amplitude and phase of synchronous vibration. This is due to bowing of the shaft caused by a temperature difference between one side of the shaft and the other. The earliest published recognition of this phenomenon was by Newkirk [1926, “Shaft Rubbing,” Mechanical Engineering, 48, pp. 830-832], where the temperature difference was a result of frictional heating from rubbing of the rotor on non-rotating elements of the machine. A rub of this type could involve labyrinth seals, armatures, impellers, slip rings, etc. Residual rotor imbalance produces vibration, which produces a rub, which generates heat locally on the shaft surface, which bows the shaft, which causes additional imbalance, which leads to increased vibration and heat, and so on. This could be described as a type of thermal runaway. Another very different way that a temperature difference in the shaft can be produced is via non-uniform viscous shearing action in the oil film of a journal bearing. This particular type of thermal action has come to be referred to as Morton Effect. Morton effect is a thermal condition which exists to various extents in all fluid film journal bearings.
A simplified analytical approach for modeling the synchronous instability phenomenon known as Morton effect was included in the paper, “Simplified Morton Effect Analysis for Synchronous Spiral Instability,” coauthored by Brian T. Murphy and Joshua A. Lorenz, and presented at the ASME 2009 Power Conference July 21-23, 2009 in Albuquerque, New Mexico.
The analysis is straight forward and easily applied to any rotor supported on fluid film bearings. The analysis in the paper clarifies the interaction of three distinct machine characteristics which combine to create a case of Morton effect. Some example calculations are shown illustrating the possible types of spiral vibration.
In addition, an analytical approach is described for estimating the magnitude of the shaft temperature difference in a journal bearing as a direct function of the shaft orbit. It is significant that this method can readily be applied to any type of journal bearing, from plain sleeve bearings to tilting pad bearings.
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