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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities 5 High-power Switching Technologies INTRODUCTION The power distribution network of combat hybrid power systems (CHPS) couples all power electronic systems and other components together, including both continuous and pulsed power systems (see Figure 1-1). This network is mediated by high-power switches that act to limit the interactions among the components. Such interactions can affect the stability and operation of the power delivery platform, hinder fault tolerance strategies, and increase the vulnerability to electrical and magnetic pulses, both from the CHPS’s own and from hostile pulsed power systems. Improved reliability of the power distribution network requires that system issues be addressed. The scope of this evaluation includes the power distribution system and the related power electronics of the CHPS platform. It does not consider the power and high-power switching requirements of specific weapon or defense systems. Existing switching technologies can meet the electrical demands of CHPS, but there are challenges related to increasing converter power density, decoupling of pulse loads, and developing more fault-tolerant architectures as weapon and defense systems are added. CONVERTER POWER DENSITY The highest-priority effort should be focused on increasing the converter power density by increasing switching frequencies and the level of component integration. As power densities increase, thermal issues also become extremely important and vital for reliable systems. The results of such an effort have application well beyond the CHPS platform and have the potential to enhance the competitive position of the national power electronic industry. Long-term Technology Needs The long-term technology needs are related to the problems of high-frequency operation, component integration, thermal management, and design tools. An increase in switching frequencies implies reduced size of components such as transformers and filters, which greatly increases the power density. On the other hand, high-frequency
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities switching magnifies the effects of system parasitic losses, which can contribute to component failures. Component integration contributes to higher power densities and greater reliability. With reduction of connections between components through integration, a major failure mode is reduced. The key components for integration are: Filter inductance and capacitance, High-frequency transformers, Sensors and gate drives, and Lowest levels of fast real-time control. Design tools for three-dimentional thermal management, packaging, system design, and manufacturability are needed to ensure that these issues are integrated in the core design. Currently, such tools are not available at the level necessary to achieve the desired power density. Performance Metrics The committee suggests that the most appropriate performance metrics for improved converter technology are these: high power density, a reduction in manufacturing complexity, and a reduction in the length of the device design and verification cycles. Research Priorities The committee suggests several research priorites for achieving higher power density converters. These are: developing processes for integrating components (e.g., distributed L and C components with active devices) and integration of high-frequency transformers; developing innovative thermal management techniques for low duty cycle power electronics; and developing an integrated computer-aided design (CAD) tool with mechanical, electromagnetic, and thermal parameter extraction. POWER ELECTRONICS FOR PULSE ENERGY STORAGE Pulse energy storage power electronics is an issue uniquely related to CHPS. The nature of pulse power requires that it be buffered from the power platform to prevent major electrical problems. Solutions include flywheel storage with related power electronics or power electronic systems with static storage.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Long-term Technology Needs There are several long-term issues related to moving power from the Combat Hybrid Power System platform to the weapons systems. Assuming that the weapons systems are pulse power loads, there is a potential that these loads will interfere with the other loads on the electrical platform through voltage drop and/or current surges. Adequate decoupling of pulse loads from the power distribution system is required through the use of high-voltage, high-current switches. Performance Metrics The most suitable performance metrics for these switches are high current density and a high level of decoupling. Research Priorities Currently, power electronics interfaces with energy storage devices are typically configured to provide for either high or low power flow capability. To increase system flexibility, power electronics interfacing with energy storage devices should be developed to provide bimodal power flow; that is, slow versus fast changes in power transfer. Interfacing systems also need to be developed with flexibility in configuring output voltage/current waveforms to meet the requirements of various weapon systems. FAULT-TOLERANT ARCHITECTURES The power distribution network couples all power electronic systems and other components, including such items as generators, storage systems, and traction motors. The interactions among subsystems may cause system performance degradation, or even instability and failures. These interactions also increase the sensitivity of the platform to component failures or damage. Implementation of a DC distribution system with a large number of parallel-connected components requires solving the problem of fast control without reliance on communication. For example, when a load changes its power demand, the energy sources must change their power output to meet the demand without communications. A power delivery system that depends on a single power component, such as a distribution bus, invites total collapse due to unplanned faults and component failures. Basic platform architectures need to be developed that allow for graceful degradation of mission-critical systems under faults and component failures.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Long-term Technology Needs A more fundamental understanding is needed of the factors that contribute to—or detract from—the stability of complex hybrid power distribution systems, especially a system that combines continuous and pulse power elements. Particularly vulnerable is the DC bus that provides the backbone of the distribution system. Special efforts to battle-harden the DC bus, such as isolating various sections and providing redundancy for system-critical elements, will be needed to create an architecture with graceful degradation of mission-critical systems under fault conditions. Performance Metrics The most appropriate performance metric of fault-tolerant power distribution architectures is the level of functionality that the system displays under unplanned faults and component failures. Research Priorities In general, a broader understanding of the factors affecting system stability should lead to design strategies for graceful degradation of systems. More specifically, dynamic models are needed for power converter interactions at the DC bus, as well as the development of controls that mitigate instabilities on the DC bus. SUMMARY A summary of the technical challenges and opportunities for improvement of high-power switching technologies is given in Table 5-1. TABLE 5-1 Technical Challenges, Performance Metrics, and Research Priorities Associated with the Application of High-Power Switching Technologies to Combat Hybrid Power Systems System/Component Technical Challenge Performance Metrics R&D Priorities Power converters Higher power densities, switching frequencies, and greater reliability High power density Manufacturing simplicity Reduced design and verification cycle times Processes for integration of distributed components with active devices Design tools for three-dimensional thermal management, packaging, system design, and manufacturability
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Power electronics for pulse energy storage Effective decoupling of pulse loads from the power distribution system High current density High level of decoupling Development of storage system interfaces with bimodal (slow and fast) power transfer capability Development of interfaces with flexibility to tailor output voltage/current waveforms to requirements of weapons systems Power distribution network Mission-critical systems that degrade gracefully under fault conditions Level of functionality under unplanned faults and component failures Fundamental understanding of factors affecting system stability Dynamic models of power converter interactions at the DC bus Controls that mitigate instabilities on the DC bus BIBLIOGRAPHY Cieski, J.G. and RW. Ashton. 2000. Selection and Stability Issues Associated with a Navy Shipboard DC Zonal Electric Distribution System. Institute of Electrical and Electronics Engineers, Inc (IEEE) Transactions on Power Delivery, Volume 15, Issue 2 (April): 665-669. Jitaru, I.D. and A. Ivascu. 1997. Quasi-integrated Magnetic: an Avenue for Higher Power Density and Efficiency in Power Converters. Applied Power Electronics Conference and Exposition, Volume I: 395-402 Katayama, Y., S. Sugahara, H. Nakazawa, and M. Edo. 2000. High-Power-Density MHz-Switching Monolithic DC-DC Converter with Thin-Film Inductor. Power Electronics Specialists Conference , Volume III: 1485-1490. Lasseter, R. H., Y. Shern, and S. G. Jalali. 1995. "Hannonic Instabilities in Static V AR Compensators, Power Electronics in Power Systems (CIGRE), Tokyo, Japan, May 22-24. Mazumder, S.K. and K. Shenai. 2002. On the Reliability of Distributed Power Systems: a Macro- to Micro- Level Overview Using a Parallel DC-DC Converter. Power Electronics Specialists Conference, Volume II: 809-814. Zhang, M.T., M.M. Jovanovic, and F. C. Lee. 1997. Design and Analysis of Thermal Management for High-power-density Converters in Sealed Enclosures. Applied Power Electronics Conference and Exposition, Volume I: 405-412.
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