Publications

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Several technical power system architectures are being evaluated for the Navy's next generation all-electric warship. One concept being considered includes a scheme to power both port and starboard busses from a single generator with dual-windings. This approach offers redundancy and reduces the effects of prime mover light loading, but it inherently couples the two busses through the common generator. In this work, dynamic issues of galvanic and electro-mechanical coupling of power systems through a single dual-wound generator are discussed. Previous works focused on harmonics and galvanic coupling. Herein, focus is placed on average-value modeling of the galvanic coupling and on evaluation for fault response. Conclusions are presented from analysis, simulation, and experimental results.

In most distributed power electronic systems, the transmission line effects associated with cabling are neglected due to the expectation that cables are sufficiently short to be modeled as a lumped parameter model. However, as converter switching speeds and control bandwidth increase, especially in large distributed power electronic based systems, the transmission line effects may become an important consideration when establishing margins of stability. In this work, immittance based stability analysis is applied to power electronic systems with long cables between source and load converter. In particular, the Energy Systems Analysis Consortium (ESAC) method is utilized to compute limits on cable length so as to maintain prescribed stability margins. Simulation results are presented in support of the approach.

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With the next generation of semiconductor materials in development, significant strides in the Size, Weight, and Power (SWaP) characteristics of power conversion systems are presently underway. In particular, much of the improvements in system-level efficiencies and power densities due to wide-bandgap (WBG) and ultra-wide-bandgap (UWBG) device incorporation are realized through higher voltage, higher frequency, and higher temperature operation. Concomitantly, there is a demand for ever smaller device footprints with high voltage, high power handling ability while maintaining ultra-low inductive/capacitive parasitics for high frequency operation. For our work, we are developing small size vertical gallium nitride (GaN) and aluminum gallium nitride (AlGaN) power diodes and transistors with breakdown and hold-off voltages as high as 15kV. The small size and high power densities of these devices create stringent requirements on both the size (balanced between larger sizing for increased voltage hold-off with smaller sizing for reduced parasitics) and heat dissipation capabilities of the associated packaging. To accommodate these requirements and to be able to characterize these novel device designs, we have developed specialized packages as well as test hardware and capabilities. This work describes the requirements of these new devices, the development of the high voltage, high power packages, and the high-voltage, high-Temperature test capabilities needed to characterize and use the completed components. In the course of this work, we have settled on a multi-step methodology for assessing the performance of these new power devices, which we also present.

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With increasing renewable penetrations and advancements in power electronics associated with smart grid technologies, distributed control of the power grid is quickly becoming a necessity. Once communications are introduced into a control system, the impacts of latency and unreliable communications quickly become a priority. Vector Lyapunov techniques are well suited for the analysis of control systems with structured perturbations. These perturbations can be employed to model uncertainty in communications as well as parameter uncertainty. In this paper, we present results for small signal stability of a simplified two area power system model for several scenarios: bandwidth limited local communications and tie line uncertainty; local communications and bandwidth limited global communications combined with tie line uncertainty; and uncertainty in global communications. These results are intended to be a starting point for the analysis of the impact of communications uncertainty on the stability of power systems.

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Future U.S. Navy ships will require power systems that meet more stringent agility, efficiency, scalability, controllability and resiliency requirements. Modularity and the ability to interconnect power systems having their own energy storage, generation, and loads is an enabling capability. To aid in the design of power system controls, much of what has been learned from advances in the control of networked microgrids is being applied. Developing alternative methods for controlling and analyzing these systems will provide insight into tradeoffs that can be made during the design phase. This paper considers the problem of electric ship power disturbances in response to pulsed loads, in particular, to electromagnetic launch systems. Recent literature has indicated that there exists a trade-off in information and power flow and that intelligent, coordinated control of power flow in a microgrid system (i.e. such as an electric ship) can modify energy storage hardware requirements. The control presented herein was developed to provide the necessary flexibility with little computational burden. It is described analytically and then demonstrated in simulation and hardware.

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The goal of this effort was to apply four potential control analysis/design approaches to the design of distributed grid control systems to address the impact of latency and communications uncertainty with high penetrations of photovoltaic (PV) generation. The four techniques considered were: optimal fixed structure control; Nyquist stability criterion; vector Lyapunov analysis; and Hamiltonian design methods. A reduced order model of the Western Electricity Coordinating Council (WECC) developed for the Matlab Power Systems Toolbox (PST) was employed for the study, as well as representative smaller systems (e.g., a two-area, three-area, and four-area power system). Excellent results were obtained with the optimal fixed structure approach, and the methodology we developed was published in a journal article. This approach is promising because it offers a method for designing optimal control systems with the feedback signals available from Phasor Measurement Unit (PMU) data as opposed to full state feedback or the design of an observer. The Nyquist approach inherently handles time delay and incorporates performance guarantees (e.g., gain and phase margin). We developed a technique that works for moderate sized systems, but the approach does not scale well to extremely large system because of computational complexity. The vector Lyapunov approach was applied to a two area model to demonstrate the utility for modeling communications uncertainty. Application to large power systems requires a method to automatically expand/contract the state space and partition the system so that communications uncertainty can be considered. The Hamiltonian Surface Shaping and Power Flow Control (HSSPFC) design methodology was selected to investigate grid systems for energy storage requirements to support high penetration of variable or stochastic generation (such as wind and PV) and loads. This method was applied to several small system models.

The anticipated high penetration of distributed photovoltaic (PV) energy sources is expected to lead to significant changes in utility interconnection requirements for PV systems. These changes will include provisions for voltage and frequency regulation capability, as well as better voltage and frequency ride through requirements. For distributed energy resources (DER), in particular PV, to provide grid support, it must participate in frequency and voltage regulation. Frequency and voltage ride through allows inverters to remain connected to ensure robust recovery in the event of voltage and frequency disturbance. Implementing these advanced capabilities is essential to mitigating the negative impacts of high penetration PV, but their integration into a typical distribution system presents significant technical challenges, one of which is the increased risk of unintentional islanding. In this paper, an island detection method is presented that relies on a continuous subharmonic signal, a power line carrier permissive (PLCP), that is injected at the transmission level or at the substation and detected by any type of DERs in any combination. Absence of the signal indicates loss of utility and possible island condition. Laboratory and simulation experiments were done to investigate feasibility of the method. The PLC system discussed herein is novel in that it utilizes a power electronics based series voltage injection method. Advantages include the ability to use a smaller and less expensive transformer and enhanced flexibility in the amplitude, waveform and frequency of the injected signal.

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