As a part of NASA's efforts in space, options are being examined for an Artemis moon base project to be deployed. This project requires a system of interconnected, but separate, DC microgrids for habitation, mining, and fuel processing. This in-place use of power resources is called in-situ resource utilization (ISRU). These microgrids are to be separated by 9-12 km and each contains a photovoltaic (PV) source, energy storage systems (ESS), and a variety of loads, separated by level of criticality in operation. The separate microgrids need to be able to transfer power between themselves in cases where there are generation shortfall, faults, or other failures in order to keep more critical loads running and ensure safety of personnel and the success of mission goals. In this work, a 2 grid microgrid system is analyzed involving a habitation unit and a mining unit separated by a tie line. A set of optimal controls that has been developed, including power flow controls on the tie line, dispatch of PV generation, and dispatch of non-critical loads, is analyzed, and validated in hardware on the Secure Scalable Microgrid Testbed (SSMTB). This testbed includes hardware emulators for a variety of energy sources, energy storage devices, pulsed loads, and other loads.
A multiple input multiple output (MIMO) power line communication (PLC) model for industrial facilities was developed that uses the physics of a bottom-up model but can be calibrated like top-down models. The PLC model considers 4-conductor cables (three-phase conductors and a ground conductor) and has several load types, including motor loads. The model is calibrated to data using mean field variational inference with a sensitivity analysis to reduce the parameter space. The results show that the inference method can accurately identify many of the model parameters, and the model is accurate even when the network is modified.
A high altitude electromagnetic pulse (HEMP) or other similar geomagnetic disturbance (GMD) has the potential to severely impact the operation of large-scale electric power grids. By introducing low-frequency common-mode (CM) currents, these events can impact the performance of key system components such as large power transformers. In this work, a solid-state transformer (SST) that can replace susceptible equipment and improve grid resiliency by safely absorbing these CM insults is described. An overview of the proposed SST power electronics and controls architecture is provided, a system model is developed, and the performance of the SST in response to a simulated CM insult is evaluated. Compared to a conventional magnetic transformer, the SST is found to recover quickly from the insult while maintaining nominal ac input/output behavior.
A high altitude electromagnetic pulse (HEMP) or other similar geomagnetic disturbance (GMD) has the potential to severely impact the operation of large-scale electric power grids. By introducing low-frequency common-mode (CM) currents, these events can impact the performance of key system components such as large power transformers. In this work, a solid-state transformer (SST) that can replace susceptible equipment and improve grid resiliency by safely absorbing these CM insults is described. An overview of the proposed SST power electronics and controls architecture is provided, a system model is developed, and the performance of the SST in response to a simulated CM insult is evaluated. Compared to a conventional magnetic transformer, the SST is found to recover quickly from the insult while maintaining nominal ac input/output behavior.
Structural modularity is critical to solid-state transformer (SST) and solid-state power substation (SSPS) concepts, but operational aspects related to this modularity are not yet fully understood. Previous studies and demonstrations of modular power conversion systems assume identical module compositions, but dependence on module uniformity undercuts the value of the modular framework. In this project, a hierarchical control approach was developed for modular SSTs which achieves system-level objectives while ensuring equitable power sharing between nonuniform building block modules. This enables module replacements and upgrades which leverage circuit and device technology advancements to improve system-level performance. The functionality of the control approach is demonstrated in detailed time-domain simulations. Results of this project provide context and strategic direction for future LDRD projects focusing on technologies supporting the SST crosscut outcome of the resilient energy systems mission campaign.
DC microgrids envisioned with high bandwidth communications may well expand their application range by considering autonomous strategies as resiliency contingencies. In most cases, these strategies are based on the droop control method, seeking low voltage regulation and proportional load sharing. Control challenges arise when coordinating the output of multiple DC microgrids composed of several Distributed Energy Resources. This paper proposes an autonomous control strategy for transactional converters when multiple DC microgrids are connected through a common bus. The control seeks to match the external bus voltage with the internal bus voltage balancing power. Three case scenarios are considered: standalone operation of each DC microgrid, excess generation, and generation deficit in one DC microgrid. Results using Sandia National Laboratories Secure Scalable Microgrid Simulink library, and models developed in MATLAB are compared.
Optimized designs were achieved using a genetic algorithm to evaluate multi-objective trade space, including Mean-Time-Between-Failure (MTBF) and volumetric power density. This work provides a foundational platform that can be used to optimize additional power converters, such as an inverter for the EV traction drive system as well as trade-offs in thermal management due to the use of different device substrate materials.
In power electronic applications, reliability and power density are a few of the many important performance metrics that require continual improvement in order to meet the demand of today's complex electrical systems. However, due to the complexity of the synergy between various components, it is challenging to visualize and evaluate the effects of choosing one component over another and what certain design parameters impose on the overall reliability and lifetime of the system. Furthermore, many areas of electronics have realized remarkable innovation in the integration of new materials of passive and active components; wide-bandgap semiconductor devices and new magnetic materials allow higher operating temperature, blocking voltage, and switching frequency; all of which enable much more compact power converter designs. However, uncertainty remains in the overall electronics reliability in different design variations. Hence, in order to better understand the relationship between reliability and power density in a power electronic system, this paper utilizes a genetic algorithm (GA) to provide pareto optimal solution sets in a multi-variate trade space that relates the Mean Time Between Failures (MTBF) and volumetric power density for the design of a 5 kW synchronous boost converter. Different designs of the synchronous boost converter based on the variation of the electrical parameters and material types for the passive (input and output capacitors, the boost inductor, and the heatsink) and active components (switches) have been studied. A few candidate designs have been evaluated and verified through hardware experiments.
Power systems with highly flexible architectures (i.e. permitting many configurations) may allow for more economic operation as well as improved reliability and resiliency. The greater number of configurations enable optimization for attaining the former benefit and redundancy for achieving the latter. Flexibility is of great importance in electric ship power systems wherein the system must ensure delivery of power to vital loads. The United States (US) Navy is currently investigating new architectures that enable a greater number of interconnection permutations. Among the new features considered are generators that may supply two buses; this may be done using conventional (single winding set) generators and two rectifiers or a dual wound machine with two rectifiers. In systems supplied by dual-wound machines, buses may not be tied directly but are linked dynamically through the shared generator dynamics. In systems with conventional generation supplying two rectifiers, the two buses are tied through a common AC bus supplying both rectifiers. This paper presents a comparison of these two approaches of supplying two buses from one generator; the evaluation considers issues associated with dynamic coupling through these two candidate architectures, including the coupled response due to faults and systems with pulsed loads. Results are based on analysis, simulation results, and hardware experiment.
This paper describes the design of a very high power density inverter drive module using aggressive high-frequency design methods and multi-objective optimization tools. This work is part of a larger effort to develop electric drive designs with >97% efficiency, power densities of 100 kW/L for the power electronics, and with predicted reliable operation to 300, 000 miles. The approach taken in this work is to develop designs that utilize wide band gap devices (SiC or GaN) and ceramic capacitors to enable high-frequency switching and a compact integrated design. The multi-objective optimization is employed to select key parameters for the design.
The U.S. Navy is investing in the development of new technologies that broaden warship capabilities and maintain U.S. naval superiority. Specifically, Naval Sea Systems Command (NAVSEA) is supporting the development of power systems technologies that enable the Navy to realise an all-electric warship. A challenge to fielding an all-electric power system architecture includes minimising the size of energy storage systems (ESS) while maintaining the response times necessary to support potential pulsed loads. This work explores the trade-off between energy storage size requirements (i.e. mass) and performance (i.e. peak power, energy storage, and control bandwidth) in the context of a power system architecture that meets the needs of the U.S. Navy. In this work, the simulated time domain responses of a representative power system were evaluated under different loading conditions and control parameters, and the results were considered in conjunction with sizing constraints of and estimated specific power and energy densities of various storage technologies. The simulation scenarios were based on representative operational vignettes, and a Ragone plot was used to illustrate the intersection of potential energy storage sizing with the energy and power density requirements of the system. Furthermore, the energy storage control bandwidth requirements were evaluated by simulation for different loading scenarios. Two approaches were taken to design an ESS: one based only on time domain power and energy requirements from simulation and another based on bandwidth (specific frequency) limitations of various technologies.
The Hawaiian Electric Company intends to procure grid-scale Battery Energy Storage System (“BESS”) capacity. The purpose of this study is to determine whether providing contingency reserve or time-of-day shifting is of more benefit to the Oahu grid, and to better understand the relationship between BESS size and level of benefit. This is an independent study by Sandia, and is not being used to support the regulatory case for BESS capacity by Hawaiian Electric. The study team created a production cost model of the Oahu grid using data primarily from the Hawaiian Electric Company. The proposed BESS supplied contingency reserve in one set of runs and time-of-day shifting in another. Supplying contingency reserve led to larger savings than time-of-day energy shifting. Assuming a renewable reserve and a quick-start reserve, and $15/MMBtu for Low-Sulphur Fuel Oil, the 50-MW/25-MWh, 100-MW/50-MWh, and 150-MW/75-MWh systems supplying contingency reserve provided, respectively, savings of 9.6, 15.6, and 18.3 million USD over system year 2018. Over the range of fuel prices tested, these cost savings were found to be directly proportional to the cost of fuel. As the focus is the operational benefit of BESS capacity, the capacity value of the BESS was not included in benefit calculations.
Many candidate power system architectures are being evaluated for the Navy’s next generation all-electric warship. One proposed power system concept involves the use of dual-wound generators to power both the Port and Starboard side buses using different 3-phase sets from the same machine (Doerry, 2015). This offers the benefit of improved efficiency through reduced engine light-loading and improved dispatch flexibility, but the approach couples the two busses through a common generator, making one bus vulnerable to faults and other dynamic events on the other bus. Thus, understanding the dynamics of cross-bus coupling is imperative to the successful implementation of a dual-wound generator system. In (Rashkin, 2017), a kilowatt-scale system was analysed that considered the use of a dual-wound permanent magnet machine, two passive rectifiers, and two DC buses with resistive loads. For this system, dc voltage variation on one bus was evaluated in the time domain as a function of load changes on the other bus. Therein, substantive cross-bus coupling was demonstrated in simulation and hardware experiments. The voltage disturbances were attributed to electromechanical (i.e. speed disturbances) as well as electromagnetic coupling mechanisms. In this work, a 25 MVA dual-wound generator was considered, and active rectifier models were implemented in Matlab both using average value modelling and switching (space vector modulation) simulation models. The frequency dynamics of the system between the load on one side and the dc voltage on the other side was studied. The coupling is depicted in the frequency domain as a transfer function with amplitude and phase and is shown to have distinct characteristics (i.e. frequency regimes) associated with physical coupling mechanisms such as electromechanical and electromagnetic coupling as well as response characteristics associated with control action by the active rectifiers. In addition, based on requirements outlined in draft Military Standard 1399-MVDC, an approach to derive specifications will be discussed and presented. This method will aid in quantifying the allowable coupling of energy from one bus to another in various frequency regimes as a function of other power system parameters. Finally, design and control strategies will be discussed to mitigate cross-bus coupling. The findings of this work will inform the design, control, and operation of future naval warship power systems.
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.