Cost-Effective DC Source Emulator for Microgrid Testing

Combining a dynamic load emulation technique with a PWM dithering technique, NASA’s technology provides a more efficient, cost-effective, and practical method to emulate complex loads. While there are commercially available electronic device loads on the market that meet basic emulation needs, these devices are limited; they are limited with respect to small input voltage changes, and to feedback signals from the device’s power system, which may lack the strength and resolution needed to emulate accurately. A common solution for the bus emulation limitation is to construct a model of an actual microgrid using representative loads and connections. But this can be complex, costly, and have limitations in performance. NASA’s approach addresses these challenges without creating an actual model microgrid to replicate the systems. As opposed to stand-alone COTS electronic load devices or model microgrids using representative loads and connections for a given test, NASA’s technology is a system constructed of an input power filter, a COTS electronic load device or load subsystem, and a power control circuit. The input power filter is designed to emulate load or bus performance at the medium to high frequency range. The power control circuit combined with the electronic load or load subsystem emulates lower frequency and constant power dynamics of the system. Lastly, the power control circuit linearizes digitization and quantization issues present with digitally controlled COTS electronic loads. The power control circuit can be set to measure a load voltage, which is divided by a determined value for power, and combined with a triangle wave dither (the power control circuit block image demonstrates how to integrate a triangle wave dither). This dither dynamically adjusts the electrical current or power to keep it constant within the commercially purchased load device, enabling accurate emulation of complex DC microgrid systems.

In general, individual fuel cells produce relatively small electrical potentials, so fuel cells are "stacked" or placed in series (anode to cathode) to increase the combined voltage and meet the application's requirements. The current is drawn off by connection points, which typically are at the extreme ends of the fuel cell stack. DC power converters reduce or boost the voltage produced at the ends of the stack into a voltage that can be used by attached device. However, these converters add cost, mass, volume, and potential failure points into the fuel cell system. With NASA Glenn's groundbreaking technique, the fuel cell stack includes a plurality of connection points to the device instead of having a fixed number of individual fuel cells. By connecting additional cells in the same stack to the device the system power can be to be tailored to produce the required voltage for the connected device. Initially, this plurality includes a ground, a first connection point, and a second connection point. Additional connection points to the device can be added as needed, resulting in various powers that are available for use. Each connection point allows power to be drawn the combined voltages of the fuel cells located between the connection point and the ground. This configuration permits the voltage to be adjusted to the system power requirements of the device without the need to add DC power converters to the fuel cell system to add additional fuel cell systems to meet the power demand of the device. For larger fuel cell configurations in particular, NASA's innovative technique results in a far less costly, more efficient means of power generation.

NASA’s Self-Bootstrapping IPC operates in either transition mode for bootstrapping or fixed frequency mode for a regulated output via closed feedback. The transition mode is initially turned on via the input (i.e., primary) voltage control of the main switch and acts as a bootstrap converter utilizing a Gallium Nitrate transistor to control peak primary inductor current. That peak current can be varied via the sensor gain and/or a precise artificially generated offset and controls the switching frequency together with the secondary load (i.e., output). The IPC operates in transition mode until the Pulse Width Modulator (PWM) Under Voltage Lockout threshold is reached and fixed frequency mode begins. Fixed frequency operation is controlled by the PWM and the normal operation mode of the converter maintains a frequency while varying the duty cycle as needed. The PWM is secondary ground referenced and controls primary switching via galvanic isolation. The peak current in transition mode is set higher than the peak current in fixed frequency operation to prevent interruption or instability while in fixed frequency operation after bootstrap is completed. However, the transition mode control can serve as the overall peak current limiter. This invention is applicable to both flyback and buck-derived topologies with similar efficiency and size advantages. While NASA originally developed the Self-Bootstrapping IPC for CubeSats and space-based electronics with strict SWaP requirements, it may also be useful for safety-critical industries (e.g., aerospace and defense) to allow for high reliability power supplies and more favorable SWaP than existing state-of-the-art high-power dc-dc converters. The reliability, efficiency, and SWaP advantages of this NASA invention could also benefit medium- and high-power commercial power supplies.

NASA’s Universal Modular Interface Converter (UMIC) is a bidirectional, modular power electronics converter that transfers power between a 120 V DC space power bus, and a medium-to-high-voltage, three-phase AC power grid. The UMIC system contains multiple parallel AC/DC UMIC modules that convert between 120 V DC and low voltage AC, as well as one or more transformers that convert power from the low voltage AC bus to the grid voltage. The UMIC module consists of multiple subsystems, including the power stage, gate driver, Field Programmable Gate Array (FPGA)-based controller, output filter, signal conditioning and sensing circuits, and thermal management subsystems. An FPGA-based controller is included within each AC/DC module and is used to regulate desired power system variables; synchronize power switching events and share load current between modules; synchronize the modules with existing service on the grid; receive commands; and share telemetry. The FPGA-based controller subsystem includes the FPGA Integrated Circuit, associated flash memory, and a controller area network (CAN) transceiver. It is envisioned that future UMIC designs can support lunar grid expansions, a Mars surface grid, or large space stations. These applications may necessitate different grid voltages or frequencies, or different control logic and communication systems. However, the core UMIC architecture and functionality will remain the same. The related patent is now available to license. Please note that NASA does not manufacture products itself for commercial sale.

Glenn's novel system supports the NASA Aeronautics Research Mission Directorate (ARMD) strategic plan to leverage advancements in technologies over the next 25 years and beyond, leading to new aircraft configurations with enhanced performance, improved energy efficiency, and reduced CO2 emissions. The electric system is a multi-megawatt micro-grid that converts mechanical energy to electric via generators, and electric energy to mechanical via motor-driven fans. This innovation would use the variation in aircraft throttle settings to produce a high-voltage (20 kilovolts), variable-frequency 9-phase AC distribution system. Using doubly fed electric machines (generator, propulsor, and flywheel) allows for field excitation that can cause variable-frequency or variable speed operation around the commanded throttle setting. The flywheel enables an energy storage system that recovers and reuses energy, while the flywheel slews with the throttle control using the electromagnetic torque produced by the doubly fed electric machine. This design permits both sub-synchronous and super-synchronous operation using limited field excitation power provided through power converters. Finally, the reduced switchgear mass facilitated through the use of a high-frequency AC system, setting-less protection zones, and simplified switches for fault clearance provides enhanced operational capability. This system can be controlled so that fault energy is minimized, preventing collateral damage to aircraft structures even with high voltage distribution. Glenn's innovative system adds performance, efficiency, reliability, and cost savings to cutting-edge hybrid electric technology. This is an early-stage technology requiring additional development, and Glenn welcomes co-development opportunities.